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Chapter 7: Mechanisms of Perception:
Hearing, Touch, Smell, Taste, and Attention
• Exteroceptive sensory systems: sensory systems that interpret stimuli from outside the body –
vision, hearing (audition), touch (somatosensory), smell (olfactory) and taste (gustatory).
7.1 PRINCIPLES OF SENSORY SYSTEM ORGANIZATION
• Primary sensory cortex: Receives input directly from thalamic relay nuclei of that system.
• Secondary sensory cortex: Receives input from primary or from other secondary areas.
• Association cortex: Receives input from more than one sensory system, via various areas of
• Interactions between these three types of sensory cortex and other sensory structures are
• Hierarchical organization: sensory structures are
organized in hierarchy based on specificity and
complexity of their function. From receptors up to
association cortex, neurons respond optimally to stimuli
of greater specificity and complexity. Each level
receives most of its input from lower levels, and adds
another layer of analysis.
• The higher the level of brain damage, the more
specific and complex the deficit.
o E.g. destruction of a sensory system’s receptors
produces complete loss of ability to perceive that
sensory modality (e.g. total blindness), while destruction of association cortex produces
complex and specific sensory deficits that leave fundamental sensory abilities intact.
o The man who mistook his wife for a hat: Dr. P had excellent visual acuity, but could not seem to
identify objects (could not recognize a glove, tried to grasp his wife’s head and put it on his
• Sensation: process of detecting presence of stimuli
• Perception: higher-order process of integrating, recognizing, and interpreting complete patterns of
sensations (Dr. P had problems with perception, not sensation).
• Not true that each area of sensory system is functionally homogeneous (all areas act together to
perform same function); instead, there is functional segregation – in each level of sensory
hierarchy, there are functionally distinct areas that specialize in different kinds of analysis.
• Levels of sensory hierarchy are not serial systems – system where information flows among the
components over just one pathway, like string through strand of beads.
• They are actually parallel systems – information flows through the components over multiple
pathways. They have parallel processing, the simultaneous analysis of a signal in different ways
by the multiple parallel pathways of a network.
• Two different kinds of parallel streams – one to influence behaviour without our conscious
awareness, and one that influences behaviour by engaging our conscious awareness. Page 2 of16
Summary Model of Sensory System Organization
• Used to be: Hierarchical, functionally homogeneous,
• Now: Hierarchical, but functionally segregated and
• Multiple specialized areas, at multiple levels, are
interconnected by multiple parallel pathways – “division of
• E.g. Complex visual stimuli are perceived as integrated
wholes due to work of different systems perceiving shape,
colour, movement, etc.
• How does brain combine individual attributes to produce
integrated perceptions? Binding problem.
o Perception is a product of combined activity of
different interconnected cortical areas.
• Not just down to up: many neurons descend through the hierarchy and carry top-down signals.
7.2 THE AUDITORY SYSTEM
• Auditory system perceives sound – perception
of objects and events through the sounds they
• Sounds are vibrations of air molecules that
stimulate the auditory system. We hear between
• Amplitude = loudness; frequency = pitch;
complexity = timbre
• Timbre, the complexity of a sound, is what
allows us to distinguish between a guitar and
violin playing at the same loudness and same
pitch. We visualize this with a spectral profile, which graphs the intensity of component frequencies.
• In reality, pure tones do not exist outside a lab; sound is always associated with complex patterns
of vibrations, a combination of sine waves of various frequencies and amplitudes.
• Fourier analysis: mathematical procedure for breaking down complex waves into component
sound waves. This may be how the auditory system analyzes complex sounds.
• Relationship between tone frequency and perceived pitch for complex sounds: we interpret its
pitch based on the fundamental frequency (highest frequency of which various component
frequencies are multiples – numerically the lowest common denominator), e.g. complex sound of
100, 200 and 300Hz has fundamental frequency of 100Hz.
o Missing fundamental: when perception of pitch not directly related to sound’s component
frequencies. E.g. Mixture of 200, 300 and 400Hz perceived as having same pitch as 100Hz
pure tone – it is the fundamental frequency, but is not a component.
