PSYC2410 – Chapter 7
Exteroceptive sensory systems: auditory (hearing), somatosensory (touch), olfactory (smell) and gustatory (taste)
7.1 Principles of Sensory System Organization
Sensory areas of the cortex are of 3 fundamental types:
Primary sensory cortex: receives most of the its input directly from the thalamic relay nuclei of that system
Secondary sensory cortex: areas of the sensory cortex that receive most of their input from the primary sensory cortex or
from other areas of secondary sensory cortex of the same system
Association cortex: receive input from more than one sensory system. Most from areas of secondary sensory cortex.
Hierarchical Organization: sensory structures are organized in a hierarchy based on the specificity and complexity of their
o Ex: Receptors thalamic relay nuclei primary sensory cortex secondary sensory cortex association cortex
o As you move from primary sensory cortex to secondary cortex, the neurons respond optimally to stimuli of greater
and greater specificity and complexity. The higher the level of damage, the more specific and complex the deficit.
Thus, psychologists divide the general process of perceiving into 2 general phases:
o Sensation: process of detecting the presence of stimuli
o Perception: higher-order process of integrating, recognizing and interpreting complete patterns of sensations.
o Each of the 3 levels of cerebral cortex: primary, secondary and association, in each sensory system contains
functionally distinct areas that specialize in different kinds of analysis.
o Systems in which information flows through the components over multiple pathways. Parallel systems feature
parallel processing: the simultaneous analysis of a signal in different ways by the multiple parallel pathways of a
o Two fundamentally different kinds of parallel streams of analysis:
One capable of influencing our behaviour without our conscious awareness
One that influences our behaviour by engaging our conscious awareness
Summary Model of Sensory System Organization
o Sensory systems are characterized by a division of labor: multiple specialized areas at multiple levels are
interconnected by multiple parallel pathways.
o Binding problem: how does the brain combine individual sensory attributes to produce integrate perceptions?
Possible solution: there is a single area of the cortex at the top of the sensory hierarchy that receives
signals from all other areas of the system and puts them together to for perceptions; however there are
no areas of cortex to which all areas of a single sensory system report.
Then perceptions must be a product of the combined activity of different interconnected cortical areas
7.2 Auditory System
Sounds are vibrations of air molecules that stimulate the auditory system; humans hear only those molecular vibrations
between 20 and 20 000 hertz (cycles per second).
o Amplitude – loudness
o Frequency – pitch
o Complexity – timbre
Pure tones: (sine wave vibrations) exist only in laboratories and sound recording studios; in real life sound is always
associated with complex patterns of vibrations. Any complex soundwave can be broken down into a series of sine waves of
various frequencies and amplitudes.
o Fourier analysis: the mathematical procedure for breaking down complex waves into their component sine waves.
One theory suggests the auditory system does this.
o Fundamental frequency: highest frequency of which the various component frequencies of a sound are multiples PSYC2410 – Chapter 7
o Missing fundamental: when the pitch of a complex sound may not be directly related to the frequency of any of
the sound’s components.
o Sound waves travel from the outer ear down the auditory canal and cause the tympanic membrane (eardrum) to
o These vibrations are then transferred to the three ossicles: malleus (hammer), incus (anvil) and the stapes
o The vibrations of the stapes trigger vibrations of the membrane called the oval window which in turn transfers the
vibrations to the fluid of the cochlea a long coiled tube with an internal membrane running almost to its tip.
o The internal membrane is the auditory receptor organ of Corti. Each pressure change at the oval window travels
along the organ of Corti as a wave. Has 2 membranes:
Basilar membrane: the auditory hair cell receptors are mounted in the basilar membrane
Tectorial membrane: rests on the hair cells
o A deflection of the organ of Corti at any point along its length produces a shearing force on the hair cells at the
same point which stimulates the hair cells, which increases the firing in axons of the auditory nerve.
o The vibrations of the cochlear fluid are ultimately dissipated by the round window an elastic membrane in the
o Cochlea is very sensitive, humans can hear differences in pure tones that differ in frequency by only 0.2%
o Different frequencies produce maximal stimulation of hair cells at different points along the basilar membrane
higher frequencies producing activation closer to the windows
lower frequencies producing greater activation at the top of the basilar membrane.
Thus complex sounds activate hair cells at many different points along the basilar membrane, and the
many signals created by a single complex sound are carried out of the ear by many different auditory
o The organization of the auditory system is primarily tonotopic.
o Semicircular cannals: the receptive organs of the vestibular system which carries information about the direction
and intensity of head movements, which helps us maintain our balance.
