How is the visual system able to operate at widely varying
levels of light intensity?
Sanjay Manohar, Cambridge 2001
Intro
vibration 500BC
pressure wave Vinci
ecology - roar, communication
humans bad at localising - 2 deg (cf vision), but can hear behind us.
mechanism in brainstem - closely related to orienting
illusions mcgurk
ill posed problem / priors
Transduction
Pinna, concha, canal, tympanum
ossicles malleus (hammer) incus (anvil) stapes (stirrup)
oval window, cochlea round window,
Cochlea:
perilymph (scala vestib), reissner's, endolymph (scala media), tectorial, basilar, scala tympani
2.5 turns 32mm long 2 mm diam
modiolus, reticular m, basilar m, tectorial m
endolymph & endocochlear potential, K+
bipolar spiral ganglion cells
corey et al 2004 - (zebrafish , mouse) TRPA cation channel
kachar and gillespie
amplification:
pinna
stapes
mechanical - lever effect at tectorial/BM
100 stereocilia, 500nm diam, moving 0.3nm to 20 nm.
OHC motor
vgcc ca++
resonance in cells characteristic freq;
IHC OHC
-------------------------------------------------
3500 15000
1 to 20 outputs 1 to 1 output
otoacoustic emissions
IHC:
K+ atpase +80mv endolymph
shearing stereocilia tiplink, cation channel; Ca++ -> glutamate.
resonance; OHC amplification; efferents; otoacoustic
Tonotopy:
oval window - low pass
BM stiffness x 1/100, width x 5 --> dispersion
Bekesy cadaver -> silver particles, tonotopy / topographical
cochlear nucleus - tonotopic
lat inh
PL -can't do multiple frequencies, >4khz (refractory)()
tonotopy - hard to distinguish below 200 Hz; convolves intensity + freq.
von bekesy - stroboscopic silver flakes microscope. hard to study - bone!
Evans 1972 - auditory afferent fibres tuned more highly than BM
Crawford and fettiplace 1981 turtle single hair cell intracellular rec- intrinsic frequencies of hair cells
hudspeth 1983 - in vitro microelectrode - bending - depolarisation.
Anatomy and localisation of sound
Intro
Mechanisms
Sound: longitudinal pressure waves. amplitude & frequency
Thump of feet 20-100Hz, rustle of leaves 10kHz
Coordinates
- Polar: azimuth, altitude (elevation), distance (range)
- head centred
- body centred
- world centred
Range:
- inverse square law of amplitude
- greater attenuation of high freq (low pass)
- echoes (cf bat)
- prior about spectrum and amplitude
Azimuth
- Sup Oliv Complex - binaural inputs from cochlear nuclei
- Interaural delay for <3kHz (interaural distance)
head size -> ITD ~ 700us
graph: temporal delay vs direction of source
graph: firing rate vs temporal delay: tuning curve
phase ambiguity. phase locking diagram. Volley theory.
draw PL
MSO coincidence detectors - delay lines - Jefress 1984
2001 challenged - presence of inhibitory input
- Interaural intensity difference >2kHz (sound shadow; diffraction)
blocking ear. High freq - temporal coding
- cone of confusion
Altitude:
- Pinna asymmetry -> transfer function
- reflection, absorption, diffraction; phase and amplitude
graph: intra-canal mic / open-air-mic gain vs frequency
- removal of pinna; plastic pinna inserts
- individual difference; learned decoding
Monaural cues
spectral colour - selective amplification, elevation-dependent
Expts: Fisher 1968 remove pinna, mould pinna, tube in ear, reduces localisation when head fixed
Hofman 1998 - months of practice improves
microphone in canal
simple attenuation of sound, and of high freq
Obrist 1993 bat pinna removed abolishes localisation
done in DCn fusiform cells
Pathway
- Cochlear n -> SO-> IC -> MGN -> A1;
IC= auditory map, sensorymotor input; combines DCN vertial and olivary horizontal.
SC= visuotactile and tectospinal maps; orienting.
only place with topographic rather than tonotopic.
coordinate reference frame - retinal, head-centred, body-centred
- rostral Inf colliculus ? head-centred map -
periodotopy (freq selective laminae); combines info from delay and intensity?
- fast neurones needed for timing; project to SC: orienting
- Medial geniculate
- A1 - sweep-selective
Higher functions
Motor integration - Populis 2006 head restsraint reduces monkey localisation
Wallach 1936
possible cortical visual integration too. dorsal stream 3D
Why
- ? conversion from head- to body-centred
- ? motion-selective
- ? applying priors about kind of sound expected (eg speech)
- ? integration with vision: auditory dominates for time, vision for location
- needs to be done quickly and automatically -> collicular orienting response