PSYC 2410 Chapter Notes - Chapter 7: Secondary Somatosensory Cortex, Ventral Posterior Nucleus, Posterior Parietal Cortex

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15 Oct 2013
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Chapter 7: Mechanisms of Perception: Hearing, Touch, Smell, Taste and Attention
- Exteroceptive sensory systems
o Auditory (hearing), somatosensory (touch), olfactory (smell) and gustatory (taste)
Principles of Sensory System Organization
- Sensory areas of the cortex are of 3 fundamental types
o Primary sensory cortex
Receives most of its input from the thalamic relay nuclei of that system
o Secondary sensory cortex
Areas of the sensory cortex that receive most of their input directly from the
primary sensory cortex or from other areas of secondary sensory cortex of the
same system
o Association cortex
Receive input from more than one sensory system. Most from areas of
secondary sensory cortex
- Hierarchical organization
o Sensory structures are organized in a hierarchy based on the specificity and complexity
of their function
Ex. Receptors thalamic relay nuclei primary sensory cortex secondary
sensory cortex association cortex
As you move from primary 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
o Psychologists divide the general process of perceiving 2 general phases
Sensation
Process of detecting the presence of stimuli
Perception
High-order process of integrating, recognizing and interpreting
complete patterns of sensations
- Functional segregation
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
- Parallel processing
o Systems in which information flows through components over multiple pathways
o The simultaneous analysis of a signal in different ways by the multiple parallel pathways
of a neural network
o 2 fundamentally different kinds of parallel streams of analysis
One capable of influencing our behaviour without our conscious awareness
- Summary model of sensory system organization
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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 systems report
Then perceptions must be a product of the combined activity of
different interconnected cortical areas
Auditory System
- Sounds
o Are vibrations of air molecules that stimulate the auditory system, humans only hear
those molecular vibrations between 20 and 20 000 hertz (cycles per second)
Amplitude- loudness
Frequency- pitch
Complexity- timbre
- Pure tones
o Single wave vibrations, they exist only in laboratories and sound recording studios, in
real life sound is always associated with complex patterns and vibrations
o Any complex sound wave can be broken down into a series of sine waves of various
frequencies and amplitudes
Fourier analysis
The mathematical procedure for breaking down complex waves into
their component sine waves, one theory suggests the auditory system
does this
Fundamental frequency
Highest frequency of which the various component frequencies of
sound are multiples
Missing fundamental
When the pitch of a complex sound may not be directly related to the
frequency of any of the sounds components
- The ear
o Sound waves travel from the outer ear down the auditory canal and cause the tympanic
membrane (ear drum) to vibrate
o These vibrations are then transferred to the three ossicles: malleus (hammer), incus
(anvil) and the stapes (stirrup)
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o The vibrations of the staples trigger vibrations of the membrane called the oval window
which in turn transfer the vibrations to the fluid of the cochlea, a long coiled tube with
an internal membrane running almost down to its tip
o The internal membrane is the auditory receptor organ of corti, each pressure change at
the oval window travels along with the organ of corti as a wave. It has two 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 of axons of the auditory nerve
o The vibrations of the cochlear fluid are ultimately dissipated by the round withdow an
elastic membrane in the cochlear wall
o Cochlea is very sensitive, humans can hear differences in pure tones that differ by
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 neurons
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 nerve synapse in ipsilateral cochlear nuclei which projects to
superior olives, which project via the later lemniscus to inferior colliculi which project to
medial geniculate nuclei to 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 auditory cortex
- 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 ing
the left ear
o Neurons in the medial superior olives respond to slight diffs in the time of arrival of
signals from two ears
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