U7: MECHANISMS OF PERCEPTION
1. Sensation and Perception - Sensation is the simple process of detecting the presence of
- Perception is the complex process of integrating,
recognizing and interpreting complex patterns of sensations.
o Ex. The man who mistook his wife for a hat: Dr. P
knew there was an object in his visual field but
had a problem in perceptual processing, which
3 Sensory areas of the Cortex resulted in an inability to recognize that object.
1. Primary sensory cortex
- Receives most of its input directly from the thalamic relay
nuclei of that system
2. Secondary Sensory Cortex
- Comprises the areas of the sensory cortex that receive most
of their input from the primary sensory cortex of that system
or from other areas of the secondary sensory cortex of the
3. Association cortex
- Receives input from more than one sensory system
- Sensory systems are characterized by a parallel, functionally
segregated and hierarchical organization.
- Parallel: information flows between different structures
simultaneously along multiple pathways
- There seems to be 2 kinds of parallel analysis, one that is
capable of influencing our behaviour without our conscious
awareness and one that influences our behaviour by
engaging our conscious awareness.
- Functionally segregated: different parts of various
structures specialize in different kinds of analysis
- It was thought that primary, secondary and association areas
were functionally homogeneous whereby they acted together
to perform the same function.
- The three levels contains functionally distinct areas that
specialize in different kinds of analysis.
- Hierarchical: information flows through brain structures in
order of the increasing neuroanatomical and functional
o Receptors Thalamic Relay Nuclei Primary
Binding problem Sensory Cortex Secondary Sensory Cortex
o The higher the level of damage, the more specific
Colour’s effect on Taste (Top-down processing) and complex the deficit
Dubose et al.
- If different types of information are processed in different
specialized zones, how are complex stimuli perceived as an
- One possible solution is that there is a single area of the
cortex at the top of the sensory hierarchy that receives
signals from all other areas of the sensory system and puts
them together to form perceptions; however there are no
areas of cortex to which all areas of a single sensory system
- Perceptions are a product of the combined activity of
different interconnected cortical areas.
- People’s ability to identify taste and smell depends to some
degree on colour.
o Ex. People’s taste of cherry, orange, and lime
beverages were judged correctly an average of 67% of the time if colours matched the beverages’
o Dropped to 37% if solutions were colourless
o 28% if colours didn’t match the taste.
2. Auditory System - The function of this system is to perceive objects and events
**The reason we hear 100 out of 200, 300, and through the sounds they make.
400 is because 100 is the greatest common - Amplitude, Frequency, and Complexity of the molecular
denominator. vibrations are most closely linked to perceptions of
loudness, pitch, and timbre.
- Fournier analysis is the mathematical procedure for
breaking down complex waves into their component sine
- The pitch of a complex sound may not be directly related to
the frequency of any of the sound’s components
o Ex. A mixture of pure tones with frequency of
200, 300, and 400 Hz would be perceived as
having the same pitch as a pure tone of 100 Hz
because it is the fundamental frequency of 200,
Auditory Pathway 300, 400.
o This is called the missing fundamental.
- Vibrations in the air are transmitted through tympanic
membrane (ear drum), ossicles (three small bones), oval
window, into fluid of the cochlea.
- Organization of organ of Corti is tonotopic because higher
frequencies excite receptors closer to oval window. This is a
- Each pressure change at the oval window travels along this
organ as a wave.
- The organ of Corti is composed of 2 membranes: the basilar
membrane that contains the auditory receptors, or hair cells
and the tectorial membrane rests on the hair cells.
- 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 them and increase firing in axons of
the auditory nerve
- The vibrations of the cochlear fluid are ultimately dissipated
by the round window
1. Hair cells synapse on neurons whose axons enter the
metencephalon and synapse in the ipsilateral cochlear
2. From the cochlear nucleus, some fibers project to the
nearby superior olives inferior colliculus via lateral
3. From the inferior colliculus, fibers ascend to the
medial geniculate nucleus of the thalamus
4. Fibers ascend to the primary auditory cortex in the
- The sensitivity of the cochlea (0.2% difference in pure tone
that differ in frequency) brings up the problem of auditory
processing whereby you can sort out individual frequency
activations on your basilar membrane when in a crowded
- Semicircular canals are the receptive organs of the
vestibular system that carries information about the
direction and intensity of head movements, to help maintain
balance. - Localization of sounds in space is mediated by the lateral
and medial superior olives
- When a sound originates on the left side, it reaches the left
ear first and is louder there, some neurons in the medial
superior olives respond to slight differences in the time of
arrival of signals from the two ears whereas some neurons in
the lateral superior olives respond to slight differences in
amplitude of sounds from the two ears.
