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Interaural time difference
From Wikipedia, the free encyclopedia
The interaural time difference (or ITD) when concerning humans
or animals, is the difference inarrival time of a sound between two
ears. It is important in the localization of sounds, as it
providesa cue to the direction or angle of the sound source from
the head. If a signal arrives at the head fromone side, the signal
has further to travel to reach the far ear than the near ear. This
pathlengthdifference results in a time difference between the
sound's arrivals at the ears, which is detected andaids the process
of identifying the direction of sound source.When a signal is
produced in the horizontal plane, its angle in relation to the head
is referred to asits azimuth, with 0 degrees (0) azimuth being
directly in front of the listener, 90 to the right, and180 being
directly behind.
Different methods for measuring ITDs For an abrupt stimulus such
as a click, onset ITDs are measured. An onset ITD is the time
difference between the onset of the signal reaching two ears. A
transient ITD can be measured when using a random noise stimulus
and is calculated as
the time difference between a set peak of the noise stimulus
reaching the ears. If the stimulus used is not abrupt but periodic
then ongoing ITDs are measured. This is
where the waveforms reaching both ears can be shifted in time
until they perfectly match upand the size of this shift is recorded
as the ITD. This shift is known as the interaural phasedifference
(IPD) and can be used for measuring the ITDs of periodic inputs
such as puretones and amplitude modulated stimuli. An amplitude
modulated stimulus IPD can beassessed by looking at either the
waveform envelope or the waveform fine structure.
Duplex theoryThe Duplex theory proposed by Lord Rayleigh (1907)
provides an explanation for the ability ofhumans to localise sounds
by time differences between the sounds reaching each ear (ITDs)
anddifferences in sound level entering the ears (interaural level
differences, ILDs). But there still lies aquestion whether ITD or
ILD is prominent.The duplex theory states that ITDs are used to
localise low frequency sounds, in particular, whileILDs are used in
the localisation of high frequency sound inputs. However, the
frequency ranges forwhich the auditory system can use ITDs and ILDs
significantly overlap, and most natural soundswill have both high
and low frequency components, so that the auditory system will in
most caseshave to combine information from both ITDs and ILDs to
judge the location of a sound source.[1] Aconsequence of this
duplex system is that it is also possible to generate so-called
"cue trading" or"timeintensity trading" stimuli on headphones,
where ITDs pointing to the left are offset by ILDspointing to the
right, so the sound is perceived as coming from the midline. A
limitation of theduplex theory is that the theory does not
completely explain directional hearing, as no explanation isgiven
for the ability to distinguish between a sound source directly in
front and behind. Also thetheory only relates to localising sounds
in the horizontal plane around the head. The theory alsodoes not
take into account the use of the pinna in localisation.(Gelfand,
2004)
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Experiments conducted by Woodworth (1938) tested the duplex
theory by using a solid sphere tomodel the shape of the head and
measuring the ITDs as a function of azimuth for
differentfrequencies. The model used had a distance between the 2
ears of approximately 2223 cm. Initialmeasurements found that there
was a maximum time delay of approximately 660 s when the
soundsource was placed at directly 90 azimuth to one ear. This time
delay correlates to the wavelength ofa sound input with a frequency
of 1500 Hz. The results concluded that when a sound played had
afrequency less than 1500 Hz the wavelength is greater than this
maximum time delay between theears. Therefore there is a phase
difference between the sound waves entering the ears
providingacoustic localisation cues. With a sound input with a
frequency closer to 1500 Hz the wavelength ofthe sound wave is
similar to the natural time delay. Therefore due to the size of the
head and thedistance between the ears there is a reduced phase
difference so localisations errors start to be made.When a high
frequency sound input is used with a frequency greater than 1500
Hz, the wavelengthis shorter than the distance between the 2 ears,
a head shadow is produced and ILD provide cues forthe localisation
of this sound.Feddersen et al. (1957) also conducted experiments
taking measurements on how ITDs alter withchanging the azimuth of
the loudspeaker around the head at different frequencies. But
unlike theWoodworth experiments human subjects were used rather
than a model of the head. The experimentresults agreed with the
conclusion made by Woodworth about ITDs. The experiments
alsoconcluded that is there is no difference in ITDs when sounds
are provided from directly in front orbehind at 0 and 180 azimuth.
The explanation for this is that the sound is equidistant from
bothears. Interaural time differences alter as the loudspeaker is
moved around the head. The maximumITD of 660 s occurs when a sound
source is positioned at 90 azimuth to one ear.
The anatomy of the ITD pathwayThe auditory nerve fibres, known
as the afferent nerve fibres, carry information from the organ
ofCorti to the brainstem and brain. Auditory afferent fibres
consist of two types of fibres called type Iand type II fibres.
Type I fibres innervate the base of one or two inner hair cells and
Type II fibresinnervate the outer hair cells. Both leave the organ
of Corti through an opening called the habenulaperforata. The type
I fibres are thicker than the type II fibres and may also differ in
how theyinnervate the inner hair cells. Neurons with large calycal
endings ensure preservation of timinginformation throughout the ITD
pathway.Next in the pathway is the cochlear nucleus, which receives
mainly ipsilateral (that is, from thesame side) afferent input. The
cochlear nucleus has three distinct anatomical divisions, known
asthe antero-ventral cochlear nucleus (AVCN), postero-ventral
cochlear nucleus (PVCN) and dorsalcochlear nucleus (DCN) and each
have different neural innervations.The AVCN contains predominant
bushy cells, with one or two profusely branching dendrites; it
isthought that bushy cells may process the change in the spectral
profile of complex stimuli. TheAVCN also contain cells with more
complex firing patterns than bushy cells called multipolar
cells,these cells have several profusely branching dendrites and
irregular shaped cell bodies. Multipolarcells are sensitive to
changes in acoustic stimuli and in particular, onset and offset of
sounds, as wellas changes in intensity and frequency. The axons of
both cell types leave the AVCN as large tractcalled the ventral
acoustic stria, which forms part of the trapezoid body and travels
to the superiorolivary complex.A group of nuclei in pons make up
the superior olivary complex (SOC). This is the first stage
inauditory pathway to receive input from both cochleas, which is
crucial for our ability to localise thesounds source in the
horizontal plane. The SOC receives input from cochlear nuclei,
primarily theipsilateral and contralateral AVCN. Four nuclei make
up the SOC but only the medial superior olive(MSO) and the lateral
superior olive (LSO) receive input from both cochlear nuclei.The
MSO is made up of neurons which receive input from the
low-frequency fibers of the left and
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right AVCN. The result of having input from both cochleas is an
increase in the firing rate of theMSO units. The neurons in the MSO
are sensitive to the difference in the arrival time of sound ateach
ear, also known as the interaural time difference (ITD). Research
shows that if stimulationarrives at one ear before the other, many
of the MSO units will have increased discharge rates. Theaxons from
the MSO continue to higher parts of the pathway via the ipsilateral
lateral lemniscustract.(Yost, 2000)The lateral lemniscus (LL) is
the main auditory tract in the brainstem connecting SOC to
theinferior colliculus. The dorsal nucleus of the lateral lemniscus
(DNLL) is a group of neuronsseparated by lemniscus fibres, these
fibres are predominantly destined for the inferior colliculus(IC).
In studies using an unanesthetized rabbit the DNLL was shown to
alter the sensitivity of the ICneurons and may alter the coding of
interaural timing differences (ITDs) in the IC.(Kuwada et al.,2005)
The ventral nucleus of the lateral lemniscus (VNLL) is a chief
source of input to the inferiorcolliculus. Research using rabbits
shows the discharge patterns, frequency tuning and dynamicranges of
VNLL neurons supply the inferior colliculus with a variety of
inputs, each enabling adifferent function in the analysis of
sound.(Batra & Fitzpatrick, 2001) In the inferior colliculus
(IC)all the major ascending pathways from the olivary complex and
the central nucleus converge. TheIC is situated in the midbrain and
consists of a group of nuclei the largest of these is the
centralnucleus of inferior colliculus (CNIC). The greater part of
the ascending axons forming the laterallemniscus will terminate in
the ipsilateral CNIC however a few follow the commissure of
Probstand terminate on the contralateral CNIC. The axons of most of
the CNIC cells form the brachium ofIC and leave the brainstem to
travel to the ipsilateral thalamus. Cells in different parts of the
IC tendto be monaural, responding to input from one ear, or
binaural and therefore respond to bilateralstimulation.The spectral
processing that occurs in the AVCN and the ability to process
binaural stimuli, as seenin the SOC, are replicated in the IC.
Lower centres of the IC extract different features of theacoustic
signal such as frequencies, frequency bands, onsets, offsets,
changes in intensity andlocalisation. The integration or synthesis
of acoustic information is thought to start in the CNIC.(Yost,
2000)
Effect of a hearing lossA number of studies have looked into the
effect of hearing loss on interaural time differences. Intheir
review of localisation and lateralisation studies, Durlach,
Thompson, and Colburn (1981),citedin Moore (1996) found a clear
trend for poor localization and lateralization in people
withunilateral or asymmetrical cochlear damage. This is due to the
difference in performance betweenthe two ears. In support of this,
they did not find significant localisation problems in
individualswith symmetrical cochlear losses. In addition to this,
studies have been conducted into the effect ofhearing loss on the
threshold for interaural time differences. The normal human
threshold fordetection of an ITD is up to a time difference of 10s
(microseconds). Studies by Gabriel, Koehnke,& Colburn (1992),
Husler, Colburn, & Marr (1983) and Kinkel, Kollmeier, &
Holube (1991)(citedby Moore, 1996) have shown that there can be
great differences between individuals regardingbinaural
performance. It was found that unilateral or asymmetric hearing
losses can increase thethreshold of ITD detection in patients. This
was also found to apply to individuals with symmetricalhearing
losses when detecting ITDs in narrowband signals. However, ITD
thresholds seem to benormal for those with symmetrical losses when
listening to broadband sounds.
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Sound localizationSound localization refers to a listener's
ability to identify the location or origin of a detected soundin
direction and distance. It may also refer to the methods in
acoustical engineering to simulate theplacement of an auditory cue
in a virtual 3D space (see binaural recording, wave field
synthesis).The sound localization mechanisms of the mammalian
auditory system have been extensivelystudied. The auditory system
uses several cues for sound source localization, including time-
andlevel-differences between both ears, spectral information,
timing analysis, correlation analysis, andpattern matching.These
cues are also used by animals, but there may be differences in
usage, and there are alsolocalization cues which are absent in the
human auditory system, such as the effects of earmovements.
Sound localization by the human auditory systemSound
localization is the process of determining the location of a sound
source. The brain utilizessubtle differences in intensity,
spectral, and timing cues to allow us to localize sound sources.[1]
[2] Localization can be described in terms of three-dimensional
position: the azimuth or horizontalangle, the elevation or vertical
angle, and the distance (for static sounds) or velocity (for
movingsounds).[3] The azimuth of a sound is signalled by the
difference in arrival times between the ears,by the relative
amplitude of high-frequency sounds (the shadow effect), and by the
asymmetricalspectral reflections from various parts of our bodies,
including torso, shoulders, and pinnae.[3] Thedistance cues are the
loss of amplitude, the loss of high frequencies, and the ratio of
the direct signalto the reverberated signal.[3] Depending on where
the source is located, our head acts as a barrier tochange the
timbre, intensity, and spectral qualities of the sound, helping the
brain orient where thesound emanated from.[2] These minute
differences between the two ears are known as interauralcues.[2]
Lower frequencies, with longer wavelengths, diffract the sound
around the head forcing thebrain to focus only on the phasing cues
from the source.[2] Helmut Haas discovered that we candiscern the
sound source despite additional reflections at 10 decibels louder
than the original wavefront, using the earliest arriving wave
front.[2] This principle is known as the Haas effect, a
specificversion of the precedence effect.[2] Haas measured down to
even a 1 millisecond difference intiming between the original sound
and reflected sound increased the spaciousness, allowing thebrain
to discern the true location of the original sound. The nervous
system combines all earlyreflections into a single perceptual whole
allowing the brain to process multiple different sounds atonce.[4]
The nervous system will combine reflections that are within about
35 milliseconds of eachother and that have a similar
intensity.[4]
Lateral information (left, ahead, right)For determining the
lateral input direction (left, front, right) the auditory system
analyzes thefollowing ear signal information:
Interaural time differences Sound from the right side reaches
the right ear earlier than the left ear. The auditory
systemevaluates interaural time differences from
Phase delays at low frequencies group delays at high
frequencies
Interaural level differences
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Sound from the right side has a higher level at the right ear
than at the left ear, because thehead shadows the left ear. These
level differences are highly frequency dependent and theyincrease
with increasing frequency.
For frequencies below 800 Hz, mainly interaural time differences
are evaluated (phase delays), forfrequencies above 1600 Hz mainly
interaural level differences are evaluated. Between 800 Hz and1600
Hz there is a transition zone, where both mechanisms play a
role.Localization accuracy is 1 degree for sources in front of the
listener and 15 degrees for sources tothe sides. Humans can discern
interaural time differences of 10 microseconds or less.[5] [6]
Evaluation for low frequenciesFor frequencies below 800 Hz, the
dimensions of the head (ear distance 21.5 cm, corresponding toan
interaural time delay of 625 s), are smaller than the half
wavelength of the sound waves. So theauditory system can determine
phase delays between both ears without confusion. Interaural
leveldifferences are very low in this frequency range, especially
below about 200 Hz, so a preciseevaluation of the input direction
is nearly impossible on the basis of level differences alone. As
thefrequency drops below 80 Hz it becomes difficult or impossible
to use either time difference orlevel difference to determine a
sound's lateral source, because the phase difference between the
earsbecomes too small for a directional evaluation.
Evaluation for high frequenciesFor frequencies above 1600 Hz the
dimensions of the head are greater than the length of the
soundwaves. An unambiguous determination of the input direction
based on interaural phase alone is notpossible at these
frequencies. However, the interaural level differences become
larger, and theselevel differences are evaluated by the auditory
system. Also, group delays between the ears can beevaluated, and is
more pronounced at higher frequencies; that is, if there is a sound
onset, the delayof this onset between the ears can be used to
determine the input direction of the correspondingsound source.
This mechanism becomes especially important in reverberant
environment. After asound onset there is a short time frame where
the direct sound reaches the ears, but not yet thereflected sound.
The auditory system uses this short time frame for evaluating the
sound sourcedirection, and keeps this detected direction as long as
reflections and reverberation prevent anunambiguous direction
estimation.The mechanisms described above cannot be used to
differentiate between a sound source ahead ofthe hearer or behind
the hearer; therefore additional cues have to be evaluated.
Sound localization in the median plane (front, above, back,
below)The human outer ear, i.e. the structures of the pinna and the
external ear canal, form direction-selective filters. Depending on
the sound input direction in the median plane, different
filterresonances become active. These resonances implant
direction-specific patterns into the frequencyresponses of the
ears, which can be evaluated by the auditory system (directional
bands) for verticalsound localization. Together with other
direction-selective reflections at the head, shoulders andtorso,
they form the outer ear transfer functions.These patterns in the
ear's frequency responses are highly individual, depending on the
shape andsize of the outer ear. If sound is presented through
headphones, and has been recorded via anotherhead with
different-shaped outer ear surfaces, the directional patterns
differ from the listener's own,and problems will appear when trying
to evaluate directions in the median plane with these foreignears.
As a consequence, frontback permutations or
inside-the-head-localization can appear whenlistening to dummy head
recordings,or otherwise referred to as binaural recordings.
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Distance of the sound sourceThe human auditory system has only
limited possibilities to determine the distance of a soundsource.
In the close-up-range there are some indications for distance
determination, such as extremelevel differences (e.g. when
whispering into one ear) or specific pinna resonances in the
close-uprange.The auditory system uses these clues to estimate the
distance to a sound source:
Sound spectrum : High frequencies are more quickly damped by the
air than lowfrequencies. Therefore a distant sound source sounds
more muffled than a close one, becausethe high frequencies are
attenuated. For sound with a known spectrum (e.g. speech)
thedistance can be estimated roughly with the help of the perceived
sound.
Loudness: Distant sound sources have a lower loudness than close
ones. This aspect can beevaluated especially for well-known sound
sources (e.g. known speakers).
Movement: Similar to the visual system there is also the
phenomenon of motion parallax inacoustical perception. For a moving
listener nearby sound sources are passing faster thandistant sound
sources.
Reflections: In enclosed rooms two types of sound are arriving
at a listener: The directsound arrives at the listener's ears
without being reflected at a wall. Reflected sound hasbeen
reflected at least one time at a wall before arriving at the
listener. The ratio betweendirect sound and reflected sound can
give an indication about the distance of the soundsource.
Signal processingSound processing of the human auditory system
is performed in so-called critical bands. Thehearing range is
segmented into 24 critical bands, each with a width of 1 Bark or
100 Mel. For adirectional analysis the signals inside the critical
band are analyzed together.The auditory system can extract the
sound of a desired sound source out of interfering noise. So
theauditory system can concentrate on only one speaker if other
speakers are also talking (the cocktailparty effect). With the help
of the cocktail party effect sound from interfering directions is
perceivedattenuated compared to the sound from the desired
direction. The auditory system can increase thesignal-to-noise
ratio by up to 15 dB, which means that interfering sound is
perceived to beattenuated to half (or less) of its actual
loudness.
Localization in enclosed roomsIn enclosed rooms not only the
direct sound from a sound source is arriving at the listener's
ears, butalso sound which has been reflected at the walls. The
auditory system analyses only the directsound[citation needed],
which is arriving first, for sound localization, but not the
reflected sound,which is arriving later (law of the first wave
front). So sound localization remains possible even inan echoic
environment. This echo cancellation occurs in the Dorsal Nucleus of
the LateralLemniscus (DNLL).In order to determine the time periods,
where the direct sound prevails and which can be used
fordirectional evaluation, the auditory system analyzes loudness
changes in different critical bands andalso the stability of the
perceived direction. If there is a strong attack of the loudness in
severalcritical bands and if the perceived direction is stable,
this attack is in all probability caused by thedirect sound of a
sound source, which is entering newly or which is changing its
signalcharacteristics. This short time period is used by the
auditory system for directional and loudnessanalysis of this sound.
When reflections arrive a little bit later, they do not enhance the
loudness
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inside the critical bands in such a strong way, but the
directional cues become unstable, becausethere is a mix of sound of
several reflection directions. As a result no new directional
analysis istriggered by the auditory system.This first detected
direction from the direct sound is taken as the found sound source
direction, untilother strong loudness attacks, combined with stable
directional information, indicate that a newdirectional analysis is
possible. (see Franssen effect)
AnimalsSince most animals have two ears, many of the effects of
the human auditory system can also befound in animals. Therefore
interaural time differences (interaural phase differences) and
interaurallevel differences play a role for the hearing of many
animals. But the influences on localization ofthese effects are
dependent on head sizes, ear distances, the ear positions and the
orientation of theears.
