Greg Detre
6th December 2002
Andy King
one ear � move head, move pinna
2 ways of detecting sounds � pressure (non-directional) waves
air movement (insects, directional, low-frequency)
longitudinal � American cockroaches � at the back end, cerci (protuberances) covered with slender hairs, different subsets activated by different directions
ventral cochlear nucleus � both types preserve time information
man can detect 3� azimuth = accuracy of 20ms ITD
an owl�s head is 1/10 size, just as accurate = 2ms disparities
even smaller flies can do it even better
the problem is not in the delay lines (in homeothermic animals)
but with the coincidence detectors
the time-course of an action potential is c. 10ms � so the summation happens too slwoly
2 dendritic trees � otherwise, don't seem special
stellate/bushy have funny synapses
some synapses release both excitatory and inhibitory transmitters
funny EPSP
2 of these matched up in time
lateral inhibition of an array of coincidence detectors
we have anatomical evidence, and we have ideas of how it could be done
inferior colliculus space-specific neurons
visual system map in tectum (mid-brain)
also snakes infrared system
the visual map seesm to be used to train the auditory map, recalibrating with growth + feather ruff etc.
azimuth ambiguity � front/back
owl sacrifices � intensity signals elevation, timing �/span> azimuth
but how do we manage elevation?
different reflections from the pinna, front/back and up/down asymmetrical
some enters directly, some reflected
CNS picks up on this, we don't know how
we�re almost as good at elevation as azimuth
false ears � relearn, reduced then improves
if we learn with false ears, then take them off, we flip straight back to the original system
this plasticity may be related to wax in ears, length of hair � still, it�s curious
why/how keep old system? different from prism glasses, relearn them then have to relearn back
microphone feed, lose elevation information, unless put ear shapes round microphone
owl medulla = equivalent to cochlear nucleus
cochlear nucleus � divided into dorsal + ventral
2 dendrites � helps with coincidence detection � how? reference in reading list
in the wild, intensity is informative about distance, especially if you recognise the sound and know how loud it should be
high frequency bits of the sound are differentially absorbed in the wild � if you recognise, you can use that spectral information for distance
moustached bats � FM squeaks with continuous tone
the FM indicates where in the tone its hearing the echo of
cochlear fovea � complex harmonic, listen for the echo of a particular one
cetacea also echolocate � also, the swifts and the South American owl(???) bird � both live in caves
mention bats for distance
Doppler shifts � distance and first derivative of distance
what/where in somatosensory system
S1 � where, good topographic maps
what � further back in PPC � definitely involved in active touch
efferent only to outer(???) � can change this size, tune up the cochlear
where does the auditory system do sound localisation?
mammals � brain stem
in bats, multiple cortical processing for elevation + azimuth etc.
also look at owls a bit, but they don't have a cerebral cortex (dolphins do???)
fMRI on dolphins? needs a big brain
dolphin = aquatic cows
fly (reading list) � negligible ITD
though there will be IID
time taken for auditory processing = very amplitude-dependent
interaction between amplitude and time delay
which accentuates the ITD
missing page???
there�s always ambiguity in sound localisation � cone of confusion
need to combine information from different techniques
in summary, the cochlear nucleus is most of all a timing device, preliminary processing for the levels above
phase-locking is produced by the mechanics of polarisation of the hair cells
< 4kHz (vowels are in this range)
can't discriminate phase-locking better with intensity � rate of firing � but phase-locking is not a rate code
may be more useful for tonotopicity for frequency discrimination
critical band � measure of the frequency resolution of the human auditory system
how far apart in frequency two signals have to be to be discriminable
many of these filters in the auditory system � they look like tuning curves
hear the fundamental frequency even when it�s removed from its harmonies � wouldn't work with place coding
early single channel cochlear implants only signalled in one place, yet some people were able to get quite good frequency discrimination from this, using phase-locking
isofrequency band � orthogonal to tonotopic axis
hearing is ID representation � the 2nd isofrequency dimension represents another parameter, e.g. intensity or time
some neurons respond to how quickly you change the frequency/amplitude
maps aren't generally useful (except for us scientists)
auditory cortex � object recognition, complex echolocation, short-term memory and temporal unilateral lesion, only thing that definitely goes is sound localisation
Railsheker, Wang, DSP used to see what it is about the stimuli that a cell is responding to
King � Nature Nov, letters, �linear processing ��
haven't fully decided whether distance is important there
found neurons in areas around A1 sensitive to pairs of sounds (echoes) in mustache bats, distance area, velocity area
the bat uses a very limited range of pure sounds
more difficult for a wide range of communication sounds
fewer auditory studies on awake mokeys
more difficult to use earphones, but anesthesia assumed that weaker but not substantially different from awake
there�s no behaviour directed in anesthetised animals
why does being poikilothermic screw up the delay lines???
what about front/back???
phase-locking???
fly sound-localiser
head-centred???
why does the auditory system but not the visual system use space-specific cells???
how do we distinguish pinnae spectral transformation from inherent characteristics of the sound???
distance/level
MLD??
easier to have 3 ears???
fish anables(???) has 4 eyes (S American), swims at the surface
do we echolocate???
Fourier???
lots of pre-cortical auditory processing � any cortical sound localisation???
where in the auditory system does sound localisation happen???
can blind people echolocate at all??? he doesn't know
tonic release??? vs phasic???
can't have a 3D frequency/amplitude/time representation � auditory cortex of mustache bat (known best)
neuroethological approaches � A1 is tonotopic, in isofrequency dimension, the non-monotonic intensity � what would the label be for time???
map??? just any systematic map???