• Sound waves travel through the ear:
1) Auditory canal
2) Vibrates tympanic membrane (eardrum)
3) Vibrates middle ear bones, the ossicles (malleus, incus, stapes) Page 3 of 16
4) Stapes triggers vibrations on the oval window
5) Vibrational energy transferred to fluid of the cochlea, a long coiled tube with an internal
membrane running through the middle. The internal membrane is the organ of Corti, the
auditory receptor organ.
• It has 3 chambers: the vestibular duct (scala vestibule), the cochlear duct (scala media) and the
tympanic duct (scala tympani).
• The cochlear duct is composed of the basilar membrane and the tectorial membrane with the
organ of Corti in between. The basilar membrane is stiff while the tectorial membrane is soft.
• The auditory receptors, the hair cells, are mounted on the basilar (base = bottom) and poke
through the tectorial (on the top).
Inner Ear Middle Ear OUTER EAR
Outer Ear Pinna funnels sound
into the auditory canal.
Sound energy vibrates
The middle ear bones
Tympanic and vestibular ducts are
perilymph (like ECF) are the smallest in the
Cochlear duct has endolymph, human body.
which has high K concentration and Stirrup presses on the
oval window, which
+80mV. connects to the inner
duct (scala ear.
Vestibular duct media) INNER EAR
(scala vestibuli) Semicircular canals –
Cochlea – audition
At (scala area of hair Hair cells are the auditory sensory
higtympani)entration. The shearing force opens
the mechanically-gated ion channels (stereocilia)
so K enters the hair cell and causeslls are connected to
Endolymph afferent nerves to send signals to
depolarization. the brain.
At the base of the cell, the surrounding fluid hasted to
+ efferent nerves from the brain.
low K concentration. To recover from +he
depolarization (receptor potential), K channels at
the base open to let K out of cell (down
Meanwhile, Ca channels also open here inenergy from
oval window initiat+s fluid waves
respond to the depolariin cochlear, organ of Corti deflects
to help repolarization.on its pivot points, produces
Perilymph shearing force on hair cells.
Action potential causes vesicles of transmitter
(maybe glutamate?) to be expelled and initiate is described
action potential. Signal is transmitted down the
auditory nerve (a branch of the auditory-vestibular Page 4 of16
• Sound energy is dissipated at the round window, an elastic membrane in the cochlea wall.
• The cochlea is very sensitive: we can hear differences in pure tones that differ in frequency by only
• Different frequencies produce maximal stimulation of hair cells at different points along the basilar
membrane with tonotopic organization.
o High frequencies are perceived (hair cells here most sensitive to) at
the base of the cochlear – high frequencies have a lot of energy
and are registered quickly; low frequencies are perceived at the
apex of the cochlear – low energy so won’t be registered till it
reaches the tip. The characteristic frequency is the frequency a
region of hair cell is most sensitive to.
o Another explanation: basilar membrane is narrow and stiff at the
base – so high frequency resonates best here; it is wide and floppy
at the apex, so low frequencies resonate here (high frequencies
o Characteristic frequencies on basilar membrane are logarithmically
spaced – for same distance, wider range of high frequency (squished closer together) at apex
than for low frequency at base (spaced widely apart). This is why the elderly lose high frequency
hearing early – damage to small area causes loss of a
wide, noticeable range of hearing.
o For a complex sound, the component frequencies would
together activate hair cells at different points along the
membrane; the many signals created are carried by
different auditory neurons to the brain.
• Tonotopic organization of Organ of Corti demonstrated by hair
cell tuning curves – testing each hair cell with pure tones of
different frequencies and of different intensities to find
characteristic frequency. Intensity can excite a
cell at a lower-than-characteristic frequency, but
for its characteristic frequency the cell will be
excited even at a low intensity.