Lateral information (left, ahead, right)If the ears are located
at the side of the head, similar lateral localization cues as for
the humanauditory system can be used. This means: evaluation of
interaural time differences (interaural phasedifferences) for lower
frequencies and evaluation of interaural level differences for
higherfrequencies. The evaluation of interaural phase differences
is useful, as long as it givesunambiguous results. This is the
case, as long as ear distance is smaller than half the
length(maximal one wavelength) of the sound waves. For animals with
a larger head than humans theevaluation range for interaural phase
differences is shifted towards lower frequencies, for animalswith a
smaller head, this range is shifted towards higher frequencies.The
lowest frequency which can be localized depends on the ear
distance. Animals with a greaterear distance can localize lower
frequencies than humans can. For animals with a smaller eardistance
the lowest localizable frequency is higher than for humans.If the
ears are located at the side of the head, interaural level
differences appear for higherfrequencies and can be evaluated for
localization tasks. For animals with ears at the top of the head,no
shadowing by the head will appear and therefore there will be much
less interaural leveldifferences, which could be evaluated. Many of
these animals can move their ears, and these earmovements can be
used as a lateral localization cue.
Sound localization by odontocetesDolphins (and other
odontocetes) rely on echolocation to aid in detecting, identifying,
localizing,and capturing prey. Dolphin sonar signals are well
suited for localizing multiple, small targets in a
three dimensional aquatic environment by utilizing highly
directional (3 dB beamwidth of about 10deg), broadband (3 dB
bandwidth typically of about 40 kHz; peak frequencies between 40
kHz and120 kHz), short duration clicks (about 40 s). Dolphins can
localize sounds both passively and
actively (echolocation) with a resolution of about 1 deg. Cross
modal matching (between vision andecholocation) suggests dolphins
perceive the spatial structure of complex objects
interrogatedthrough echolocation, a feat that likely requires
spatially resolving individual object features andintegration into
a holistic representation of object shape. Although dolphins are
sensitive to small,
binaural intensity and time differences, mounting evidence
suggests dolphins employ positiondependent spectral cues derived
from well developed head related transfer functions, for sound
localization in both the horizontal and vertical planes. A very
small temporal integration time (264s) allows localization of
multiple targets at varying distances. Localization adaptations
includepronounced asymmetry of the skull, nasal sacks, and
specialized lipid structures in the forehead andjaws, as well as
acoustically isolated middle and inner ears.
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Sound localization in the median plane (front, above, back,
below)For many mammals there are also pronounced structures in the
pinna near the entry of the ear canal.As a consequence,
direction-dependent resonances can appear, which could be used as
an additionallocalization cue, similar to the localization in the
median plane in the human auditory system. Thereare additional
localization cues which are also used by animals.
Head tiltingFor sound localization in the median plane
(elevation of the sound) also two detectors can be used,which are
positioned at different heights. In animals, however, rough
elevation information isgained simply by tilting the head, provided
that the sound lasts long enough to complete themovement. This
explains the innate behavior of cocking the head to one side when
trying to localizea sound precisely. To get instantaneous
localization in more than two dimensions from time-difference or
amplitude-difference cues requires more than two detectors.
Localization with one ear (flies)The tiny parasitic fly Ormia
ochracea has become a model organism in sound
localizationexperiments because of its unique ear. The animal is
too small for the time difference of soundarriving at the two ears
to be calculated in the usual way, yet it can determine the
direction of soundsources with exquisite precision. The tympanic
membranes of opposite ears are directly connectedmechanically,
allowing resolution of sub-microsecond time differences[7] [8] and
requiring a newneural coding strategy.[9] Ho[10] showed that the
coupled-eardrum system in frogs can produceincreased interaural
vibration disparities when only small arrival time and sound level
differenceswere available to the animals head. Efforts to build
directional microphones based on the coupled-eardrum structure are
underway.
Bi-coordinate sound localization in owlsMost owls are nocturnal
or crepuscular birds of prey. Because they hunt at night, they must
rely onnon-visual senses. Experiments by Roger Payne[11] have shown
that owls are sensitive to thesounds made by their prey, not the
heat or the smell. In fact, the sound cues are both necessary
andsufficient for localization of mice from a distant location
where they are perched. For this to work,the owls must be able to
accurately localize both the azimuth and the elevation of the sound
source.
ITD and ILDOwls must be able to determine the necessary angle of
descent, i.e. the elevation, in addition toazimuth (horizontal
angle to the sound). This bi-coordinate sound localization is
accomplishedthrough two binaural cues: the interaural time
difference (ITD) and the interaural level difference(ILD), also
known as the interaural intensity difference (IID). The ability in
owls is unusual; inground-bound mammals such as mice, ITD and ILD
are not utilized in the same manner. In thesemammals, ITDs tend to
be utilized for localization of lower frequency sounds, while ILDs
tend tobe used for higher frequency sounds.ITD occurs whenever the
distance from the source of sound to the two ears is different,
resulting indifferences in the arrival times of the sound at the
two ears. When the sound source is directly infront of the owl,
there is no ITD, i.e. the ITD is zero. In sound localization, ITDs
are used as cuesfor location in the azimuth. ITD changes
systematically with azimuth. Sounds to the right arrivefirst at the
right ear; sounds to the left arrive first at the left ear.In
mammals there is a level difference in sounds at the two ears
caused by the sound-shadowingeffect of the head. But in many
species of owls, level differences arise primarily for sounds that
are
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shifted above or below the elevation of the horizontal plane.
This is due to the asymmetry inplacement of the ear openings in the
owl's head, such that sounds from below the owl reach the leftear
first and sounds from above reach the right ear first.[12] IID is a
measure of the difference in thelevel of the sound as it reaches
each ear. In many owls, IIDs for high-frequency sounds (higher
than4 or 5 kHz) are the principal cues for locating sound
elevation.
Parallel processing pathways in the brainThe axons of the
auditory nerve originate from the hair cells of the cochlea in the
inner ear.Different sound frequencies are encoded by different
fibers of the auditory nerve, arranged alongthe length of the
auditory nerve, but codes for the timing and level of the sound are
not segregatedwithin the auditory nerve. Instead, the ITD is
encoded by phase locking, i.e. firing at or near aparticular phase
angle of the sinusoidal stimulus sound wave, and the IID is encoded
by spike rate.Both parameters are carried by each fiber of the
auditory nerve.[13]The fibers of the auditory nerve innervate both
cochlear nuclei in the brainstem, the cochlearnucleus
magnocellularis (mammalian anteroventral cochlear nucleus) and the
cochlear nucleusangularis (see figure; mammalian posteroventral and
dorsal cochlear nuclei). The neurons of thenucleus magnocellularis
phase-lock, but are fairly insensitive to variations in sound
pressure, whilethe neurons of the nucleus angularis phase-lock
poorly, if at all, but are sensitive to variations insound
pressure. These two nuclei are the starting points of two separate
but parallel pathways to theinferior colliculus: the pathway from
nucleus magnocellularis processes ITDs, and the pathwayfrom nucleus
angularis processes IID.
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Parallel processing pathways in the brain for time and level for
sound localization in the owlIn the time pathway, the nucleus
laminaris (mammalian medial superior olive) is the first site
ofbinaural convergence. It is here that ITD is detected and encoded
using neuronal delay lines andcoincidence detection, as in the
Jeffress model; when phase-locked impulses coming from the leftand
right ears coincide at a laminaris neuron, the cell fires most
strongly. Thus, the nucleuslaminaris acts as a delay-line
coincidence detector, converting distance traveled to time delay
andgenerating a map of interaural time difference. Neurons from the
nucleus laminaris project to thecore of the central nucleus of the
inferior colliculus and to the anterior lateral lemniscal
nucleus.In the sound level pathway, the posterior lateral lemniscal
nucleus (mammalian lateral superiorolive) is the site of binaural
convergence and where IID is processed. Stimulation of
thecontralateral ear inhibits and that of the ipsilateral ear
excites the neurons of the nuclei in each brainhemisphere
independently. The degree of excitation and inhibition depends on
sound pressure, andthe difference between the strength of the
inhibitory input and that of the excitatory inputdetermines the
rate at which neurons of the lemniscal nucleus fire. Thus the
response of theseneurons is a function of the difference in sound
pressure between the two ears.The time and sound-pressure pathways
converge at the lateral shell of the central nucleus of theinferior
colliculus. The lateral shell projects to the external nucleus,
where each space-specific
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neuron responds to acoustic stimuli only if the sound originates
from a restricted area in space, i.e.the receptive field of that
neuron. These neurons respond exclusively to binaural signals
containingthe same ITD and IID that would be created by a sound
source located in the neurons receptivefield. Thus their receptive
fields arise from the neurons tuning to particular combinations of
ITDand IID, simultaneously in a narrow range. These space-specific
neurons can thus form a map ofauditory space in which the positions
of receptive fields in space are isomorphically projected ontothe
anatomical sites of the neurons.[14]
Significance of asymmetrical ears for localization of
elevationThe ears of many species of owls are asymmetrical. For
example, in barn owls (Tyto alba), theplacement of the two ear
flaps (operculi) lying directly in front of the ear canal opening
is differentfor each ear. This asymmetry is such that the center of
the left ear flap is slightly above a horizontalline passing
through the eyes and directed downward, while the center of the
right ear flap isslightly below the line and directed upward. In
two other species of owls with asymmetrical ears,the saw-whet Owl
and the long-eared owl, the asymmetry is achieved by different
means: in sawwhets, the skull is asymmetrical; in the long-eared
owl, the skin structures lying near the ear formasymmetrical
entrances to the ear canals, which is achieved by a horizontal
membrane. Thus, earasymmetry seems to have evolved on at least
three different occasions among owls. Because owlsdepend on their
sense of hearing for hunting, this convergent evolution in owl ears
suggests thatasymmetry is important for sound localization in the
owl.Ear asymmetry allows for sound originating from below the eye
level to sound louder in the left ear,while sound originating from
above the eye level to sound louder in the right ear. Asymmetrical
earplacement also causes IID for high frequencies (between 4 kHz
and 8 kHz) to vary systematicallywith elevation, converting IID
into a map of elevation. Thus, it is essential for an owl to have
theability to hear high frequencies. Many birds have the
neurophysiological machinery to process bothITD and IID, but
because they have small heads and low frequency sensitivity, they
use bothparameters only for localization in the azimuth. Through
evolution, the ability to hear frequencieshigher than 3 kHz, the
highest frequency of owl flight noise, enabled owls to exploit
elevationalIIDs, produced by small ear asymmetries that arose by
chance, and began the evolution of moreelaborate forms of ear
asymmetry.[15]Another demonstration of the importance of ear
asymmetry in owls is that, in experiments, owlswith symmetrical
ears, such as the screech owl (Otus asio) and the great horned owl
(Bubovirginianus), could not be trained to locate prey in total
darkness, whereas owls with asymmetricalears could be
trained.[16]
Neural interactionsIn vertebrates, inter-aural time differences
are known to be calculated in the superior olivarynucleus of the
brainstem. According to Jeffress,[17] this calculation relies on
delay lines: neurons inthe superior olive which accept innervation
from each ear with different connecting axon lengths.Some cells are
more directly connected to one ear than the other, thus they are
specific for aparticular inter-aural time difference. This theory
is equivalent to the mathematical procedure ofcross-correlation.
However, because Jeffress' theory is unable to account for the
precedence effect,in which only the first of multiple identical
sounds is used to determine the sounds' location (thusavoiding
confusion caused by echoes), it cannot be entirely used to explain
the response.Furthermore, a number of recent physiological
observations made in the midbrain and brainstem ofsmall mammals
have shed considerable doubt on the validity of Jeffress' original
ideas [18]Neurons sensitive to ILDs are excited by stimulation of
one ear and inhibited by stimulation of theother ear, such that the
response magnitude of the cell depends on the relative strengths of
the twoinputs, which in turn, depends on the sound intensities at
the ears.
-
In the auditory midbrain nucleus, the inferior colliculus (IC),
many ILD sensitive neurons haveresponse functions that decline
steeply from maximum to zero spikes as a function of ILD.However,
there are also many neurons with much more shallow response
functions that do notdecline to zero spikes.
Binaural fusion
Binaural fusion (or binaural integration) is a cognitive process
that involves the "fusion" ofdifferent auditory information
presented binaurally, or to each ear. In humans, this process
isessential in understanding speech as one ear may pick up more
information about the speech stimulithan the other.The process of
binaural fusion is important for computing the location of sound
sources in thehorizontal plane (sound localization), and it is
important for sound segregation.[1] Soundsegregation refers the
ability to identify acoustic components from one or more sound
sources.[2]The binaural auditory system is highly dynamic and
capable of rapidly adjusting tuning propertiesdepending on the
context in which sounds are heard. Each eardrum moves
one-dimensionally; theauditory brain analyzes and compares
movements of both eardrums to extract physical cues andsynthesize
auditory objects.[3]When stimulation from a sound reaches the ear,
the eardrum deflects in a mechanical fashion, andthe three middle
ear bones (ossicles) transmit the mechanical signal to the cochlea,
where hair cellstransform the mechanical signal into an electrical
signal. The auditory nerve, also called thecochlear nerve, then
transmits action potentials to the central auditory nervous
system.[3]In binaural fusion, inputs from both ears integrate and
fuse to create a complete auditory picture atthe brainstem.
Therefore, the signals sent to the central auditory nervous system
are representativeof this complete picture, integrated information
from both ears instead of a single ear.Binaural fusion is
responsible for what is known as the cocktail party effect, the
ability of a listenerto hear a particular speaker against other
interfering voices.[3]The binaural squelch effect is a result of
nuclei of the brainstem processing timing, amplitude, andspectral
differences between the two ears. Sounds are integrated and then
separated into auditoryobjects. For this effect to take place,
neural integration from both sides is required.[4]
-
AnatomyAs sound travels into the inner eardrum of vertebrate
mammals, it encounters the hair cells that linethe basilar membrane
of the cochlea in the inner ear.[5] The cochlea receives auditory
informationto be binaurally integrated. At the cochlea, this
information is converted into electrical impulses thattravel by
means of the cochlear nerve, which spans from the cochlea to the
ventral cochlear nucleus,which is located in the pons of the
brainstem.[6] The lateral lemniscus projects from the
cochlearnucleus to the superior olivary complex (SOC), a set of
brainstem nuclei that consists primarily oftwo nuclei, the medial
superior olive (MSO) and the lateral superior olive (LSO), and is
the majorsite of binaural fusion. The subdivision of the ventral
cochlear nucleus that concerns binaural fusionis the anterior
ventral cochlear nucleus (AVCN).[3] The AVCN consists of spherical
bushy cells andglobular bushy cells and can also transmit signals
to the medial nucleus of the trapezoid body(MNTB), whose neuron
projects to the MSO. Transmissions from the SOC travel to the
inferiorcolliculus (IC) via the lateral lemniscus. At the level of
the IC, binaural fusion is complete. Thesignal ascends to the
thalamocortical system, and sensory inputs to the thalamus are then
relayed tothe primary auditory cortex.[3] [7] [8] [9]
FunctionThe ear functions to analyze and encode a sounds
dimensions.[10] Binaural fusion is responsible
-
for avoiding the creation of multiple sound images from a sound
source and its reflections. Theadvantages of this phenomenon are
more noticeable in small rooms, decreasing as the
reflectivesurfaces are placed farther from the listener.[11]
Central auditory systemThe central auditory system converges
inputs from both ears (inputs contain no explicit
spatialinformation) onto single neurons within the brainstem. This
system contains many subcortical sitesthat have integrative
functions. The auditory nuclei collect, integrate, and analyze
afferent supply,[10] the outcome is a representation of auditory
space.[3] The subcortical auditory nuclei areresponsible for
extraction and analysis of dimensions of sounds.[10]The integration
of a sound stimulus is a result of analyzing frequency (pitch),
intensity, and spatiallocalization of the sound source.[12] Once a
sound source has been identified, the cells of lowerauditory
pathways are specialized to analyze physical sound parameters.[3]
Summation is observedwhen the loudness of a sound from one stimulus
is perceived as having been doubled when heardby both ears instead
of only one. This process of summation is called binaural summation
and is theresult of different acoustics at each ear, depending on
where sound is coming from. [4]The cochlear nerve spans from the
cochlea of the inner ear to the ventral cochlear nuclei located
inthe pons of the brainstem, relaying auditory signals to the
superior olivary complex where it is to bebinaurally
integrated.
Medial superior olive and lateral superior oliveThe MSO contains
cells that function in comparing inputs from the left and right
cochlear nuclei.[13] The tuning of neurons in the MSO favors low
frequencies, whereas those in the LSO favorhigh
frequencies.[14]GABAB receptors in the LSO and MSO are involved in
balance of excitatory and inhibitory inputs.The GABAB receptors are
coupled to G proteins and provide a way of regulating synaptic
efficacy.Specifically, GABAB receptors modulate excitatory and
inhibitory inputs to the LSO.[3] Whetherthe GABAB receptor
functions as excitatory or inhibitory for the postsynaptic neuron,
depends onthe exact location and action of the receptor.[1]
Sound localizationSound localization is the ability to correctly
identify the directional location of sounds. A soundstimulus
localized in the horizontal plane is called azimuth; in the
vertical plane it is referred to aselevation. The time, intensity,
and spectral differences in the sound arriving at the two ears are
usedin localization. Localization of low frequency sounds is
accomplished by analyzing interaural timedifference (ITD).
Localization of high frequency sounds is accomplished by analyzing
interaurallevel difference (ILD).[4]
Mechanism
Binaural hearingAction potentials originate in the hair cells of
the cochlea and propagate to the brainstem; both thetiming of these
action potentials and the signal they transmit provide information
to the SOC aboutthe orientation of sound in space. The processing
and propagation of action potentials is rapid, andtherefore,
information about the timing of the sounds that were heard, which
is crucial to binaural
-
processing, is conserved.[15] Each eardrum moves in one
dimension, and the auditory brainanalyzes and compares the
movements of both eardrums in order to synthesize auditory
objects.[3]This integration of information from both ears is the
essence of binaural fusion. The binaural systemof hearing involves
sound localization in the horizontal plane, contrasting with the
monaural systemof hearing, which involves sound localization in the
vertical plane.[3]
Superior olivary complexThe primary stage of binaural fusion,
the processing of binaural signals, occurs at the SOC,
whereafferent fibers of the left and right auditory pathways first
converge. This processing occurs becauseof the interaction of
excitatory and inhibitory inputs in the LSO and MSO.[3] [13] [1]
The SOCprocesses and integrates binaural information, in the form
of ITD and ILD, entering the brainstemfrom the cochleae. This
initial processing of ILD and ITD is regulated by GABAB
receptors.[1]
ITD and ILDThe auditory space of binaural hearing is constructed
based on the analysis of differences in twodifferent binaural cues
in the horizontal plane: sound level, or ILD, and arrival time at
the two ears,or ITD, which allow for the comparison of the sound
heard at each eardrum.[1] [3] ITD is processedin the LSO and
results from sounds arriving earlier at one ear than the other;
this occurs when thesound does not arise from directly in front or
directly behind the hearer. ILD is processed in theMSO and results
from the shadowing effect that is produced at the ear that is
farther from the soundsource. Outputs from the SOC are targeted to
the dorsal nucleus of the lateral lemniscus as well asthe
IC.[3]
Lateral superior oliveLSO neurons are excited by inputs from one
ear and inhibited by inputs from the other, and aretherefore
referred to as IE neurons. Excitatory inputs are received at the
LSO from spherical bushycells of the ipsilateral cochlear nucleus,
which combine inputs coming from several auditory nervefibers.
Inhibitory inputs are received at the LSO from globular bushy cells
of the contralateralcochlear nucleus.[3]
Medial superior oliveMSO neurons are excited bilaterally,
meaning that they are excited by inputs from both ears, andthey are
therefore referred to as EE neurons.[3] Fibers from the left
cochlear nucleus terminate onthe left of MSO neurons, and fibers
from the right cochlear nucleus terminate on the right of
MSOneurons.[13] Excitatory inputs to the MSO from spherical bushy
cells are mediated by glutamate,and inhibitory inputs to the MSO
from globular bushy cells are mediated by glycine. MSO
neuronsextract ITD information from binaural inputs and resolve
small differences in the time of arrival ofsounds at each ear.[3]
Outputs from the MSO and LSO are sent via the lateral lemniscus to
the IC,which integrates the spatial localization of sound. In the
IC, acoustic cues have been processed andfiltered into separate
streams, forming the basis of auditory object recognition.[3]
Binaural fusion abnormalities in autismCurrent research is being
performed on the dysfunction of binaural fusion in individuals
withautism. The neurological disorder autism is associated with
many symptoms of impaired brainfunction, including the degradation
of hearing, both unilateral and bilateral.[16] Individuals
withautism who experience hearing loss maintain symptoms such as
difficulty listening to backgroundnoise and impairments in sound
localization. Both the ability to distinguish particular speakers
frombackground noise and the process of sound localization are key
products of binaural fusion. They
-
are particularly related to the proper function of the SOC, and
there is increasing evidence thatmorphological abnormalities within
the brainstem, namely in the SOC, of autistic individuals are
acause of the hearing difficulties.[17] The neurons of the MSO of
individuals with autism displayatypical anatomical features,
including atypical cell shape and orientation of the cell body as
well asstellate and fusiform formations.[18] Data also suggests
that neurons of the LSO and MNTBcontain distinct dysmorphology in
autistic individuals, such as irregular stellate and fusiform
shapesand a smaller than normal size. Moreover, a significant
depletion of SOC neurons is seen in thebrainstem of autistic
individuals. All of these structures play a crucial role in the
proper functioningof binaural fusion, so their dysmorphology may be
at least partially responsible for the incidence ofthese auditory
symptoms in autistic patients.[17]
Acoustic location
Swedish soldiers operating an acoustic locator in 1940Acoustic
location is the science of using sound to determine the distance
and direction ofsomething. Location can be done actively or
passively, and can take place in gases (such as theatmosphere),
liquids (such as water), and in solids (such as in the earth).
Active acoustic location involves the creation of sound in order
to produce an echo, which isthen analyzed to determine the location
of the object in question.
Passive acoustic location involves the detection of sound or
vibration created by the objectbeing detected, which is then
analyzed to determine the location of the object in question.
Both of these techniques, when used in water, are known as
sonar; passive sonar and active sonarare both widely used.Acoustic
mirrors and dishes, when using microphones, are a means of passive
acoustic localization,but when using speakers are a means of active
localization. Typically, more than one device is used,and the
location is then triangulated between the several devices.As a
military air defense tool, passive acoustic location was used from
mid-World War I[1] to theearly years of World War II to detect
enemy aircraft by picking up the noise of their engines. It
wasrendered obsolete before and during World War II by the
introduction of radar, which was far moreeffective (but
interceptable). Acoustic techniques had the advantage that they
could 'see' aroundcorners and over hills, due to sound
refraction.The civilian uses include locating wildlife[2] and
locating the shooting position of a firearm.[3]
-
Antiaircraft defence Sweden 1940Swedish soldiers operating an
acoustic locatorSoldiers operating an acoustic airplane locator
during World War 2, Trelleborg, Sweden, 1940.Before radar was
invented, acoustic locator equipment was used to detect approaching
enemyaircraft by listening for the sound of their engines.
Military uses have included locating submarines[4] and
aircraft.[5]The air-defense instruments usually consisted of large
horns or microphones connected to theoperators ears using tubing,
much like a very large stethoscope.[6] [7] Most of the work on
anti-aircraft sound ranging was done by the British. They developed
anextensive network of sound mirrors that were used from World War
I through World War II.[8] [9] Sound mirrors normally work by using
moveable microphones to find the angle that maximizes theamplitude
of sound received, which is also the bearing angle to the target.
Two sound mirrors atdifferent positions will generate two different
bearings, which allows the use of triangulation todetermine a sound
source's position.As World War II neared, radar began to become a
credible alternative to the sound location ofaircraft. For typical
aircraft speeds of that time, sound location only gave a few
minutes of warning.[5] The acoustic location stations were left in
operation as a backup to radar, as exemplified duringthe Battle of
Britain.[10] Today, the abandoned sites are still in existence and
are readily accessible.[8]After World War II, sound ranging played
no further role in anti-aircraft operations.For enemy artillery
spotting, see sound ranging.
Active / passive locatorsActive locators have some sort of
signal generation device, in addition to a listening device. Thetwo
devices do not have to be located together.
SonarSONAR (Sound Navigation And Ranging) or sonar is a
technique that uses soundpropagation under water (or occasionally
in air) to navigate, communicate or to detect other vessels.There
are two kinds of sonar active and passive. A single active sonar
can localize in range andbearing as well as measuring radial speed.
However, a single passive sonar can only localize inbearing
directly, though target motion analysis can be used to localize in
range, given time. Multiplepassive sonars can be used for range
localization by triangulation or correlation, directly.For more
information on this item, see the article on Sonar.
Biological echo locationDolphins, whales and bats use
echolocation to detect prey and avoid obstacles.
Time-of-arrival localizationHaving speakers/ultrasonic
transmitters emitting sound at known positions and time, the
position ofa target equipped with a microphone/ultrasonic receiver
can be estimated based on the time of
-
arrival of the sound. The accuracy is usually poor under
non-line-of-sight conditions, where thereare blockages in between
the transmitters and the receivers. [11]
Seismic surveysSeismic surveys involve the generation of sound
waves to measure underground structures. Sourcewaves are generally
created by percussion mechanisms located near the ground or water
surface,typically dropped weights, vibroseis trucks, or explosives.
Data are collected with geophones, thenstored and processed by
computer. Current technology allows the generation of 3D images
ofunderground rock structures using such equipment.For more
information, see Reflection seismology.
EcotracerEcotracer is an acoustic locator that was used to
determining the presence and position of ships infog. Some could
detect targets at distances up to 12 kilometers. Static walls could
detect aircraft upto 30 miles away.
TypesThere were four main kinds of system:[12]
Personal/wearable horns Transportable steerable horns Static
dishes Static walls
ImpactAmerican acoustic locators were used in 1941 to detect the
Japanese attack on the fortress island ofCorregidor in the
Philippines.
OtherBecause the cost of the associated sensors and electronics
is dropping, the use of sound rangingtechnology is becoming
accessible for other uses, such as for locating wildlife.[13]
Coincidence detection in neurobiologyFor the electronic device,
see Coincidence circuit.Coincidence detection in the context of
neurobiology is a process by which a neuron or a neuralcircuit can
encode information by detecting the occurrence of timely
simultaneous yet spatiallyseparate input signals. Coincidence
detectors are important in information processing by
reducingtemporal jitter, reducing spontaneous activity, and forming
associations between separate neuralevents. This concept has led to
a greater understanding of neural processes and the formation
ofcomputational maps in the brain.
-
Principles of coincidence detectionCoincidence detection relies
on separate inputs converging on a common target. Consider a
basicneural circuit with two input neurons, A and B, that have
excitatory synaptic terminals convergingon a single output neuron,
C (Fig. 1). If each input neuron's EPSP is subthreshold for an
actionpotential at C, then C will not fire unless the two inputs
from A and B are temporally close together.Synchronous arrival of
these two inputs may push the membrane potential of a target neuron
overthe threshold required to create an action potential. If the
two inputs arrive too far apart, thedepolarization of the first
input may have time to drop significantly, preventing the
membranepotential of the target neuron from reaching the action
potential threshold. This exampleincorporates the principles of
spatial and temporal summation. Furthermore, coincidence
detectioncan reduce the jitter formed by spontaneous activity.
While random sub-threshold stimulations byneuronal cells may not
often fire coincidentally, coincident synaptic inputs derived from
a unitaryexternal stimulus will ensure that a target neuron fires
as a result of the stimulus.