From the Ear to the Primary Auditory Cortex
o No major auditory pathway, but a network of them
o Axons of each auditory never synapse in ipsilateral cochlear nuclei which projects to superior olives, which project
via the lateral lemniscus to inferior colliculi which project to medial geniculate nuclei of the thalamus, which
projects to primary auditory cortex
o Signals from each ear are combined at a very low level and are transmitted to both ipsilateral and contralateral
Subcortical Mechanisms of Sound Localization
o Localization of sounds in space is mediated by the lateral and medial superior olives.
o When a sound originates to a person’s left it reaches the left ear first, and it is louder at the left ear.
o Neurons in the medial superior olives respond to slight diffs in the time of arrival of signals from the two ears
o Neurons in the lateral superior olives respond to slight differences in the amplitude of sounds from the two ears
o The medial and lateral superior olives project to the superior colliculus and the inferior colliculus.
The deep layers of the superior colliculi are laid out according to a map of auditory space (instead of
The superficial layers, which receive visual input, are organized retinotopically. So the general function of
the superior colliculi is locating sources of sensory input in space.
Barn owls: amazing at locating sounds, auditory neurons of the superior colliculus are finely tuned, each
neuron responds to sounds from only a particular location in the range of the owl’s hearing.
o Primary auditory cortex receives the majority of its input from the medial geniculate nucleus, is located in the
temporal lobe, hidden from view within the lateral fissure
o Core region: PAC + two adjacent areas in each hemisphere PSYC2410 – Chapter 7
o Belt: surrounds core region, area of secondary cortex
o Parabelt: areas of secondary cortex outside the belt
o Organization of Primate Auditory Cortex
Organized in functional columns (like PVC): All neurons encountered during a vertical microelectrode
penetration of primary auditory cortex tend to respond optimally to sounds in the same frequency range
Organized tonotopically (like cochlea): each are of primary and secondary auditory cortex appears to be
organized on the basis of frequency
o What sounds should be used to study auditory cortex?
Major reason for research on auditory cortex lagging is a lack of clear understanding of the dimensions
along which auditory cortex evaluates sound
There is clear evidence of hierarchal organization in auditory cortex: neural responses of secondary
auditory cortex are more complex than in the PAC, but responses at PAC are complex themselves
Catch 22: no idea what auditory cortex does, don’t know what questions to ask or what sounds to use
o Two streams of Auditory Cortex
Proposed that like the 2 mainstreams of visual cortex (dorsal, ventral) there’s 2 in the auditory cortex too
Auditory signals are conducted to 2 large areas of association cortex: prefrontal cortex and posterior
parietal cortex. Hypothesized:
Anterior auditory pathway involved in identifying sounds (what)
Posterior auditory pathway involved in locating sounds (where)
o Auditory-Visual Interactions
In one study of monkeys: posterior parietal neurons were found to have visual receptive fields, some to
have auditory receptive fields and some both. In areas with both, both fields covered the same location of
the subject’s immediate environment.
Functional brain imaging studies have confirmed that sensory interactions do occur in association cortex,
and have repeatedly found evidence of sensory interactions in areas of primary sensory cortex.
Leading toward new way of thinking: sensory system interaction is an early and integral part of sensory
o Where does the Perception of Pitch Occur?
Small area just anterior to PAC that contained many neurons that responded to pitch, regardless of the
quality of the sound. It also contained neurons that responded to frequency, Bender and Wang suggested
this is likely the place where frequencies of sound were converted to perception of pitch.
A similar pitch areas has been identified by fMRI studies in a similar location in the human brain
Effects of Damage to the Auditory System
o Auditory Cortex Damage:
Effects of auditory cortex damage have relied largely on the study of surgically placed lesions in
nonhumans and most study the effect of large lesions involving the core region and most of the belt and
Surprising lack of severe permanent deficits suggesting subcortical circuits serve more complex and
important fns than thought: few permanent hearing deficits have been reliably detected in rats following
auditory cortex lesions. Effect on humans and monkeys seems to be similar.
