sound and hearing

sound

collisions created when objects vibrate; in gas (air), it is changed in pressure (compression and rarefaction)

frequency

the number of times per second that a pattern of pressure repeats; related to wavelength of sound: wavelength=speed/frequency; related to pitch - perceptual quality of how “high” or “low” a sound is

amplitude

the magnitude of displacement of sound pressure waves; - related to loudness - the perceived intensity of a sound

loudness

the perceived magnitude of sound intensity

decibel

a unit for measuring sound intensity - represent the log of the ratio of sound pressure level (spl) of the measured sound to that of that of the absolute threshold (barely detectable) sound

sine waves

pure tones; the waveform for which variation as a function of time is a sine function

fourier analysis

breaking down complex signals or functions into simpler components

harmonic spectrum

the spectrum of a complex sound in which energy is at integer multiples of the fundamental frequency

fundamental frequency

the lowest-frequency component of a complex period sound

timbre

the psychological sensation by which a listener can judge that two sound with the same loudness and pitch are different (quality conveyed by harmonics and other high frequencies

outer ear, middle ear, inner ear

peripheral auditory system

pinna and ear canal

outer ear

ossicles (bones)

middle ear

cochlea and auditory nerve

inner ear

outer ear

collects and filters sound

middle ear

amplifies sound

inner ear

transduces sound into neural responses

purpose of pinnae

– Sounds are first collected from the environment by the pinnae

– The directional sound filtering properties of the pinnae are very important for sound localization

– The pinnae funnel sound waves into the ear canal

purpose of ear canals

– The length and shape of the ear canal enhances certain sound frequencies

– To collect sound waves and funnel them to the tympanic membrane

– To insulate and protect the tympanic membrane

tympanic membrane (ear drum)

the border between outer ear and middle ear; thin sheet of skin at the end of the outer ear canal, vibrates in response to sound

ossicles

smallest bones in your body, located in the middle ear; malleus, incus, stapes

malleus

hammer; receives vibrations from the tympanic membrane and is attached to the incus

incus

anvil; the middle ossicle; transmits vibrations from the malleus to the stapes

stapes

stirrup; connected to the incus on one end and the oval window

oval window

the border between the middle ear and inner ear

organ of corti

spiral structure of inner ear

vestibular canal, tympanic canal, middle canal

three canals in the cochlea

vestibular canal

extends from oval window at base of cochlea to helicotrema at the apex - canal closest to ossicles and through which pressure waves move first

tympanic canal

extends from the helicotrema at the apex to the round window at the base of the cochlea

middle canal

sandwiched between the vestibular and tympanic canals and contains the cochlear partition

basilar membrane

plate of fibers that forms the base of the cochlear partition and separates the middle and tympanic canals in the cochlea s

hair cells

cells that support the stereocilia, which transduce mechanical movement in the cochlea into neural activity sent to the brain stem - some hair cells also receive input from the brain - arranged in four rows that run down length of basilar membrane

stereocilia

hairlike extensions on the tips of the hair cells in the cochlea that initiate the release of neurotransmitters when they are flexed; the tip of each stereocilium is connected to the side of its neighbor by a tiny filament called a tip link

tectorial membrane

extends into the middle canal of the ear, floating above inner hair cells and touching outer hair cells

inner hair cells

convey almost all info about sound waves to the brain (using afferent fibers)

outer hair cells

convey info from the brain (using efferent fibers)

outer ear

pinna → ear canal → tympanic membrane

middle ear

malleus → incus → stapes

inner ear

oval window → vestibular canal → helicotrema → tympanic membrane → round window

place code

tuning of different parts of the cochlea to different frequencies (info about the particular frequency of an incoming sound wave is coded by the place along the cochlear partition with the greatest mechanical displacement, info about amplitude (intensity) at a particular frequency is coded by the firing rate of neurons tuned to that frequency)

coding of frequency

info of incoming sound wave coded along the place along the cochlear partition with the greatest mechanical displacement

coding of amplitude

info about sound intensity at a particular frequency coded by the firing rate of neurons tuned to that frequency

threshold tuning curve

a function plotting the response threshold of a neuron as a function of frequency

characteristic frequency

frequency to which the neuron is most sensitive

high spontaneous fibers

respond more easily to low SPL, but reach saturation more quickly

rate saturation

nerve fiber is firing as quickly as possible and thus incapable of increasing the firing rate

low spontaneous fibers

require more SPL to fire and continue increasing firing rate to higher SPLs

phase locking

tendency of a neuron to fire at particular phases of an ongoing periodic sound waveform - hair cells are only depolarized when moved in one direction, not in the other

temporal coding

auditory systems ability to process and encode info based on the timing of soundwaves rather than just their frequency and intensity

cochlear nucleus

first synapse of the AN fibers in the brain (in brain stem)

superior olive

brain stem region in the auditory pathway where inputs from both ears first converge

inferior colliculus

midbrain nucleus in auditory pathway

medial geniculate nucleus (MGN)

part of the thalamus that relays auditory signals to the cortex (analogous to the LGN in vision) s

primary auditory cortex (a1)

first cortical area for processing audition (located in temporal lobe)

belt area

region of cortex directly adjacent to a1 (and receives its inputs) where neurons respond to more complex sound characteristics

parabelt area

region of cortex adjacent to belt area where neurons respond to more complex characteristics of sounds as well as input from other senses

volley principle

explains how the auditory system encodes high frequency sounds by having groups of neurons fire action potentials slightly out of sync with each other - synchronized firing creates a “volley” of impulses that, when combined, effectively represent frequencies higher than what a single neuron could individually encode

locus of processing

large portion of processing occurs beyond v1

topographic organization

tonotopic (frequency - based) organization); starts in cochlea, maintained all the way through primary auditory cortex (a1)

psychoacoustics

study of the psychological correlates of the physical dimensions

physical properties

frequency, amplitude/intensity (SPL)

psychological percept

pitch, loudness

masking

using a second ( e.g., noise ) to make the detection of another sound more difficult

critical bandwidth

range of frequencies conveyed within a channel

conductive hearing loss

hearing loss caused by impairment of the mechanical transmission of sound waves to the cochlea

sensorineural hearing loss

hearing loss caused by damage to the cochlea or to the auditory nerve

cochlear implant

tiny flexible coils with mini electrode contacts, surgeons thread implant through round window toward cochlear apex , tiny mic transmits radio signals to receiver int the scalp, computer chip performs Fourier transform and stimulates appropriate location in cochlea for each frequency

hearing aids

earliest devices were horns, today, electronic