PSYC-2900 Chapter 7.1

audition

the process of hearing. serves 3 purposes:

• detecting sounds: translating frequency of a wave into a neural signal

• localising source of sound

• perception/recognition of what the sound is associated with

sound waves

object moves → air compresses & vibrates → waves of air pressure

• air molecules are packed densely in high pressure areas and spread out in low pressure areas

3 physical properties that relate to perceptual properties of sound:

• frequency

• amplitude/intensity

• complexity

outer ear

a region of the ear with structures that focus sound waves onto the tympanic membrane

middle ear

a region of the ear behind the tympanic membrane that contains a set of bones (ossicles) that vibrates

• malleus → incus + stapes → cochlea

  • stapes presses against oval window of cochlea

inner ear

a region of the ear that has the cochlea and the organ of corti

cochlea

a snail-shaped, fluid-filled structure in the inner ear, divided into 3 sections:

• scala vestibuli, scala media, scala tympani

organ of corti

a receptive organ in the inner ear composed of a basilar membrane, hair cells, and a tectorial membrane

deiter’s cells

cells that anchor hair cells to the basilar membrane of the cochlea

inner ear sound transduction

• sound waves cause the basilar membrane to move, bending the cilia of hair cells, producing receptor potentials

  • portion that moves the most is determine by sound frequency (high frequency bends portion closest to the oval window to bend)

  • pressure changes in the fluid under basilar membrane move towards the round window

  • when stapes pushes in toward oval window, round window bulges

inner hair cells

a type of auditory receptor with a role in sound transduction. they are required for hearing.

• ~3500 cells in humans

outer hair cells

a type of auditory receptor with a role in sound transduction. they are effector cells, which are mechanical characteristics of the basilar membrane that influence the effects of vibrations on inner hair cells.

• ~12000 cells in humans

hair cells

auditory receptors found in the cochlea that transduce sounds.

• have cilia that are attached to the basilar membrane at the base

• vibrations move basilar and tectorial membranes, causing these receptors to bend

  • bending can also occur by electric stimulation

• surrounding fluid is rich in potassium (K)

cilia

portion of hair cells that are attached to the basilar membrane.

• rigid due to actin and myosin

• linked together by tip links

• receptor potentials initiated at insertional plaques

insertional plaques

the site of tip links of cilia that initiates potentials when tension pulls on tip links due to movement of cilia

• contains TRPA1 (transient receptor potential cation channel) channel

cochlear nerve

a branch of the auditory nerve that brings auditory info to the brain

• formation of afferent fibres of axons of neurons with a shared synapse with hair cells

• efferent fibres create olivocochlear bundle

the auditory pathway

• axons of hair cells synapse in the cochlear nuclei

• project bilaterally (more contralateral) → superior olivary nucleus

• fibres → lateral lemniscus → inferior colliculus

• project ipsilaterally → MGN (thalamus) → auditory cortex (temporal lobe)

• each hemisphere receives info from both ears, but more from contralateral

auditory cortex

consists of a hierarchical organisation:

• core region (primary auditory cortex)

• belt region (association cortex)

• parabelt region

and 2 processing streams

• anterior stream

• posterior stream

has tonotopic representation

tonotopic representation

hair cells along certain points of the basilar membrane respond to certain frequencies mapped to the cortex

anterior stream

a processing stream in the auditory cortex that analyses complex sounds

posterior stream

a processing stream in the auditory cortex involved in sound localisation

frequency

a physical dimension of sound that relates to the perceptual dimension of pitch

• low = low pitch

• high = high pitch

amplitude/intensity

a physical dimension of sound that relates to the perceptual dimension of volume

• high = loud

• low = quiet

complexity

a physical dimension of sound that relates to the perceptual dimension of timbre, a mixture of different frequencies

• linear = simple

• spiky = complex

place coding

detection of moderate to high frequency sound waves in pitch perception

• frequencies coded by locations in basilar membrane

• high dose antibiotic use degenerates hair cells, high frequency loss first

rate coding

detection of lower frequency sound waves in pitch perception

• neurons fire in synchrony with movement of basilar membrane

cochlear implants

a microphone and processor, connected to a wire resting along the basilar membrane

• wire contains stimulating electrodes that stimulate the membrane when signalled by processor

perceiving volume

louder sounds → more intense vibrations in ossicles → greater force on cilia

• larger force = more neurotransmitter release → higher firing rate of cochlear nerve axons

more complicated version of this process for low frequencies (rate coding)

• signalled by number of axons from neurons in basilar membrane

fundamental frequency

in perceiving timbre, the perceived pitch of the noise

overtones

in perceiving timbre, frequencies of more complex tones

• start (attack) of note contain several frequencies, trailing off in different rates (decay)

perceiving timbre

the auditory cortex analyses complex sequence of multiple frequencies that appear, change in amplitude, and disappear

• angle of pinna (external ear) enhances different frequencies

• elevation/direction of sound hitting pinna results in dominant frequencies

perceiving spatial location

most accurate at detecting if the sound comes from left or right side. based on:

• phase differences

• intensity differences

• analysis of timbre

phase differences

neurons in superior olivary complex work as coincidence detectors: respond only when they receive simultaneous input from both ears

• sound coming from 1 side will be out of phase when it hits each ear

• sound from in front/behind will be in phase when it reaches each ear

perceiving complex sounds

the auditory system uses pattern recognition. 2 auditory stream serving 2 functions:

• anterior stream: perception of identity of sounds (what)

• posterior stream: perception of location of sounds (where)

these project to areas with overlapping functions

perceiving music

a complex perceptual task involving several brain areas:

• inferior frontal cortex: recognition of harmony

• right auditory cortex: perception of beat

• left auditory cortex: perception of rhythmic patterns on beat

• cerebellum & basal ganglia: timing

recruits wider neural networks (auditory & motor) and stimulates plasticity

• suggested that neural circuits that process music are present at birth

vestibular system

involved in aspects of unconscious maintenance of body positioning (e.g., balance, head upright, vesitbulo-ocular reflex)

• low frequency stimulation of vestibular sacs can produce nausea

• stimulating semicircular canals can produce dizziness and nystagmus (rhythmic eye movements)

vestibulo-ocular reflex

adjusting eye position in response to head movement to keep a steady retinal image

vestibular sacs

comprises a labyrinth of the inner ear, includes the utricle and saccule, responsible for head orientation

• hair cells with cilia within receptive tissue of floor and walls

  • cilia fixed in a mass contain otoconia (calcium crystals)

    • weight of otoconia causes mass to shift with orientation of head

semicircular canals

comprises a labyrinth of the inner ear, includes 3 tunnels representing the 3 planes of the head: rotation, changes in position, and linear acceleration

• canals contain endolymph fluid

• ampulla contains contains hair cells

  • cilia of hair cells embedded in cupula

• rotation causes inertial resistance, pushing endolymph against cupula

  • stopping moves cupula in opposite direction

vestibular pathway

axons of receptors form vestibular nerve (other branch of auditory nerve, CN VIII), and end in vestibular nuclei in medulla. cell bodies found in vestibular ganglion

• vestibular nuclei → cerebellum, spinal cord, medulla, pons, temporal cortex (unclear), CN controlling eye muscles