Chapter 7: Sensory Systems, Perception,
and Attention
→ There are five human exteroceptive sensory systems: the visual system, sensory
systems that detect stimuli outside of our bodies, the auditory (hearing), somatosensory
(touch), olfactory (smell), and gustatory (taste) systems.
→ There are additional exteroceptive senses that we do not have but other species
do.
Principles of Sensory System Organization
→ Each exteroceptive sensory system is organized like the visual system in
fundamental ways.
Types of Sensory Areas of Cortex
→ The sensory areas of the cortex are considered to be of three fundamentally
different types:
- Primary Sensory Cortex: the area that receives most of its input directly from
the thalamic relay nuclei of that system. For example the primary visual cortex
is the area of the cerebral cortex that receives most of its input from the
lateral geniculate nucleus of the thalamus.
- Secondary Sensory Cortex: 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 secondary sensory cortex of the same system.
- Association Sensory Cortex: receives input from more than one sensory
system. Most input to areas of the association cortex comes via areas of
secondary sensory cortex.
Receptors → Thalamic Relay Nuclei → Primary Sensory Cortex → Secondary
Sensory Cortex → Association Cortex
→ The interactions among these three types of sensory cortex and other sensory
structures are characterized by three major principles: hierarchical organization,
functional segregation, and parallel processing.
Features of Sensory System Organization
Hierarchical Organization: Sensory structures are organized in a hierarchy on the
basis of the specificity and complexity of their function.
- As stimuli move through the sensory system neurons respond optimally to
stimuli of greater and greater specificity and complexity.
- Each level of a sensory hierarchy receives a majority of its input from lower
levels and adds another layer of analysis before passing it on up the hierarchy
- The higher the level of damage, the more specific and complex the deficit.
- For example, destruction of a sensory system’s receptors produces a
complete loss of ability to perceive in that sensory modality where
, destruction of an area of association or secondary sensory cortex typically
produces complex and specific sensory deficits, while leaving fundamental
sensory abilities intact.
Functional Segregation: It was once assumed that all areas of cortex at any given
level of a sensory hierarchy acted together to perform the same function. Research
has shown that functional segregation, rather than functional homogeneity,
characterizes the organization of sensory systems.
- All three levels of the cerebral cortex (primary, secondary, and association)
contain functionally distinct areas that specialize in different kinds of analysis.
Parallel Processing: It was once believed that the different levels of a sensory
hierarchy were connected in a serial fashion where information flows among the
components over just one pathway. However, sensory systems are parallel systems
in which information flows through the components over multiple pathways.
- Parallel systems feature parallel processing which is the simultaneous
analysis of a signal in different ways by the multiple parallel pathways of a
neural network.
Descending Pathways: this is how higher levels of sensory systems can influence
sensory input (not shown in model). Many neurons descend through the sensory
hierarchies where they carry information from lower to higher levels of their
respective hierarchies,
- They conduct information in the opposite direction (from higher to lower
levels). These are known as top-down signals
The Auditory System
The function of the auditory system is the perception of sound. Sounds are
vibrations of air molecules that stimulate the auditory system. Humans hear only
those molecular vibrations between about 20 and 20,000 hertz (cycles per second).
,Physical and Perceptual Dimensions of Sound
→ The amplitude, frequency, and complexity of the
molecular vibrations are most closely linked to
perceptions of loudness, pitch, and timbre, respectively.
→ Pure tones (sine wave vibrations) exist only in
laboratories and sound recording studios as in real life,
sound is always associated with complex patterns of
vibrations. There is a close relationship between
frequency and perceived pitch
→ Fourier analysis: the mathematical procedure for
breaking down complex waves into their component
sine waves of various frequencies and amplitudes. These component sine waves
produce the original sound when they are added together.
- One theory of audition is that the auditory system performs a Fourier-like
analysis of complex sounds in terms of their component sine waves.
→ Natural Sounds: the relation between the frequencies (always composed of a
mixture of frequencies) and their perceived pitch is complex
- The pitch of such sounds is related to their fundamental frequency: the
frequency that is the highest common divisor (a number that divides another
number) for the various component frequencies.
- For example, a sound that is a mixture of 100, 200, and 300 Hz frequencies
normally has a pitch related to 100 Hz because 100 Hz is the highest common
divisor of the three components.
- Pitch perception missing fundamental: the pitch of a complex sound may not
be directly related to the frequency of any of the sound’s components.
- For example, a mixture of pure tones with frequencies of 200, 300, and 400
Hz would be perceived as having the same pitch as a pure tone of 100 Hz.
This is because 100 Hz is the fundamental frequency (the highest common
divisor) of 200, 300, and 400 Hz.