• The vestibular system, with its semicircular
canals, is involved in the maintenance of
From the Ear to the Primary Auditory Cortex
• Network of auditory pathways:
1. Axons of each auditory nerve synapse in
the ipsilateral cochlear nuclei
2. To the superior olive in the brain stem
(here signals from each ear are combined)
3. Olivary neurons project to the lateral
4. To o the inferior colliculi in the tectum
(mesencephalon) where they synapse
onto the next neurons
5. Project to the medial geniculate of the
nuclei in the thalamus
6. To the primary auditory cortex Page 5 of16
• There are at least 5 synapses from cochlea to cortex
• Signals are transmitted both ipsilaterally and contralaterally to the primary auditory cortex.
Subcortical Mechanisms of Sound Localization
• Medial superior olives: respond to differences in theLateral timing of arrival of
sound signals between the two ears (phase Olive difference). For
frequencies < 800Hz.
• Lateral superior olives: respond to differential loudness
(amplitude) of sounds received by the two ears. For frequencies >
1600Hz (we have poor ability to localize sounds between 800 and
• The olives project to both the inferior and superior colliculi; the
layers of the superior colliculi are organized not tonotopically, but
according to a map of auditory space.
• The general function of the superior colliculi is locating
(localizing) sources of sensory input in space; it also receives
visual input, and is organized retinotopically for that modality.
• There is not only an ascending pathway of auditory information,
but a descending pathway which exercises inhibitory control over
lower-level organs to determine what will be picked up (e.g.
attention to one person at a loud party).
• The auditory neurons of a barn owl`s superior colliculus are very
finely tuned with small receptive fields.
Auditory Cortex WHAT
• Primary auditory cortex is located in the
superior temporal lobe, in Heschl’s gyrus (HG),
and hidden from view in the lateral fissure.
• Adjacent to the primary auditory area two other
areas, together called the core; surrounding it is
a belt region of secondary cortex (over 6 distinct
areas). Surrounding that is the parabelt.
• Organization of Primary Auditory Cortex:
Organized in functional columns with neurons in Lateral
the same vertical area (all neurons encountered
during vertical microelectrode penetration) fissure
responding optimally to sounds in same
frequency range. It is also arrange tonotopically, based on frequency.
• What sounds should be used to study the auditory cortex? Progress in research of auditory system
is limited by complexity and variety of system even at low levels.
• Two Streams of the Auditory Cortex:
o WHAT – ANTERIOR: Recognition (“ventral”) = identifying sounds, using the anterior auditory
pathway to the prefrontal cortex.
o WHERE – POSTERIOR: Spatial (“dorsal”) = localizing sound, using the posterior auditory
pathway to the posterior parietal cortex.
o However, newest evidence suggests that the WHERE pathway may not really project to the
posterior parietal cortex – instead projects to the posterior part of the prefrontal cortex.
o Evidence of imaging of fibre (white matter) tracts to look at connections. Page 6 of 16
• Auditory-Visual Interactions: Interactions take place in the association cortex (especially the
posterior parietal cortex) - combination of visual and auditory receptive fields, of the same location in
the subject’s environment.
• fMRI studies allow recording of activity throughout the brain: show us that interactions occur not
only in the association cortex, but even at the lowest level of the primary sensory cortex. These
interactions are an early and integral part of sensory processing, not after unimodal analyses, but
• Where Does the Perception of Pitch Occur? Observation that when stimuli have different
frequency and pitch (using missing fundamentals), neurons respond to changes in frequency, not
pitch. An area anterior to the primary auditory cortex responds to pitch most, while also containing
neurons that respond to frequency – this may be where frequencies are converted to the perception
of pitch (Bendor & Wang, 2005).
Effects of Damage to the Auditory System
• Auditory Cortex Damage: Auditory cortex rarely entirely destroyed due to its being hidden in the
lateral fissure, and when it is, it’s also accompanied by extensive damage to surrounding tissues; we
rely on lesion-based research on animals.
• Large lesions surprisingly do not produce severe permanent deficits – subcortical circuits must
serve complex and important processing functions before signal reaches cortex (at tectum,
• In humans with bilateral lesions, there is often complete loss of hearing, but it recovers in ensuing
weeks – deafness must have due to shock of the lesion. Permanent effects include loss of ability to
localize sounds, and impairment of ability to discriminate frequencies.