Initially, there is loss of hearing from shock of lesions, but is restored after a few weeks
Major permanent effects are loss of the ability to localize sounds and impairment of the ability to
Effects of unilateral auditory cortex lesions suggest that the system is partially contralateral: it disrupts
ability to localize sounds in space contralateral to the lesion; however other dificits produced this way
only slightly contralateral
o Deafness in Humans
250 million people suffer from disabling hearing impairment (2005)
Total deafness only 1% of the time PSYC2410 – Chapter 7
Because of the parallel organization of the auditory system, severe hearing problems usually result from
damage to the inner or middle ear or to the nerves leading from them rather than more central damage.
Damage to ossicles (conductive deafness) and damage to the cochlea or auditory nerve (nerve deafness –
caused by loss of hair cell receptors). If only part of cochlea damaged, may only have nerve deafness to
some frequencies and not others. Characteristic of age-related hearing loss (first is perceiving high
Tinnitus: ringing of the ears. Cutting the nerve does not stop the ringing, suggests that changes to the
central auditory system caused by the deafness are the cause of tinnitus
Cochlear implants: benefit people with nerve deafness, they bypass damage to hair cells by converting
sounds picked up by a microphone on the patient’s ear to electrical signals which are carried into the
cochlea by a bundle of electrodes. The signals excite the auditory nerve. Do not restore normal hearing.
Sooner you get an implant the better because disuse leads to degeneration of auditory neural pathways.
7.3 Somatosensory System: Touch and Pain
Somatosensory system is actually 3 separate but interacting systems:
1) Exteroceptive system which sense external stimuli that are applied to the skin,
mechanical stimuli (touch)
thermal stimuli (temperature)
nociceptive stimuli (pain)
2) Proprioceptive system which monitors info about the position of the body that comes from receptors in the muscles, joints
and organs of balance
3) Interoceptive system which provides general info about conditions within the body.
o Free nerve endings (simplest) sensitive to temperature change and pain
o Pacinian corpuscles (largest and deepest) adapt rapidly, respond to sudden displacements of the skin, not to
o Merkel’s disks respond to gradual skin indentation
o Ruffini endings respond to skin stretch
o Stereognosis: identification of objects by touch. Moving object around to change pattern of stimulation to keep all
o Stimuli applied to the skin deform or change the chemistry of the receptor and this in turn changes the
permeability of the receptor cell membrane to various ions, results in neural signal
o Neural fibers that carry info from cutaneous receptors and other somatosensory receptors gather together in
nerves and enter the spinal cord via the dorsal roots.
o Dermatome: area of the body that is innervated by the left and right dorsal roots of a given segment of the spinal
o Lots of overlap, so destruction of a single dorsal root produces little somatosensory loss * FIG7.11 p. 175
Two Major Somatosensory Pathways
o Somatosensory info ascends from each side of the body to the human cortex over 2 major pathways:
Dorsal-column medial-lemniscus system: tends to carries info about touch and proprioception
Neurons enter the spinal cord via dorsal roots
Ascend ipsilaterally in dorsal columns
Synapse in dorsal column nuclei of the medulla
Axons of dorsal column nuclei decussate (cross over) PSYC2410 – Chapter 7
Ascend in medial lemniscus to contralateral ventral posterior nucleus of thalamus(also receives
input from trigeminal nerve which carry somatosensory info from contralateral areas of face)
Most neurons of ventral posterior nucleus project to primary somatosensory cortex(SI)
Others project to secondary somatosensory cortex(SII) or the posterior parietal cortex
Dorsal column neurons from toes are longest neurons in the body
Antero-lateral system: tends to carries info about pain and temperature
Dorsal root neurons synapses as soon as they enter the spinal cord
Axons of second-order neurons decussate then ascend to the brain in the contralateral
anterolateral portion of the spinal cord; some don’t decussate and ascend ipsilaterally
Has 3 tracts:
Spinothalamic: projects to ventral posterior nucleus of thalamus
Spinoreticular: projects to reticular formation, then parafascicular nuclei & intralaminar
nuclei of thalamus
Spinotectal: projects to tectum (colliculi)
Trigeminal nerve branches carry pain and temperature information from face to same thalamic
sites, then info is distributed to SI SII and posterior parietal cortex and other parts
o If both ascending somatosensory paths are completely transected by a spinal injury, the patient can feel no body
sensation from below the cut
o Lesions to the ventral posterior nuclei (receive input from both spinothalamic tract and dorsal-column medial-
lemniscus system) produced some loss of cutaneous sensitivity to touch, temperature change and chronic pain.
o Lesions to parafascicular and intralaminar nuclei (input from spinoreticular tract) reduced chronic