How is the visual system able to operate at widely varying
levels of light intensity?
Sanjay Manohar, Cambridge 2001
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
ill posed problem / priors
Pinna, concha, canal, tympanum
ossicles malleus (hammer) incus (anvil) stapes (stirrup)
oval window, cochlea round window,
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
mechanical - lever effect at tectorial/BM
100 stereocilia, 500nm diam, moving 0.3nm to 20 nm.
resonance in cells characteristic freq;
1 to 20 outputs 1 to 1 output
K+ atpase +80mv endolymph
shearing stereocilia tiplink, cation channel; Ca++ -> glutamate.
resonance; OHC amplification; efferents; otoacoustic
oval window - low pass
BM stiffness x 1/100, width x 5 --> dispersion
Bekesy cadaver -> silver particles, tonotopy / topographical
cochlear nucleus - tonotopic
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
Sound: longitudinal pressure waves. amplitude & frequency
Thump of feet 20-100Hz, rustle of leaves 10kHz
- Polar: azimuth, altitude (elevation), distance (range)
- head centred
- body centred
- world centred
- inverse square law of amplitude
- greater attenuation of high freq (low pass)
- echoes (cf bat)
- prior about spectrum and amplitude
- 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.
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
- 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
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
- 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
Motor integration - Populis 2006 head restsraint reduces monkey localisation
possible cortical visual integration too. dorsal stream 3D
- ? 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