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