The Ear
→ Sound waves travel from the outer ear down the auditory canal and cause the
tympanic membrane (the eardrum) to vibrate. These vibrations are then transferred
to the three ossicles in the middle ear (the malleus (the hammer), the incus (the anvil),
and the stapes (the stirrup)). The vibrations of the stapes trigger vibrations of the
membrane called the oval window, which transfers the vibrations to the fluid of the
cochlea. The cochlea is a long, coiled tube with internal structures that are the
auditory receptor organs called the organ of Corti. Each pressure change at the oval
window travels along the organ of Corti as a wave of stimulus. The organ of Corti is
composed of several membranes: the basilar membrane and the tectorial membrane.
The auditory receptors, the hair cells, are mounted in the basilar membrane, and the
, tectorial membrane rests on the hair cells. Wave of stimulus
in the organ of Corti produces a shearing force on the hair
cells which stimulates them. This causes an increase firing in
axons of the auditory nerve (a branch of the
auditory-vestibular cranial nerve). The vibrations of the
cochlear fluid are ultimately dissipated by the round window,
an elastic membrane in the cochlear wall.
→ The cochlea is very sensitive which allows us to hear
differences in pure tones that differ in frequency by only 0.2
percent. The major principle of cochlear coding is that
different frequencies produce maximal stimulation of hair cells
at different points along the basilar membrane
- Higher frequencies produce greater activation closer to the windows
- Lower frequencies produce greater activation at the tip of the basilar
membrane.
→ Similar to the cochlea, most other structures of the auditory system are arrayed
according to frequency. **Organization of the visual system is largely retinotopic, the
organization of the auditory system is largely tonotopic
→ Complex acoustic environments have many component frequencies where each
individual sound activates many sites along the basilar membrane. The number of
sites simultaneously activated can be enormous. But somehow your auditory system
manages to sort these individual frequencies into separate categories and combine
them so that you hear each source of complex sounds independently. The
mechanism underlying this important ability has yet to be identified, but one theory
is that it is due to the synchronous relationship over time of the frequency elements
of each sound source.
→ The semicircular canals: receptive organs of the vestibular system. This carries
information about the direction and intensity of head movements, which helps us
maintain our balance.
From the Ear to the Primary Auditory Cortex
→ There is no major auditory pathway to the cortex. Instead, there is a network of
auditory pathways.
- The axons of each auditory nerve synapse in the ipsilateral cochlear nuclei,
from where projections lead to the superior olives on both sides of the
brainstem at the same level.
- The axons of the olivary neurons project via the lateral lemniscus to the inferior
colliculi, where they synapse on neurons that project to the medial geniculate
nuclei of the thalamus, which in turn project to the primary auditory cortex.
- Signals from each ear are combined at a very low level (in the superior olives)
and are transmitted to both ipsilateral and contralateral auditory cortex.
and Attention
→ There are five human exteroceptive sensory systems: the visual system, sensory
systems that detect stimuli outside of our bodies, the auditory (hearing), somatosensory
(touch), olfactory (smell), and gustatory (taste) systems.
→ There are additional exteroceptive senses that we do not have but other species
do.
Principles of Sensory System Organization
→ Each exteroceptive sensory system is organized like the visual system in
fundamental ways.
Types of Sensory Areas of Cortex
→ The sensory areas of the cortex are considered to be of three fundamentally
different types:
- Primary Sensory Cortex: the area that receives most of its input directly from
the thalamic relay nuclei of that system. For example the primary visual cortex
is the area of the cerebral cortex that receives most of its input from the
lateral geniculate nucleus of the thalamus.
- Secondary Sensory Cortex: 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 secondary sensory cortex of the same system.
- Association Sensory Cortex: receives input from more than one sensory
system. Most input to areas of the association cortex comes via areas of
secondary sensory cortex.
Receptors → Thalamic Relay Nuclei → Primary Sensory Cortex → Secondary
Sensory Cortex → Association Cortex
→ The interactions among these three types of sensory cortex and other sensory
structures are characterized by three major principles: hierarchical organization,
functional segregation, and parallel processing.
Features of Sensory System Organization
Hierarchical Organization: Sensory structures are organized in a hierarchy on the
basis of the specificity and complexity of their function.
- As stimuli move through the sensory system neurons respond optimally to
stimuli of greater and greater specificity and complexity.
- Each level of a sensory hierarchy receives a majority of its input from lower
levels and adds another layer of analysis before passing it on up the hierarchy
- The higher the level of damage, the more specific and complex the deficit.
- For example, destruction of a sensory system’s receptors produces a
complete loss of ability to perceive in that sensory modality where
, destruction of an area of association or secondary sensory cortex typically
produces complex and specific sensory deficits, while leaving fundamental
sensory abilities intact.
Functional Segregation: It was once assumed that all areas of cortex at any given
level of a sensory hierarchy acted together to perform the same function. Research
has shown that functional segregation, rather than functional homogeneity,
characterizes the organization of sensory systems.
- All three levels of the cerebral cortex (primary, secondary, and association)
contain functionally distinct areas that specialize in different kinds of analysis.
Parallel Processing: It was once believed that the different levels of a sensory
hierarchy were connected in a serial fashion where information flows among the
components over just one pathway. However, sensory systems are parallel systems
in which information flows through the components over multiple pathways.
