Greg Detre
2/12/99
vibration of an object in a material �/span> rarefaction + compression
within the range of an auditory system�s frequency range
3 parameters of sound:
frequency (pitch � number of cycles/second)
amplitude (degree of displacement around the resting level of pressure)
timing (where it starts � phase sens in humans)
all real sounds are spectrally complex
simplest = sinusoidal������� usually use tone burst
audiologist tests hearing��������� use audiogram (plot of minimum audible sound level)
dB SPL (reference level ≈ 20mN/cm min level audible)
can have negative dB
hearing most sensitive between 500-4000Hz � most important information
outer ear � amplifies sounds (optimal direction = 45� �from midlines)
gain 20-30dB (cupped hands)
c� (bony bit at the entrance)
sound localisation � use both ears
outer ear also tells whether front/back � different spectra � sound origin
middle ear � impedence matching device/between air/water
concentrate energy striking the drum � smaller oral window
freq gain (midrange boosted compared with high/low)
hence u-shape of audiogram
auditory system is different because other sensory systems have � receptors
auditory system uses transformation of input
cochlea (Latin � snail), spiralling
auditory nerve
scala vestibula/media/tympani
organ of Corti
basilar membrane � divides membrane���������� coiled
first processing stage of auditory system
von Bekesy����������� 1920s telephone engineer
travelling wave of displacement
envelope � maximum displacement
the lower the frequency, the further the shapes the max displacement
experimentation � different because very small, sensitive to disruption, very very hard temporal bone, million mechanical parts
transduction
hair cell � shearing motion so that stereocilia move backwards/forwards
Hudspeth � sac of bullfrog
used probe to prod the hair cell + record electro
resting membrane potential of -60mV
slightly hyper + depolarising elements
sensitive to orientation of deflkection of stereocilia
1 row of inner hair cells��������� 3 rows of OHC
ion gates � simple mechanical system������ heap up with very high frequencies
Helen Dain � modern auditory physiologist
2 types of hair cells
different patterns of innervation of auditory nerve
IHCs afferent�������� OHC efferent (into the cells from the brain)
(major output from cochlea)��������� (to enhance the mechanics of the cochlea)
OAEs (acoustic emissions) � sensitive microphone near ear canal � detect sounds
coning � - active process in cochlea
use to determine young babies� audition
OHC respond to stimulation by twitching
unique biological motor
IHC = transducer of auditory system � vibration of basilar membrane � neural
OHC � feedback to enhance (change local mechanisms) to sharpen the travelling wave frequency
using anaesthetised animals � much tighter envelope
auditory nerve
extra-cellular recording (can't record synaptic, only AP)
earphones
spontaneous activity produce AP with sound input
IHC always releasing neurotransmitter
tuning � narrow-band filters respnod only to narrow band at high dB, broad range
saturation
phase-locking � all auditory nerve fibres at low frequencies
tone-burst gating???
OHC???