- These olives travel to the superior colliculus and inferior
colliculus which take auditory input and lay it out according
to a map of auditory space
- The primary auditory cortex receives the majority of input
from the medial geniculate nucleus is located in the
temporal lobe, within the lateral fissure.
Localization of Sound - This cortex, along with 2 other areas make up the core
region, which is surrounded by the belt, or secondary cortex
and areas outside this are parabelt areas
- Located in the lateral fissure that separates the temporal and
- Tonotopically organized into functional columns such that
neurons in a given column all respond maximally to tones in
the same frequency range.
- Lesions in the auditory cortex don’t cause deafness, even if
the lesions include secondary auditory cortex.
- Can cause difficulty localizing brief stimuli or recognize
Auditory Cortex rapid sequences of sounds.
- The major permanent effects of bilateral lesions are loss of
ability to localize sounds and impairment of the ability to
- Research has lagged behind in this field because we have no
idea what the auditory cortex since neurons respond only
weakly to simple stimuli such as pure tones.
- The effects of unilateral auditory cortex lesions suggest that
the system is partially contralateral
- Unilateral lesions disrupts the ability to localize sounds in
space, contralateral but not ipsilateral to the lesion
Deafness in Humans
- Severe hearing problems usually result from damage to the
inner ear or middle ear or nerves leading from them, instead
of central damage.
- There are 2 classes of hearing impairments:
1. Conductive deafness – associated with damage to the
2. Nerve deafness – damage to the cochlea or auditory
nerve, occurring as a loss of hair cell receptors.
- If only part of the cochlea is damaged, individuals may have
nerve deafness for some frequencies but not others, usually
due to age.
- Tinnitus – ringing of the ears. When nerves from the
ringing ear are cut, this has no effects which suggests that
changes to the central auditory system that were caused by
the deafness are the cause of tinnitus. Two Streams of Auditory Cortex
- There are 2 streams of auditory analysis – anterior auditory
pathway is involved in identifying sounds (what) and the
posterior auditory pathway is involved in locating sounds
Auditory Visual Interactions
- Some posterior parietal neurons have both visual and
auditory receptive fields
- fMRI results showed that sensory system interaction is not
tagged on after unimodal analysis are complete, it is an early
and integral part of sensory processing.
- Bypass damage to the auditory hair cells by converting
sounds picked up by the microphone on the ear to electrical
signals which are then carried to the cochlea by a bundle of
- Small, complex device that provides a sense of sound to a
person who is profoundly deaf or severely hard of hearing
- Electrodes are inserted in the cochlea to create hearing by
electrically stimulating auditory nerve fibres
- Implants are helpful for patients with damaged hair cells
because they only amplify sounds but don’t cause that sound
to be translated into electrical signals in hair cells.
Parts of a Cochlear Implant: (External – behind ear and Internal portion –
1. Microphone – picks up sound from environment
Cochlear Implants 2. Speech processor – which selects and arranges sounds picked up
3. Transmitter/ Receiver – receive signals from the speech processor
and convert them into electric impulses
4. Electrode array – group of electrodes that collects the impulses
from the stimulator and sends them to different regions of the
3. Somatosensory System: Touch and Pain - Sensations from your body are called somatosensations
- The somatosensory system is made up of three separate but
1. Exteroceptive system – senses external stimuli that are applied to
2. Proprioceptive system – which monitors information about the
position of the body that comes from receptors in the muscles,
joints, and organs of balance
3. Interoceptive system – general information about conditions
within the body
Skin Receptors - This chapter will deal with the exteroceptive system.
- The simplest cutaneous/ skin receptors are free nerve
endings (neuron endings with no specialized structures on
them), which are particularly sensitive to temperature
change and pain.
- Pacinian corpuscles are large and onionlike receptors that
respond to sudden displacements of the skin but not to
- Merkel’s discs and Ruffini endings both respond to
gradual skin indentation and skin stretch respectively.
- When you identify objects by touch (Stereognosis), you
manipulate them in your hands so that the pattern of
stimulation continually changes
- Neural fibers that carry information from skin receptors and
other somatosensory receptors gather together in nerves and enter the spinal cord via dorsal roots
- Area of the body that is innervated by the left and right
Two Major Somatosensory Pathways dorsal roots is called a dermatome and can be plotted on a
- The dorsal-column medial lemniscus system carries
information about touch and proprioception (knowing the
presence of your own body parts)
- The anterolateral system tends to carry information about
pain and temperature
1. The sensory neurons of the dorsal column enter the spinal cord
via a dorsal root, ascend ipsilaterally in the dorsal columns and
synapse in the dorsal column nuclei
2. The axons decussate/ cross over and then ascend in the medial
lemniscus to the contralateral ventral posterior nucleus of the
3. The ventral posterior nuclei also receives input from the three
branches of the trigenimnal nerve which carries somatosensory
information from contralateral areas of the face.
1. Most dorsal root neurons of the anterolateral system synapse as
soon as they enter the spinal cord
2. Second order neurons decussate/cross over but then ascend to the
brain in the contralateral/opposite side. However, some do not
decussate but ascend to the brain on the ipsilateral side.
3. There are 3 tracts
1. Spinothalamic tract – projects to the ventral posterior
nucleus of the thalamus
2. Spinoreticular tract – projects to reticular formation
3. Spinotectal tract – projects to the tectum
4. Three branches of the trigeminal nerve carry the pain and
temperature information from the face.
- Lesions to ventral posterior nuclei, which receive input from
spinalthalamic tract and dorsal-column system produce loss
of skin sensitivity to touch, temperature change and sharp
- Lesions of parafascicular and intralaminar tract receive input
from spinoreticular tract reduced deep chronic pain without
disrupting cutaneous sensitivity.
Cortical Areas and Homunculus
Penfield - Penfield applied electrical stimulation to various sites on the
cortex and patients described what they felt.
- Discovered that human primary somatosensory cortex(SI)
was somatotopic – organized as a map of the body surface
- The greatest proportion of SI is dedicated to receiving input
from the parts of the body that we use to make tactile
discriminations (hands, lips, tongue) whereas a small areas
of SI receives input from large body areas like the back.
- A second somatotopically organized area SII is ventral to SI
and extends into lateral fissure, receiving most input from SI
- SII also receives substantial input from both sides of the
body and the output of SI and SII goes to the association
cortex of the posterior parietal lobe.
- Primary somatosensory cortex is composed of 4 functional
strips, each with a similar but separate somatotopic
- If one were to record from neurons in a horizontal line
across the four strips, one would find neurons that preferred
4 different kinds of tactile stimulation, all to the same part of
- As one moves from anterior to posterior, the preferences of
the neurons would tend to become more complex and
specific, suggesting an anterior-posterior hierarchical
Q: Clarify this left hand thing? - There have been 2 steams of perception that projects to
posterior parietal cortex and participates in multisensory
integration and direction of attention and a ventral stream
that project to SII and participates in perception of objects’
- Somatosensory signals are ultimately conducted to the
Effects of Damage to the Primary highest level for the sensory hierarchy, to areas of
Somatosensory Cortex association cortex in prefrontal and posterior parietal cortex.
- The posterior parietal cortex has bimodal neurons that
respond to both somatosensory and visual stimuli.
- The visual and somatosensory receptive fields are spatially
related – if a neuron has a somatosensory receptive field
Somatosensory Agnosia centered in the left hand, its visual field is adjacent to the left
- Damage to primary somatosensory cortex are mild because
this system has numerous parallel pathways
- In a unilateral excision that included SI, the patients
displayed 2 minor contralateral deficits: a reduced ability to
detect light touch and a reduced ability to identify objects by
- There are 2 major types: