1. Psychology 355 1
Introduction
Sensory Systems
A. Sense of hearing, audition
1. Detect sound
2. Perceive and interpret nuances
B. Sense of balance, vestibular system
1. Head and body location
2. Head and body movements
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The Nature of Sound
Sound
A. Audible variations in air pressure
B. Sound frequency: Number of cycles per
second expressed in units called Hertz (Hz)
C. Cycle: Distance between successive
compressed patches
D. Range: 20 Hz to 20,000 Hz
E. Pitch: High and Low
F. Intensity: Difference in pressure between
compressed and rarefied patches of air
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The Structure
of the Auditory System
I. Auditory pathway stages
A. Sound waves
B. Tympanic membrane
C. Ossicles
D. Oval window
E. Cochlea fluid
F. Sensory neuron response
II. Brain stem nuclei output
A. Thalamus to MGN to A1
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I. Sound Force Amplification by the Ossicles
A. Pressure: Force by surface area
B. Greater pressure at oval window than
tympanic membrane, moves fluids
II. The Attenuation Reflex
A. Response where onset of loud sound
causes tensor tympani and stapedius
muscle contraction
B. Function: Adapt ear to loud sounds,
understand speech better
The Middle Ear
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Physiology of the Cochlea
Pressure at oval window,
pushes perilymph into scala
vestibuli, round window
membrane bulges out
The Inner Ear
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I. The Innervation of Hair Cells
A. One spiral ganglion fiber: One inner hair
cell, numerous outer hair cells
II. Amplification by Outer Hair Cells
A. Function: Sound transduction
B. Motor proteins: Change length of outer
hair cells
C. Prestin: Required for outer hair cell
movements
The Inner Ear
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The Inner Ear
The Basilar Membrane
Structural
properties: Wider
at apex, stiffness
decreases from
base to apex
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Central Auditory Processes
Auditory Pathway
A. More synapses at nuclei than visual
pathway, more alternative pathways
B. Anatomy
1. Dorsal cochlear nucleus, ventral
cochlear nucleus, superior olive,
inferior colliculus, MGN, lateral
lemniscus, auditory nerve fiber
2. Primary pathway: Ventral cochlear
nucleus to superior olive to inferior
colliculus to MGN to auditory
cortex
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Response Properties of Neurons in
Auditory Pathway
A. Characteristic frequency
Frequency at which neuron is most
responsive
B. Response
More complex and diverse on
ascending auditory pathway in
brain stem
Central Auditory Processes
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Encoding Sound
Intensity and Frequency
I. Encoding Information About Sound Intensity
A. Firing rates of neurons
B. Number of active neurons
II. Stimulus Frequency, Tonotopy, Phase Locking
A. Frequency sensitivity: Basilar membrane
B. Frequency: Highest at base, lowest at
cochlea apex
C. Tonotopy: Systematic organization of
characteristic frequency within auditory
structure
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Phase Locking
Consistent
firing of cell at
same sound
wave phase
Encoding Sound
Intensity and Frequency
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I. Techniques for Sound Localization
A. Horizontal: Left-right, Vertical: Up-down
II. Localization of Sound in Horizontal Plane
A. Interaural time delay: Time taken for
sound to reach from ear to ear
B. Interaural intensity difference: Sound at
high frequency from one side of ear
C. Duplex theory of sound localization:
1. Interaural time delay: 20-2000 Hz
2. Interaural intensity difference: 2000-
20000 Hz
Mechanisms of Sound Localization
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The Sensitivity of Binaural Neurons to
Sound Location
Monaural: Sound in one ear
Binaural: Sound at both ears
Superior olive: Cochlear nuclei input to
superior olive, greatest response to
specific interaural delay
Mechanisms of Sound Localization
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Mechanisms of Sound Localization
I. Delay Lines and Neuronal Sensitivity to Interaural
Delay
A. Sound from left side, activity in left cochlear
nucleus, sent to superior olive
B. Sound reaches right ear, activity in right
cochlear nucleus, first impulse far
C. Impulses reach olivary neuron at the same
time summation action potential
II. Localization of Sound in Vertical Plane
A. Sweeping curves of outer ear
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Mechanisms of
Sound Localization
A given binaural
neuron indicates the
amount of phase
disparity between
inputs from the left
and right ear.
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I. Acoustic Radiation
A. Axons leaving MGN project to auditory
cortex via internal capsule in an array
B. Structure of A1 and secondary auditory
areas: Similar to corresponding visual
cortex areas
II. Neuronal Response Properties
A. Frequency tuning: Similar characteristic
frequency
B. Isofrequency bands: Similar
characteristic frequency, diversity
among cells
Auditory Cortex
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Principles in Study of Auditory Cortex
Tonotopy, columnar organization of cells with similar
binaural interaction
Auditory Cortex
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I. Importance of Vestibular System
A. Balance, equilibrium, posture, head, body,
eye movement
II. The Vestibular Labyrinth
Lateral line Organs
Small pits or tubes
Function
Sense vibration
or pressure
changes
The Vestibular System
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The Vestibular System
Head Angle
Linear Acceleration
Head Rotation
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The Vestibular System
I. The Semicircular Canals
A. Function: Detect head movements
II. Structure
A. Crista: Sheet of cells where hair
cells of semicircular canals clustered
B. Ampulla: Bulge along canal,
contains crista
C. Cilia: Project into gelatinous cupula
D. Kinocili oriented in same direction
so all excited or inhibited together
E. Semicircular canals: Filled with
endolymph
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I. Push-Pull Activation of Semicircular Canals
A. Three semicircular canals on one side
1. Helps sense all possible head-rotation
angles
B. Canal: Each paired with another on
opposite side of head
C. Push-pull arrangement of vestibular
axons: Rotation causes excitation on
one side, inhibition on the other
The Vestibular System
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I. The Vestibulo-Ocular Reflex (VOR)
A. Function: Line of sight fixed on visual
target
B. Mechanism: Senses rotations of head,
commands compensatory movement of
eyes in opposite direction
C. Connections from semicircular canals, to
vestibular nucleus, to cranial nerve
nuclei excite extraocular muscles
The Vestibular System
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The Vestibular System
Vestibular Pathology
A. Drugs (e.g., antibiotics) can damage
vestibular system
B. Effects:
1. Trouble fixating on visual targets
2. Walking and standing difficult
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Concluding Remarks
Hearing and Balance
A. Nearly identical sensory receptors (hair
cells)
B. Movement detectors: Periodic waves,
rotational, and linear force
C. Auditory system: Senses external
environment
D. Vestibular system: Senses movements
of itself
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Concluding Remarks
Hearing and Balance
A. Auditory Parallels Visual System
1. Tonotopy (auditory) and Retinotopy
(visual) preserved from sensory cells
to cortex code
B. Convergence of inputs from lower levels
Neurons at higher levels have more
complex responses