- Parallel systems feature parallel processing which is the simultaneous
analysis of a signal in different ways by the multiple parallel pathways of a
neural network.
Descending Pathways: this is how higher levels of sensory systems can influence
sensory input (not shown in model). Many neurons descend through the sensory
hierarchies where they carry information from lower to higher levels of their
respective hierarchies,
- They conduct information in the opposite direction (from higher to lower
levels). These are known as top-down signals
The Auditory System
The function of the auditory system is the perception of sound. Sounds are
vibrations of air molecules that stimulate the auditory system. Humans hear only
those molecular vibrations between about 20 and 20,000 hertz (cycles per second).
,Physical and Perceptual Dimensions of Sound
→ The amplitude, frequency, and complexity of the
molecular vibrations are most closely linked to
perceptions of loudness, pitch, and timbre, respectively.
→ Pure tones (sine wave vibrations) exist only in
laboratories and sound recording studios as in real life,
sound is always associated with complex patterns of
vibrations. There is a close relationship between
frequency and perceived pitch
→ Fourier analysis: the mathematical procedure for
breaking down complex waves into their component
sine waves of various frequencies and amplitudes. These component sine waves
produce the original sound when they are added together.
- One theory of audition is that the auditory system performs a Fourier-like
analysis of complex sounds in terms of their component sine waves.
→ Natural Sounds: the relation between the frequencies (always composed of a
mixture of frequencies) and their perceived pitch is complex
- The pitch of such sounds is related to their fundamental frequency: the
frequency that is the highest common divisor (a number that divides another
number) for the various component frequencies.
- For example, a sound that is a mixture of 100, 200, and 300 Hz frequencies
normally has a pitch related to 100 Hz because 100 Hz is the highest common
divisor of the three components.
- Pitch perception missing fundamental: the pitch of a complex sound may not
be directly related to the frequency of any of the sound’s components.
- For example, a mixture of pure tones with frequencies of 200, 300, and 400
Hz would be perceived as having the same pitch as a pure tone of 100 Hz.
This is because 100 Hz is the fundamental frequency (the highest common
divisor) of 200, 300, and 400 Hz.
The Ear
→ Sound waves travel from the outer ear down the auditory canal and cause the
tympanic membrane (the eardrum) to vibrate. These vibrations are then transferred
to the three ossicles in the middle ear (the malleus (the hammer), the incus (the anvil),
and the stapes (the stirrup)). The vibrations of the stapes trigger vibrations of the
membrane called the oval window, which transfers the vibrations to the fluid of the
cochlea. The cochlea is a long, coiled tube with internal structures that are the
auditory receptor organs called the organ of Corti. Each pressure change at the oval
window travels along the organ of Corti as a wave of stimulus. The organ of Corti is
composed of several membranes: the basilar membrane and the tectorial membrane.
The auditory receptors, the hair cells, are mounted in the basilar membrane, and the
, tectorial membrane rests on the hair cells. Wave of stimulus
in the organ of Corti produces a shearing force on the hair
cells which stimulates them. This causes an increase firing in
axons of the auditory nerve (a branch of the
auditory-vestibular cranial nerve). The vibrations of the
cochlear fluid are ultimately dissipated by the round window,
an elastic membrane in the cochlear wall.
→ The cochlea is very sensitive which allows us to hear
differences in pure tones that differ in frequency by only 0.2
percent. The major principle of cochlear coding is that
different frequencies produce maximal stimulation of hair cells
at different points along the basilar membrane
- Higher frequencies produce greater activation closer to the windows
- Lower frequencies produce greater activation at the tip of the basilar
membrane.
→ Similar to the cochlea, most other structures of the auditory system are arrayed
according to frequency. **Organization of the visual system is largely retinotopic, the
organization of the auditory system is largely tonotopic
→ Complex acoustic environments have many component frequencies where each
individual sound activates many sites along the basilar membrane. The number of
sites simultaneously activated can be enormous. But somehow your auditory system
manages to sort these individual frequencies into separate categories and combine
them so that you hear each source of complex sounds independently. The
mechanism underlying this important ability has yet to be identified, but one theory
is that it is due to the synchronous relationship over time of the frequency elements
of each sound source.
→ The semicircular canals: receptive organs of the vestibular system. This carries
information about the direction and intensity of head movements, which helps us
maintain our balance.
From the Ear to the Primary Auditory Cortex
→ There is no major auditory pathway to the cortex. Instead, there is a network of
auditory pathways.
- The axons of each auditory nerve synapse in the ipsilateral cochlear nuclei,
from where projections lead to the superior olives on both sides of the
brainstem at the same level.
- The axons of the olivary neurons project via the lateral lemniscus to the inferior
colliculi, where they synapse on neurons that project to the medial geniculate
nuclei of the thalamus, which in turn project to the primary auditory cortex.
- Signals from each ear are combined at a very low level (in the superior olives)
and are transmitted to both ipsilateral and contralateral auditory cortex.
