Four stages: 1. Active searching (seeking a target) 2. Approach/stalk (target acquired moving to intercept or waiting to pounce). 3. Active tracking (chasing) 4. Terminal stage (capture) • As the behavior progress to stage 4 FM pulse rate increases, FM range decreases location and foraging behavior in bats Final
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Four stages: 1.Active searching (seeking a target) 2.Approach/stalk (target acquired moving to intercept or waiting to pounce). 3.Active tracking (chasing)
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Four stages:1. Active searching (seeking a target)2. Approach/stalk (target acquired moving to
intercept or waiting to pounce).3. Active tracking (chasing)4. Terminal stage (capture)
• As the behavior progress to stage 4 FM pulse rate increases, FM range decreases
The Inferior Colliculus: A pulse-echo delay Filter (encoder)
Neurons in the inferior colliculus are sensitive to the time interval between a call and its echo. •Individual cells are sharply tuned to one particular frequency in an FM sweep•Low action potential thresholds•Invariant response latencies to calls and echoes. As the pulse-echo intensity shifts, the response latency and size remain constant. •Invariant response magnitude to calls and echoes. 1 to 2 spikes per call or echo.•FM sweeps are encoded by an ensemble of responding neurons. Each cell is tuned to narrow frequency range and responds to both the call and echo at that frequency.
•A subset of cells also project back to the ear to control tensor tympani (a muscle that attenuates amplitude on middle ear bones.
•Disproportionate representation of CF as well (acoustic fovea)
Note that FM1 is the lowest frequency and smallest amplitude; this is the part of the pulse that the bat monitors. For the echo, these bats monitor the other harmonics
Cortical organization
The cortex is made up of a matrix of cells (called columns) like a honeycomb Each column contains numerous neurons and can be considered a discrete functional unit. The columns are all layered (typically 6) in a common pattern
• layers based on differential concentrations of cell bodies of a particular type (subpopulation) or the dendrites from a specific subpopulation of cells.
Typically sensory representations are topographically very organized in a manner reflective of the salient perceptual dimensions of that sensory mode.
Recording from the cortex
•Neurons in this area respond specifically to time separated pairs of FM1 pulses and an FM(2,3) harmonic echo. (FM1call & FM2 echo)•Neurons are tuned to
•best delay: indicates distance •amplitude: indicate subtend angle of target
•Collectively these cells could encode absolute size.
•Neurons are frequency tolerant - enables cells to respond to broadband components of pulses and echoes.
•Because they are frequency tolerant neurons of the FM-FM region are blind to Doppler shift.
•Selective disruption of this area results in loss of delay discrimination in behavioral tests
FM-FM region:
FM1/FM2
FM1/FM3
FM2 Delaylow (close) High (far)
FM
1 am
pl.
FM
1 am
pl.
FM3 Delaylow (close) high (far)
•Regions contain neurons that are sensitive to varying combinations of the CF1 of the pulse and a harmonic of the CF echo (i.e. CF1 & CF2). •Respond only to pulse-echo combinations with differing fundamental frequencies (i.e. different amounts of Doppler shift). •Thus each P-E pair of frequencies encodes a different relative velocity and is coded in a physical place in the cortex.•Velocity range represented is typically -2 to 9 m/s, most sensitive to 0-4 m/s
CF
CF-CF region
CF1/CF2
CF1/CF3
CF2 freqlow (loosing) High (gaining)
====C
F1 fr
eqC
F1 fr
eq
CF3 freqlow (loosing) High (gaining)
•Largest single cortical region in bat auditory system •Neurons responds only to echo CF2 regardless of call frequency. •Develops specifically to the individual bats call.•Sensitive to faint echoes from distant targets. •Very selective frequency and amplitude discrimination •Represents the acoustic fovea: