NEURO217 Audition

functions of the auditory system

localizing objects in space

orienting to unseen stimuli

parsing complex environment

communicating socially (language, culture, oral history)

identifying individuals and groups

expressing emotions

inducing moods

appreciating nature/beauty/artistry/aesthetic

sound waves

pressure waves that vary over time

main features:

amplitude, frequency, phase, waveform (amplitude across time) — timbre

sound wave analysis

pure tones are sine waves (never heard in nature)

periodic sound stimuli emanate from resonating structures that produce repeated waveforms over time (these are important for vocal communication and music, but are not really in nature)

complex sounds are decomposed into harmonics with differing power at particular frequencies

auditory objects and scene analysis

process of grouping ongoing stream of auditory time-series information

filter relevant sounds from background noise/2 simultaneous melodies

parse meaningful words/sentences

how are 2 simultaneous streams distinguished?

differences in pitch, intensity, timbre, etc

parts of the ear

outer, middle, inner

outer ear

pinna, ear canal, ear drum

funnels sounds to tympanic membrane

aids sound localization (elevation and front-back)

middle ear

3 bones/ossicles: malleus, incus, stapes

impedance matching: matches forces of air to that of water, which allows the sound through to inner ear

takes the sound from tympanic membrane and focuses it onto much smaller oval window

inner ear

cochlea (out pouching of vestibular labyrinth)

site of auditory transduction

aqueous

frequency analyzer that decodes complex sound waves into constituent parts

parts of the cochlea

basilar membrane & hair cells — receptor cells

primary auditory nerve is made from axons that receive neurotransmitters from the single row of inner hair cells

outer hair cells play a feedback/tuning function, amplifying lower sounds/aiding in sound discrimination

auditory signal transduction

traduce pressure wave into neural signal

cilia tips aligned by height

pressure wave deflects stereo cilia towards top, which opens K+ channels and depolarizes membrane, and causes neurotransmitter release via graded potentials and depolarization of auditory nerve from inner hair cells

sound transmission along basilar membrane

displacement of oval window causes fluid movement in scala tympani

basilar membrane widens and becomes more malleable going from base to apex, which is the opposite of cochlea tapering

tonotopy: frequency tuning along membrane — high pitches are at the basal end, low pitches are at the apex

complex sound is broken up into constituent parts along length

tuning is determined by passive resonance properties and active biochemical properties via outer hair cells

properties of auditory nerve transmission

1:1 receptor and inner hair sound along basilar membrane to maintain tonotopy along the cochlea

neurons have limited capacity in how fast they can signal — biomechanics of cell firing

the brain knows whether sound is high or low-pitched, phase locking occurs below 3kHz

cochlear implants

active stimulation of nerve endings along the length of the basilar membrane according to frequencies present in the sound waveform → sounds very synthesized, artificial

central auditory pathways

parallel pathways (at least 3 from the cochlea to thalamus)

1) cochlear nuclei in medulla

2) nucleus of lateral lemniscus/superior olive/inferior colliculus

3/4/5) medial geniculate nuclear of thalamus

→ primary auditory cortex

marked crossing along pathways

how is the organization of the auditory system different from the visual system?

no direct synapses on thalamus

collicular relay is part of primary pathway

not organized by space but instead by pitch (tonotopy)

combining inputs across sensory organs happens earlier (superior olive and inferior colliculus)

mechanisms of sound localization (2)

interaural time differences (low frequency)

interaural intensity differences (high frequency)

interaural time differences

medial superior olive is critical structure

freq < 3kHz, rate where neurons can code for each pulse of sound wave traveling

range: 10µs-700µs

acuity of 1 degree (2 identical sounds must be at least this far apart to be differentiated)

medial superior olive interaural time differences

receives bilateral input so the neuron has maximum activation

dendrites get input from both right and left cochlear nuclei, prefers coincident timing

cell is left-ear leading if axon length is shorter from right ear to left

only works if left and right inputs are the same organization (conduction velocities, transduction sensitivity, phase-locking occurs in auditory nerve — only works up to 3 kHz)

interaural intensity differences

in the lateral superior olive (LSO) and medial nucleus of the trapezoid body (MNTB) in pons

freq > 2kHz

human head causes intensity drop (acoustic shadow)

input from contralateral side is inhibited by interneuron in MNTB, which yields net excitation in the ipsilateral inferior colliculus — combined excitatory ipsilateral projection from LSO and inhibitory contralateral projection from LSO

auditory cortex

located along hidden part of STG (Heschel’s grays)

cortical magnification (over-representation) of speech frequencies

medial geniculate nucleus of the thalamus projects to other cortical/subcortical areas directly as well (limbic regions — amygdala/insula for emotional reactions)

ERPs

detect sensory feature processing in auditory cortex

mismatch negativity (MMN) detects deviant auditory stimuli in a series

peaks ~160-200ms after stimulus onset

localized to auditory cortex (superior temporal gyrus)

reflects auditory sensory feature analysis

occurs without overt judgments of sensory deviation

sensitive to changes in pitch, timbre, rhythm, etc. in more widespread frontotemporal brain regions

auditory object recognition

moving beyond primary auditory cortex → specializations for different categories of sound (more lateralized)

speech sounds (more left-lateralized)

affective vocalizations (more right-lateralized)

auditory motion

Environmental sounds (more right-lateralized

speech and language lateralization

more left lateralized for speech content analysis and production

right lateralized for affective prosody

how do we hear sound? (general)

features of the ear convert sound waves and their properties (pitch, timbre, amplitude) into a neural code and facilitate the prioritization of human speech sounds

what does the cochlea do/act as?

acts as a frequency analyzer to parse fluid movements into neural codes based on pitch

what is the superior olivary complex?

a subcortical structure that facilitates sound localization via two mechanisms: interaural time differences and interaural intensity differences

where is a tonotopic map? what does it show?

the primary auditory cortex contains a tonotopic map that demonstrates cortical magnification for human speech and exhibits experience-dependent plasticity

what does the MMN (mismatch negativity) show?

shows how deviant features of an auditory stimulus series (pitch, loudness, etc.) are coded by the auditory cortex and other structures even in the absence of awareness

what do secondary auditory cortices do?

differentiate more complex aspects of auditory scene processing such as discriminating speech from affective tone or environmental sounds

music processing — only auditory system?

depends on many brain regions beyond the auditory system for coding prediction errors, emotional responses to music, etc