Fig. 1: Two EPSP's innervated in rapid successionsum to produce
a larger EPSP or even an action potential in the postsynaptic
cell.
Fig. 2: If a sound arrives at the left ear before the right
ear,the impulse in the left auditory tract will reach X soonerthan
the impulse in the right auditory tract reaches Y.Neurons 4 or 5
may therefore receive coincident inputs.
Synaptic plasticity and associativityn 1949, Donald Hebb
postulated that synaptic efficiency will increase through repeated
andpersistent stimulation of a postsynaptic cell by a presynaptic
cell. This is often informallysummarized as "cells that fire
together, wire together". The theory was validated in part by
thediscovery of long-term potentiation. Studies of LTP on multiple
presynaptic cells stimulating apostsynaptic cell uncovered the
property of associativity. A weak neuronal stimulation onto a
-
pyramidal neuron may not induce long-term potentiation. However,
this same stimulation pairedwith a simultaneous strong stimulation
from another neuron will strengthen both synapses. Thisprocess
suggests that two neuronal pathways converging on the same cell may
both strengthen ifstimulated coincidentally.
Molecular mechanism of long-term potentiationLTP in the
hippocampus requires a prolonged depolarization that can expel the
Mg2+ block ofpostsynaptic NMDA receptors. The removal of the Mg2+
block allows the flow of Ca2+ into thecell. A large elevation of
calcium levels activate protein kinases that ultimately increase
the numberof postsynaptic AMPA receptors. This increases the
sensitivity of the postsynaptic cell to glutamate.As a result, both
synapses strengthen. The prolonged depolarization needed for the
expulsion ofMg2+ from NMDA receptors requires a high frequency
stimulation (Purves 2004). Associativitybecomes a factor because
this can be achieved through two simultaneous inputs that may not
bestrong enough to activate LTP by themselves.Besides the
NMDA-receptor based processes, further cellular mechanisms allow of
the associationbetween two different input signals converging on
the same neuron, in a defined timeframe. Upon asimultaneous
increase in the intracellular concentrations of cAMP and Ca2+, a
transcriptionalcoactivator called TORC1 (CRTC1) becomes activated,
that converts the temporal coincidence ofthe two second messengers
into long term changes such as LTP (Kovacs KA; Steullet, P;
Steinmann,M; Do, KQ; Magistretti, PJ; Halfon, O; Cardinaux, JR
(2007), "TORC1 is a calcium- and cAMP-sensitive coincidence
detector involved in hippocampal long-term synaptic plasticity.",
PNAS 104(11): 47005, doi:10.1073/pnas.0607524104, PMC 1838663, PMID
17360587). This cellularmechanism, through calcium-dependent
adenylate cyclase activation, might also account for thedetection
of the repetitive stimulation of a given synapse.
Molecular mechanism of long-term depressionLong-term depression
also works through associative properties although it is not always
thereverse process of LTP. LTD in the cerebellum requires a
coincident stimulation of parallel fibersand climbing fibers.
Glutamate released from the parallel fibers activates AMPA
receptors whichdepolarize the postsynaptic cell. The parallel
fibers also activate metabotropic glutamate receptorsthat release
the second messengers IP3 and DAG. The climbing fibers stimulate a
large increase inpostsynaptic Ca2+ levels when activated. The Ca2+,
IP3, and DAG work together in a signaltransduction pathway to
internalize AMPA receptors and decrease the sensitivity of the
postsynapticcell to glutamate (Purves 2004).
-
Animal echolocation
A depiction of the ultrasound signals emitted by a bat, and the
echo from a nearby object.Echolocation, also called biosonar, is
the biological sonar used by several kinds of animals.Echolocating
animals emit calls out to the environment and listen to the echoes
of those calls thatreturn from various objects near them. They use
these echoes to locate and identify the objects.Echolocation is
used for navigation and for foraging (or hunting) in various
environments. Someblind humans have learned to find their way using
clicks produced by a device or by mouth.Echolocating animals
include some mammals and a few birds; most notably microchiropteran
batsand odontocetes (toothed whales and dolphins), but also in
simpler form in other groups such asshrews, one genus of
megachiropteran bats (Rousettus) and two cave dwelling bird groups,
the so-called cave swiftlets in the genus Aerodramus (formerly
Collocalia) and the unrelated OilbirdSteatornis caripensis.
Early researchThe term echolocation was coined by Donald
Griffin, whose work with Robert Galambos was thefirst to
conclusively demonstrate its existence in bats in 1938.[1] [2] Long
before that, however, the 18th century Italian scientist Lazzaro
Spallanzani had, by means of aseries of elaborate experiments,
concluded that bats navigate by hearing and not by
vision.[3]Echolocation in odontocetes was not properly described
until two decades later, by Schevill andMcBride.[4]
PrincipleEcholocation is the same as active sonar, using sounds
made by the animal itself. Ranging is doneby measuring the time
delay between the animal's own sound emission and any echoes that
returnfrom the environment. The relative intensity of sound
received at each ear as well as the time delaybetween arrival at
the two ears provide information about the horizontal angle
(azimuth) fromwhich the reflected sound waves arrive.[5]Unlike some
man-made sonars that rely on many extremely narrow beams and many
receivers tolocalize a target (multibeam sonar), animal
echolocation has only one transmitter and two receivers(the ears).
Echolocating animals have two ears positioned slightly apart. The
echoes returning to thetwo ears arrive at different times and at
different loudness levels, depending on the position of the
-
object generating the echoes. The time and loudness differences
are used by the animals to perceivedistance and direction. With
echolocation, the bat or other animal can see not only where it is
goingbut also how big another animal is, what kind of animal it is,
and other features.[citation needed]
BatsMicrobats use echolocation to navigate and forage, often in
total darkness. They generally emergefrom their roosts in caves,
attics, or trees at dusk and hunt for insects into the night. Their
use ofecholocation allows them to occupy a niche where there are
often many insects (that come out atnight since there are fewer
predators then) and where there is less competition for food, and
wherethere are fewer other species that may prey on the bats
themselves.Microbats generate ultrasound via the larynx and emit
the sound through the open mouth or, muchmore rarely, the nose. The
latter is most pronounced in the horseshoe bats (Rhinolophus
spp.).Microbat calls (helpinfo) range in frequency from 14,000 to
well over 100,000 Hz, mostly beyondthe range of the human ear
(typical human hearing range is considered to be from 20 Hz
to20,000 Hz). Bats may estimate the elevation of targets by
interpreting the interference patternscaused by the echoes
reflecting from the tragus, a flap of skin in the external ear.[6]
There are twohypotheses about the evolution of echolocation in
bats. The first suggests that laryngealecholocation evolved twice
in Chiroptera, once in the Yangochiroptera and once in the
Horseshoebats (Rhinolophidae).[7] [8] The second proposes that
laryngeal echolocation had a single origin inChiroptera, was
subsequently lost in the family Pteropodidae (all megabats), and
later evolved as asystem of tongue-clicking in the genus
Rousettus.[9]Individual bat species echolocate within specific
frequency ranges that suit their environment andprey types. This
has sometimes been used by researchers to identify bats flying in
an area simply byrecording their calls with ultrasonic recorders
known as "bat detectors". However echolocation callsare not always
species specific and some bats overlap in the type of calls they
use so recordings ofecholocation calls cannot be used to identify
all bats. In recent years researchers in several countrieshave
developed "bat call libraries" that contain recordings of local bat
species that have beenidentified known as "reference calls" to
assist with identification.Since the 1970s there has been an
ongoing controversy among researchers as to whether bats use aform
of processing known from radar termed coherent cross-correlation.
Coherence means that thephase of the echolocation signals is used
by the bats, while cross-correlation just implies that theoutgoing
signal is compared with the returning echoes in a running process.
Today most - but not all- researchers believe that they use
cross-correlation, but in an incoherent form, termed a filter
bankreceiver.When searching for prey they produce sounds at a low
rate (10-20 clicks/second). During the searchphase the sound
emission is coupled to respiration, which is again coupled to the
wingbeat. Thiscoupling appears to dramatically conserve energy as
there is little to no additional energetic cost ofecholocation to
flying bats.[10] After detecting a potential prey item, microbats
increase the rate ofpulses, ending with the terminal buzz, at rates
as high as 200 clicks/second. During approach to adetected target,
the duration of the sounds is gradually decreased, as is the energy
of the sound.
Calls and ecologyBats belonging to the suborder Microchiroptera
(microbats) occupy a diverse set of ecologicalconditions - they can
be found living in environments as different as Europe and
Madagascar, andhunting for food sources as different as insects,
frogs, nectar, fruit, and blood. Additionally, thecharacteristics
of an echolocation call are adapted to the particular environment,
hunting behavior,and food source of the particular bat. However,
this adaptation of echolocation calls to ecologicalfactors is
constrained by the phylogenetic relationship of the bats, leading
to a process known as
-
descent with modification, and resulting in the diversity of the
Microchiroptera today.[11] [12] [13] [14] [15] [16]
Acoustic featuresDescribing the diversity of bat echolocation
calls requires examination of the frequency andtemporal features of
the calls. It is the variations in these aspects that produce
echolocation callssuited for different acoustic environments and
hunting behaviors.[17] [18] [19] [20] [21]
Frequency Modulation and Constant Frequency: Echolocation calls
can be composed oftwo different types of frequency structures:
frequency modulated (FM) sweeps, and constantfrequency (CF) tones.
A particular call can consist of one, the other, or both
structures. AnFM sweep is a broadband signal that is, it contains a
downward sweep through a range offrequencies. A CF tone is a
narrowband signal: the sound stays constant at one
frequencythroughout its duration.
Intensity: Echolocation calls have been measured at intensities
anywhere between 60 and140 decibels.[22] Certain microbat species
can modify their call intensity mid-call, loweringthe intensity as
they approach objects that reflect sound strongly. This prevents
the returningecho from deafening the bat.[23] Additionally, the
so-called "whispering bats" have adaptedlow-amplitude echolocation
so that their prey, moths, which are able to hear
echolocationcalls, are less able to detect and avoid an oncoming
bat[24]
Harmonic composition: Calls can be composed of one frequency, or
multiple frequenciescomprising a harmonic series. In the latter
case, the call is usually dominated by a certainharmonic
("dominant" frequencies are those present at higher intensities
than otherharmonics present in the call).[citation needed]
Call duration: A single echolocation call (a call being a single
continuous trace on a soundspectrogram, and a series of calls
comprising a sequence or pass) can last anywhere from 0.2to 100
milliseconds in duration, depending on the stage of prey-catching
behavior that thebat is engaged in. For example, the duration of a
call usually decreases when the bat is in thefinal stages of prey
capture this enables the bat to call more rapidly without overlap
of calland echo. Reducing duration comes at the cost of having less
total sound available forreflecting off objects and being heard by
the bat.[citation needed]
Pulse interval: The time interval between subsequent
echolocation calls (or pulses)determines two aspects of a bat's
perception. First, it establishes how quickly the bat'sauditory
scene information is updated. For example, bats increase the
repetition rate of theircalls (that is, decrease the pulse
interval) as they home in on a target. This allows the bat toget
new information regarding the target's location at a faster rate
when it needs it most.Secondly, the pulse interval determines the
maximum range that bats can detect objects. Thisis because bats can
only keep track of the echoes from one call at a time; as soon as
theymake another call they stop listening for echoes from the
previously made call.[25] Forexample, a pulse interval of 100 ms
(typical of a bat searching for insects) allows sound totravel in
air roughly 34 meters so a bat can only detect objects as far away
as 17 meters (thesound has to travel out and back). With a pulse
interval of 5 ms (typical of a bat in the finalmoments of a capture
attempt), the bat can only detect objects up to 85 cm away.
Thereforethe bat constantly has to make a choice between getting
new information updated quicklyand detecting objects far away.
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FM Signal AdvantagesThe major advantage conferred by an FM
signal is extremely precise range discrimination, orlocalization,
of the target. J.A. Simmons demonstrated this effect with a series
of elegantexperiments that showed how bats using FM signals could
distinguish between two separate targetseven when the targets were
less than half a millimeter apart. This amazing ability is due to
thebroadband sweep of the signal, which allows for better
resolution of the time delay between the calland the returning
echo, thereby improving the cross correlation of the two.
Additionally, if harmonicfrequencies are added to the FM signal,
then this localization becomes even more precise.[26] [27] [28]
[29] One possible disadvantage of the FM signal is a decreased
operational range of the call. Because theenergy of the call is
spread out among many frequencies, the distance at which the FM-bat
candetect targets is limited.[30] This is in part because any echo
returning at a particular frequency canonly be evaluated for a
brief fraction of a millisecond, as the fast downward sweep of the
call doesnot remain at any one frequency for long.[31]
CF Signal AdvantagesThe structure of a CF signal is adaptive in
that it allows the CF-bat to detect both the velocity of atarget,
and the fluttering of a target's wings as Doppler shifted
frequencies. A Doppler shift is analteration in sound wave
frequency, and is produced in two relevant situations: when the bat
and itstarget are moving relative to each other, and when the
target's wings are oscillating back and forth.CF-bats must
compensate for Doppler shifts, lowering the frequency of their call
in response toechoes of elevated frequency - this ensures that the
returning echo remains at the frequency towhich the ears of the bat
are most finely tuned. The oscillation of a target's wings also
producesamplitude shifts, which gives a CF-bat additional help in
distinguishing a flying target from astationary one. (Schnitzler
and Flieger 1983; Zupanc 2004; Simmons and Stein 1980;
Grinnell1995; Neuweiler 2003; Jones and Teeling 2006)Additionally,
because the signal energy of a CF call is concentrated into a
narrow frequency band,the operational range of the call is much
greater than that of an FM signal. This relies on the factthat
echoes returning within the narrow frequency band can be summed
over the entire length of thecall, which maintains a constant
frequency for up to 100 milliseconds.[32] [33]
Acoustic environments of FM and CF signals
FM: An FM component is excellent for hunting prey while flying
in close, clutteredenvironments. Two aspects of the FM signal
account for this fact: the precise targetlocalization conferred by
the broadband signal, and the short duration of the call. The first
ofthese is essential because in a cluttered environment, the bats
must be able to resolve theirprey from large amounts of background
noise. The 3D localization abilities of the broadbandsignal enable
the bat to do exactly that, providing it with what Simmons and
Stein (1980)call a "clutter rejection strategy." This strategy is
further improved by the use of harmonics,which, as previously
stated, enhance the localization properties of the call. The
shortduration of the FM call is also best in close, cluttered
environments because it enables thebat to emit many calls extremely
rapidly without overlap. This means that the bat can get analmost
continuous stream of information essential when objects are close,
because theywill pass by quickly without confusing which echo
corresponds to which call. (Neuweiler2003; Simmons and Stein 1980;
Jones and Teeling 2006; Fenton 1995)
CF: A CF component is often used by bats hunting for prey while
flying in open, clutter-free
-
environments, or by bats that wait on perches for their prey to
appear. The success of theformer strategy is due to two aspects of
the CF call, both of which confer excellent prey-detection
abilities. First, the greater working range of the call allows bats
to detect targetspresent at great distances a common situation in
open environments. Second, the length ofthe call is also suited for
targets at great distances: in this case, there is a decreased
chancethat the long call will overlap with the returning echo. The
latter strategy is made possible bythe fact that the long,
narrowband call allows the bat to detect Doppler shifts, which
wouldbe produced by an insect moving either towards or away from a
perched bat. (Neuweiler2003; Simmons and Stein 1980; Jones and
Teeling 2006; Fenton 1995)
Neural mechanisms in the brainBecause bats use echolocation to
orient themselves and to locate objects, their auditory systems
areadapted for this purpose, highly specialized for sensing and
interpreting the stereotypedecholocation calls characteristic of
their own species. This specialization is evident from the innerear
up to the highest levels of information processing in the auditory
cortex.
Inner ear and primary sensory neuronsBoth CF and FM bats have
specialized inner ears which allow them to hear sounds in the
ultrasonicrange, far outside the range of human hearing. Although
in most other aspects, the bat's auditoryorgans are similar to
those of most other mammals, certain bats (horseshoe bats,
Rhinolophus spp.and the moustached bat, Pteronotus parnelii) with a
constant frequency (CF) component to theircall (known as high duty
cycle bats) do have a few additional adaptations for detecting
thepredominant frequency (and harmonics) of the CF vocalization.
These include a narrow frequency"tuning" of the inner ear organs,
with an especially large area responding to the frequency of
thebat's returning echoes (Neuweiler 2003).The basilar membrane
within the cochlea contains the first of these specializations for
echoinformation processing. In bats that use CF signals, the
section of membrane that responds to thefrequency of returning
echoes is much larger than the region of response for any other
frequency.For example, in [34], the horseshoe bat, there is a
disproportionately lengthened and thickenedsection of the membrane
that responds to sounds around 83 kHz, the constant frequency of
the echoproduced by the bat's call. This area of high sensitivity
to a specific, narrow range of frequency isknown as an "acoustic
fovea".[35]Odontocetes (toothed whales and dolphins) have similar
cochlear specializations to those found inbats. Odontocetes also
have the highest neural investment of any cochleae reported to date
withratios of greater than 1500 ganglion cells/mm of basilar
membrane.Further along the auditory pathway, the movement of the
basilar membrane results in thestimulation of primary auditory
neurons. Many of these neurons are specifically "tuned"
(respondmost strongly) to the narrow frequency range of returning
echoes of CF calls. Because of the largesize of the acoustic fovea,
the number of neurons responding to this region, and thus to the
echofrequency, is especially high.[36]
Inferior colliculusIn the Inferior colliculus, a structure in
the bat's midbrain, information from lower in the
auditoryprocessing pathway is integrated and sent on to the
auditory cortex. As George Pollak and othersshowed in a series of
papers in 1977, the interneurons in this region have a very high
level ofsensitivity to time differences, since the time delay
between a call and the returning echo tells thebat its distance
from the target object. Especially interesting is that while most
neurons respondmore quickly to stronger stimuli, collicular neurons
maintain their timing accuracy even as signal
-
intensity changes.These interneurons are specialized for time
sensitivity in several ways. First, when activated, theygenerally
respond with only one or two action potentials. This short duration
of response allowstheir action potentials to give a very specific
indication of the exact moment of the time when thestimulus
arrived, and to respond accurately to stimuli that occur close in
time to one another. Inaddition, the neurons have a very low
threshold of activation they respond quickly even to weakstimuli.
Finally, for FM signals, each interneuron is tuned to a specific
frequency within the sweep,as well as to that same frequency in the
following echo. There is specialization for the CFcomponent of the
call at this level as well. The high proportion of neurons
responding to thefrequency of the acoustic fovea actually increases
at this level.[37] [38] [39]
Auditory cortexThe auditory cortex in bats is quite large in
comparison with other mammals.[40] Variouscharacteristics of sound
are processed by different regions of the cortex, each providing
differentinformation about the location or movement of a target
object. Most of the existing studies oninformation processing in
the auditory cortex of the bat have been done by Nobuo Suga on
themustached bat, Pteronotus parnellii. This bat's call has both CF
tone and FM sweep components.Suga and his colleagues have shown
that the cortex contains a series of "maps" of auditoryinformation,
each of which is organized systematically based on characteristics
of sound such asfrequency and amplitude. The neurons in these areas
respond only to a specific combination offrequency and timing
(sound-echo delay), and are known as combination-sensitive
neurons.The systematically organized maps in the auditory cortex
respond to various aspects of the echosignal, such as its delay and
its velocity. These regions are composed of "combination
sensitive"neurons that require at least two specific stimuli to
elicit a response. The neurons varysystematically across the maps,
which are organized by acoustic features of the sound and can betwo
dimensional. The different features of the call and its echo are
used by the bat to determineimportant characteristics of their
prey. The maps include:
FM-FM area: This region of the cortex contains FM-FM
combination-sensitive neurons. Thesecells respond only to the
combination of two FM sweeps: a call and its echo. The neurons in
theFM-FM region are often referred to as "delay-tuned," since each
responds to a specific time delaybetween the original call and the
echo, in order to find the distance from the target object
(therange). Each neuron also shows specificity for one harmonic in
the original call and a differentharmonic in the echo. The neurons
within the FM-FM area of the cortex of Pteronotus areorganized into
columns, in which the delay time is constant vertically but
increases across thehorizontal plane. The result is that range is
encoded by location on the cortex, and increasessystematically
across the FM-FM area.[41] [42] [43] [44]
CF-CF area: Another kind of combination-sensitive neuron is the
CF-CF neuron. Theserespond best to the combination of a CF call
containing two given frequencies a call at30 kHz (CF1) and one of
its additional harmonics around 60 or 90 kHz (CF2 or CF3) andthe
corresponding echoes. Thus, within the CF-CF region, the changes in
echo frequencycaused by the Doppler shift can be compared to the
frequency of the original call to calculatethe bat's velocity
relative to its target object. As in the FM-FM area, information is
encodedby its location within the map-like organization of the
region. The CF-CF area is first splitinto the distinct CF1-CF2 and
CF1-CF3 areas. Within each area, the CF1 frequency isorganized on
an axis, perpendicular to the CF2 or CF3 frequency axis. In the
resulting grid,each neuron codes for a certain combination of
frequencies that is indicative of a specificvelocity[45] [46]
[47]
-
DSCF area: This large section of the cortex is a map of the
acoustic fovea, organized byfrequency and by amplitude. Neurons in
this region respond to CF signals that have beenDoppler shifted (in
other words, echoes only) and are within the same narrow
frequencyrange to which the acoustic fovea responds. For
Pteronotus, this is around 61 kHz. This areais organized into
columns, which are arranged radially based on frequency. Within a
column,each neuron responds to a specific combination of frequency
and amplitude. Suga's studieshave indicated that this brain region
is necessary for frequency discrimination.[48] [49] [50]
-
Toothed whalesBiosonar is valuable to Toothed whales (suborder
odontoceti), including dolphins, porpoises, riverdolphins, killer
whales and sperm whales, because they live in an underwater habitat
that hasfavourable acoustic characteristics and where vision is
extremely limited in range due to absorptionor turbidity.Cetacean
evolution consisted of three main radiations. Throughout the middle
and late Eoceneperiods (49-31.5 million years ago), archaeocetes,
primitive toothed Cetacea that arose fromterrestrial mammals with
the creation of aquatic adaptations, were the only known archaic
Cetacea.[51] These primitive aquatic mammals did not possess the
ability to echolocate, although they didhave slightly adapted
underwater hearing.[52] The morphology of acoustically isolated ear
bones inbasilosaurid archaeocetes indicates that this order had
directional hearing underwater at low to midfrequencies by the late
middle Eocene.[53] However, with the extinction of archaeocete at
the onsetof the Oligocene, two new lineages in the early Oligocene
period (31.5-28 million years ago)compromised a second radiation.
These early mysticete (baleen whales) and odontocete can bedated
back to the middle Oligocene in New Zealand.[51] Based on past
phylogenies, it has beenfound that the evolution of odontocetes is
monophyletic, suggesting that echolocation evolved onlyonce 36 to
34 million years ago.[53] Dispersal rates routes of early
odontocetes includedtransoceanic travel to new adaptive zones. The
third radiation occurred later in the Neogene, whenpresent dolphins
and their relatives evolved to be the most common species in the
modern sea.[52]The evolution of echolocation could be attributed
several theories. There are two proposed drivesfor the hypotheses
of cetacean radiation, one biotic and the other abiotic in nature.
The first,adaptive radiation, is the result of a rapid divergence
into new adaptive zones. This results indiverse, ecologically
different clades that are incomparable.[54] Clade Neocete (crown
cetacean)has been characterized by an evolution from archaeocetes
and a dispersion across the world'soceans, and even estuarites and
rivers. These ecological opportunities were the result of
abundantdietary resources with low competition for hunting.[55]
This hypothesis of lineage diversification,however, can be
unconvincing due to a lack of support for rapid speciation early in
cetacean history.A second, more abiotic drive is better supported.
Physical restructuring of the oceans has played arole in
echolocation radiation. This was a result of global climate change
at the Eocene-Oligoceneboundary; from a greenhouse to an icehouse
world. Tectonic openings created the emergence of theSouthern ocean
with a free flowing Antarctic Circumpolar current.[56] [57] [58]
[59] These eventsallowed for a selection regime characterized by
the ability to locate and capture prey in turbid riverwaters, or
allow odontocetes to invade and feed at depths below the photic
zone. Further studieshave found that echolocation below the photic
zone could have been a predation adaptation to dielmigrating
cephalopods.[53] [60] Since its advent, there has been adaptive
radiation especially in theDelphinidae family (dolphins) in which
echolocation has become extremely derived.[61]One specific type of
echolocation, narrow-band high frequency (NBHF) clicks, evolved at
least fourtimes in groups of odontocetes, including pygmy sperm
whale (Kogiidae) and porpoise(Phocoenidae) families, Pontoporia
blainvillei, the genus Cephalorhynchus, and part of the
genusLagenorhynchus.[62] [63] These high frequency clicks likely
evolved as adaptation of predatoravoidance, as they inhabit areas
that have many killer whales and the signals are inaudible to
killerwhales due to the absence of energy below 100 kHz. Another
reason for variation in echolocationfrequencies is habitat. Shallow
waters, where many of these species live, tend to have more
debris;a more directional transmission reduces clutter in
reception.[63]Toothed whales emit a focused beam of high-frequency
clicks in the direction that their head ispointing. Sounds are
generated by passing air from the bony nares through the phonic
lips.[64]These sounds are reflected by the dense concave bone of
the cranium and an air sac at its base. Thefocused beam is
modulated by a large fatty organ known as the 'melon'. This acts
like an acousticlens because it is composed of lipids of differing
densities. Most toothed whales use clicks in aseries, or click
train, for echolocation, while the sperm whale may produce clicks
individually.
-
Toothed whale whistles do not appear to be used in echolocation.
Different rates of click productionin a click train give rise to
the familiar barks, squeals and growls of the bottlenose dolphin. A
clicktrain with a repetition rate over 600 per second is called a
burst pulse. In bottlenose dolphins, theauditory brain response
resolves individual clicks up to 600 per second, but yields a
gradedresponse for higher repetition rates.It has been suggested
that some smaller toothed whales may have their tooth arrangement
suited toaid in echolocation. The placement of teeth in the jaw of
a bottlenose dolphin, as an example, arenot symmetrical when seen
from a vertical plane, and this asymmetry could possibly be an aid
inthe dolphin sensing if echoes from its biosonar are coming from
one side or the other.[65] [66] However, this idea lacks
experimental support.Echoes are received using complex fatty
structures around the lower jaw as the primary receptionpath, from
where they are transmitted to the middle ear via a continuous fat
body.[67] [68] Lateralsound may be received though fatty lobes
surrounding the ears with a similar density to water.Some
researchers believe that when they approach the object of interest,
they protect themselvesagainst the louder echo by quietening the
emitted sound. In bats this is known to happen, but herethe hearing
sensitivity is also reduced close to a target.Before the
echolocation abilities of "porpoises" were officially discovered,
Jacques Yves Cousteausuggested that they might exist. In his first
book, The Silent World (1953, pp. 206207), he reportedthat his
research vessel, the lie Monier, was heading to the Straits of
Gibraltar and noticed a groupof porpoises following them. Cousteau
changed course a few degrees off the optimal course to thecenter of
the strait, and the porpoises followed for a few minutes, then
diverged toward mid-channelagain. It was obvious that they knew
where the optimal course lay, even if the humans didn't.Cousteau
concluded that the cetaceans had something like sonar, which was a
relatively new featureon submarines.
Oilbirds and swiftletsOilbirds and some species of swiftlet are
known to use a relatively crude form of echolocationcompared to
that of bats and dolphins. These nocturnal birds emit calls while
flying and use thecalls to navigate through trees and caves where
they live.[69] [70]
Shrews and tenrecsMain article: Shrews#EcholocationTerrestrial
mammals other than bats known to echolocate include two ge