• A unilateral lesion disrupts ability to localize sounds in space contralateral, but no ipsilateral, to the
lesion; this effect is observable but smaller for other auditory deficits. This suggests the system is
• Deafness in Humans: Total deafness is rare, affecting 1% of the hearing-impaired; diffuse parallel
network of auditory pathways ensures that if one is destroyed, alternative pathways can still transmit
• Severe hearing problems typically result from damage to inner ear, middle ear, or the nerves
leading from them – rarely central damage.
• Conductive hearing impairment is due to damage to the outer or middle ear; this blocks the signal
from getting to the cochlea. Can be caused by earwax buildup, tear in tympanic membrane,
problems with ossicles or oval window (such as due to birth defects).
• Sensorineural hearing impairment is due to damage to cochlea and loss of receptive hair cells, or
to the auditory nerve. Causes include noise exposure, aging process, or ototoxic drugs.
• If only part of the cochlea is damaged, may have nerve deafness for some frequencies but not
others. In age-related hearing loss, high-frequency perception is often lost first.
• Tinnitus: ringing of the ears without outside stimulation, common in hearing loss. Observed that
cutting the nerve from the ringing ear does not stop the ringing – suggests changes to the central
auditory system caused by deafness is the cause of tinnitus. Can be one or both ears, temporary or
• Cochlear implants bypass damage to the auditory hair cells, replacing function of middle and
• They convert sounds picked up by microphone in patient’s ear to electrical signals; a speech
processor selects and arranges sounds picked up by the microphone; a transmitter and
receiver/stimulator converts this into an electrical signal; a bundle of electrodes collect electrical
impulses and excite the auditory nerve.
• There are many varieties – behind the ear, full or half shell, in the canal, or “in the crest”. Page 7 of 16
• Cochlear implants do not restore normal hearing, but implantation soon after deafness can be very
beneficial (disuse leads to degeneration of auditory neural pathways).
Vision vs. Audition (Lecture)
• Vision has better spatial resolution than audition. However, audition has better temporal resolution:
when we see a quick flash and hear a quick beep simultaneously, and sensory modalities conflict
(see 1 flash, hear 2 beeps) we trust audition because it has better temporal resolution (and we think
that we saw two flashes).
• Vision has limited spatial extent – we can only see what’s in front of us – while audition is omni-
7.3 SOMATOSENSORY SYSTEM: TOUCH & PAIN
• The somatosensory system is made up of 3 separate but interacting systems: an exteroceptive
system that senses external stimuli applied to the skin; 2) a proprioceptive system that monitors
information about the position of the body, from receptors in the muscles, joints, and organs; 3) an
interoceptive system that provides general information about conditions within the body, like
temperature and blood pressure.
• The exteroceptive system comprises of 3 divisions: one for perceiving mechanical stimuli (touch),
one for thermal stimuli (temperature), and one for nociceptive stimuli (pain).
• Free nerve endings: The simplest receptors, nerve
endings with no specialized structures on them. Most
sensitive to temperature change and pain.
• Pacinian Corpuscles: Largest and deepest receptors,
enclosed in an onion-like structure of tissue. They adapt
rapidly and respond to sudden displacements of the skin
(fast-adapting), though not to constant pressure. They are
mechanically-gated Na channels.
• Ruffini endings and Merkel’s disks adapt slowly and
respond to gradual indentation and stretch, respectively.
• Stereognosis is the identification of objects by touch.
When we do this, we manipulate the object in our hands so
the pattern of stimulation continually changes – otherwise
only the slow-adapting receptors would be active and the
sensation would be markedly different.
• Having information from both quick and slow-adapting
receptors provides information about both the dynamic and
static qualities of tactual stimuli.
• Somatosensory receptors are specialized, being most
sensitive to a particular type of tactual stimulation. However,
the general mechanism is similar: stimuli changes the chemistry of the receptor, changing
permeability of various io