Sensation and perception

SENSATION AND PERCEPTIONThe Auditory, Somatosensory, Olfactory and Gustatory Systems

Introduction• Sensation refers to the detection of stimuli: the sensory

organ in question must be able to pick up stimuli which it must then successfully transmit to the brain in order to evoke sensation.

• Perception refers to the neural pathways in the brain that integrate, recognise and interpret these sensory signals and give the sensation meaning.

• Example: a bell is struck behind a patient. If the subject can hear the sound but not recognise it as coming from a bell then this indicates good sensation but implicates a lesion in the neural pathways involved in perception

• Sensory systems are orgainsed in a hierarchial manner such that stimuli detected by primary receptors (specialised cells in the eyes, ears, skin, nose and tongue) such as light, vibrations/pressure changes and chemicals are converted into action potentials which almost always travel from the sensory organ to the thalamus/hypothalamus in the diencephalon and from there to the primary sensory cortex before the information is relayed to the secondary sensory cortex and then association cortex all of 3 which are in the brain.

Introduction

• Above the level of the receptors, it has been discovered that there is intercommunication within and between the different levels of the hierarchy: parallel processing.

• In addition to this, sensory cortices have been to shown to be functionally segragated in that different parts sense the different characteristics of a given stimulus.

• Example: different neurons in the auditory cortex are responsible for pitch and amplitude sensation, damage to one population of these will mean that overall sensation is kept but perception would suffer.

• Thus it can be concluded that sensation is “low-order” and involves the lower levels of the hierarchy while perception is “high-order” and involves the higher levels.

Introduction

The Hierarchical Organisation of Sensation

Figure 1 J. Pinel Biopsychology 2011

The Auditory System: Intro• Sounds are vibrations of air molecules.• Human ears can sense sounds of frequencies from 20Hz - 20KHz.• The amplitude, frequency and characteristics of sound are

perceived as the loudness, pitch and timbre, respectively.• Sounds made up of more

than one frequency are

perceived at that sound’s

fundamental frequency,

the highest common

multiple of a sound’s

constituent frequencies,

even if that frequency is not

a constituent of that sound.

Figure 2 J. Pinel Biopsychology 2011

The Auditory System: Anatomy

Figure 3. Outer and Middle ear

The Auditory System: Anatomy• Sound waves are funneled through the auditory canal until

they impact on and displace the tympanic membrane.• The vibrations then are propagated through the 3 solid

ossicles: malleus to incus to stapes.• The stapes displaces the oval window (a membrane) of

the coiled, fluid-filled cochlea where the auditory receptor is (organ of corti).

• Vibrations travel through the cochlea and are eventually dissipated at the round window thus re-equilibrating the pressure within the cochlea.

The Auditory System: Anatomy• Sound waves travel through the cochlea, their wavefronts

eventually reach and displace the basilar membrane of the organ of corti.

• Hair cells are lifted up and

tectorial membrane shears

through their cilia.• Results in depolarisation

and an action potential being sent via CN VIII.• Hair cells arranged along basilar membrane: different

frequencies activate corresponding populations of cells.

Figure 4. Uncoiled view of cochlea

The Auditory System: Anatomy

Figure 5. Transverse view of cochlea

Figure 6. Differentiation of pitch

Auditory Pathways• Auditory nerve of CNVIII carries

action potential to cochlear

nucleus of ipsilateral ear.• Cochlear nuclei to ipsi and

contra superior olives, and to

contra inferior colliculus.• Superior olives to ipsi and

contra inf. and sup. Colliculi.• Inf. Colliculi to ipsi and contra

medial geniculate nuclei of

thalamus• Thalamus to ipsi primary

auditory cortex.Figure 6. J. Pinel Biopsychology 2011

Auditory Pathways• Lateral and medial superior olives responsible for the

localisation of sound.• Medial responds to difference in time of arrival of signal,

lateral to amplitude differences.• Superior colliculi also believed to serve same purpose.

Organisation of Auditory Cortex• Organised tonotopically (according to different

frequencies) and in functional columns.• 2 streams:

1. Anterior auditory pathway transmits to pre-frontal association cortex involved in identification of sounds

2. Posterior auditory pathway transmits to parietal association cortex involved in localisation of sounds.

• There is intercommunication between the visual and auditory systems.

• Pitch is perceived by a small population of neurons anterior to the primary auditory cortex.

Damage to the Auditory System• Total deafness rare• Even a complete unilateral lesion of auditory cortex only

causes loss of perception for the affected side and not complete sensation loss due to the complex parallel pathways and intercommunication involved which can compensate for the affected region/pathway.

• 2 types of deafness: Conductive and Neural• The former implicates there is a problem in the

transmitting of the sound wave to the hair cells whereas the latter implicates a lesion in the neural pathways or damage to the hair cells (as occurs with ageing).

Somatosensory System: Intro• Comprised of 3 separate systems:

1. Exteroceptive – senses external stimuli.

2. Proprioceptive – monitors position of body in space.

3. Interoceptive – senses the body’s internal environment (e.g. blood pressure, blood pH, O2 saturation etc.)

• The exteroceptive itself has 3 divisions responsible for the sensation of pressure/vibration changes (touch), temperature and one specifically for nociceptive (painful) stimuli.

Receptors• Many different types to reflect the various types of “touch”• Broadly can be divided into slow and fast adapting• Most nociceptors and thermoceptors are slow adapting

and high threshold meaning the continue to create action potentials for the duration of the stimulus and are generally not very sensitive (require an intense stimulus to elicit an AP).

• This is unlike mechanoceptors which are usually fast adapting and low threshold meaning a continually applied stimulus will stop being registered (stop eliciting APs) but even the slightest of touches can depolarise them.

Somatosensory Neural Pathways• Neurones innervating given parts of skin enter the spinal

cord at a given level: the area of skin innervated by all of those neurones that all enter the spinal cord at that given level is know as the dermatome.

• 3 neurons are involved in the transmission of touch/pain information from skin to cortex with cell body loaction, axon tract location and synapses being different for these 2 modalities.

Somatosensory Neural Pathways• Touch: • From receptor terminal, axon enters spinal cord via dorsal

root at a given level (the lower the area of skin innervated, the lower the level). Axon goes up in the dorsal column region of the spinal cord via either the cuneate (for hands/arms) or gracile (for legs/feet) fasciculus to synapse with the cuneate/gracile nuclei respectively, in the medulla. Cell body of receptor is in periphery.

• This 2nd neuron’s axon now decussates and goes up and synapses with a neuron in the ventral posterior nucleus of the thalamus (contralateral to the dermatome).

• This 3rd and final neuron then synapses with a nerve cell body in the primary/secondary somatosensory cortex.

Somatosensory Neural Pathways• Pain and temperature:• From the receptor, the axon of the 1st neuron synapses

immediately in the dorsal gray matter of the spinal cord as soon as it enters it.

• The 2nd neuron’s axon then immediately decussates and travels up the antero-lateral tracts until it again synapses with VPN on the contralateral thalamus.

• The 3rd neuron then again synapses with the contralateral somatosensory cortex like in the touch pathway.

• VPN involved in touch, acute pain and temperature.• Parafascicular and intralaminar nuclei involved in chronic

pain.

Somatosensory Neural Pathways

Figure 7. J. Pinel Biopsychology 2011

Figure 8. J. Pinel Biopsychology 2011

Somatosensory Sensation• Postcentral gyrus is SI.• Organised somatotopically (somatosensory/motor

homunculus) with each small area being further divided into even smaller ones corresponding to the type of stimulus that area received (pain, temperature, touch etc).

• SII is just slightly ventral to this and each receives inputs from both primary SSCs.

• Information is then relayed to parietal association cortex. • 2 streams:

1. Dorsal stream to parietal cortex involved in multisensory integration and direction of attention.

2. Ventral stream to SII involved in interpretation of objects’ shapes.

Somatosensory Lesions• As in the auditory system, full damage to SI only affects

some of the touch perception of the affected side (contralateral side of the body) as the patient can still feel when stimuli are presented on the affected side. Again this is most likely due to neural intercommunication and the parallel pathways.

• Damage to the association cortex through, for example, stroke among other lesions can result in various agnosias and anopias and these illustrate how connected the somatosensory system is to the visual system.

Pain Perception and Control• Pain is beneficial –important to know when to seek

help/what to avoid.• Failure to discover a single “pain centre” in the brain

though the anterior cingulate gyrus is most implicated.• Pain can be suppressed through the analgesic effect of

the periaqueductal grey matter synapsing with the raphe nuclei whose axons extend down into the spinal cord, inhibiting excitatory and painful transmission and through the endogenous release of endorphins acting on opiate receptors. Recent research has shown that pathways exist to increase pain perception as well.

Neuropathic Pain• Neuropathic pain is the sensation of pain in the absence

of a stimulus.• Thought originate with ectopic stimulation of the CNS at

higher levels than the receptor.• Research points to aberrant glial cell inappropriately

stimulating CNS neurones involved in pain perception.

Olfactory System Receptors• Receptors located in olfactory mucosa.• Axons pass through cribiform plate and synapse with cells

in olfactory bulb of brain (this collection of synapses onto cell bodies is known as a glomerulus) which themselves project via olfactory tract.

• Several thousand olfactory receptor cells per glomerulus• Humans have almost 1000 receptor proteins.• 2 glomeruli in each bulb for each receptor protein.• 1:1 olfactory cell to receptor protein ratio.

Olfactory System Receptors• Receptors scattered across mucosa however receptors

with same receptor protein all generally synapse in the same glomerulus in the olfactory bulb.

• Glomeruli seem to be topographically organised as in other sensory systems but principle governing this organisation still undiscovered.

• Olfactory receptor cells are replaceable and each has a lifespan of a few weeks.

Olfactory System Pathways• Each tract projects to medial temporal lobe including

amygdala and piriform cortex.• Do NOT synapse in thalamus.• 2 pathways from amygdala-piriform:

1. To limbic system

2. To orbitofrontal cortex via medial dorsal nuclei of thalamus

• Former involved in emotional response to smells, latter in conscious perception of odours.

Gustatory System• Taste sensation picked up by taste buds.• Approx. 50 taste cells per taste bud.• Each taste bud has its own neuron, therefore ratio of 50

taste receptors to each 2ndary neuron.• Generally accepted that there are 5 receptor sub-types

corresponding to a given taste: sweet, salty, bitter, sour, unami (meaty).

• Receptors for sweet, umami and bitter have been identified with other flavours thought to arise from foods with those flavours having a direct effect on the ion channels of the taste cell (not receptor).

Taste Sensation• sensation for anterior 2/3 is trigeminal nerve (V3), taste is

facial (VII). • Both taste and touch is glossopharyngeal nerve for

posterior 1/3 (IX).• And finally just a few taste buds furthest back and on

epiglottis which are supplied by vagus nerve (X).• Taste fibres go through facial nerve to nucleus solitarius

(via chorda tympani) which is key taste centre.• From medullar solitary nucleus to thalamic VPN.• From VPN to gustatory cortex on lateral fissure.

Taste Sensation

Chemical Sense Loss• Anosmia – inability to smell.• Ageusia – inability to taste.• Complete loss of this extremely uncommon though

trauma causing shearing of olfactory bulb can be a reason for the former.

• Many disease result in partial anosmia of which: Parkinson’s, Down’s, Kallman’s, epilepsy, Alzheimer’s, Korsakoff.

• Damage to CN VII can result in ageusia depending on where along the nerve the damage has occurred.

Selective Attention• Definition: The ability to focus on and divert attention to a specific

subset of stimuli in the presence of a large number of competing stimuli.

• Works by improving perception of stimuli attention is diverted towards and reducing perception of competing stimuli.

• Evolutionarily important in that it allows individual to focus on important matters and discard trivial ones.

• Can occur because of endogenous or exogenous attention.• The former refers to the individual his/herself having a reason to

divert attention to given stimuli whereas the latter refers to an external factor having diverted the individual’s attention to a given stimulus.

• Endogenous attention is top-down whereas exogenous attention is bottom-up.

• Neurally, this corellates to increased or decreased activity in the dorsal or ventral streams depending on what was being focused on.