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How is the visual system able to operate at widely varying
levels of light intensity?
Sanjay Manohar, Cambridge 2001
1. Refraction
focusing power 65D: 45D in cornea + accom; shape of eye
hypermetropia emmetropia myopia presbyopia
sph aberration, chromatic, dispersion, scattering
2. Lucency
cataract, corneal vascularisation,
3. IOP / glaucoma
4. Light collection vs depth of field
photopic 10^5 mesopic scotopic 10^-3 => 10^8 (firing rate 0-400Hz)
pupil diameter (30x)
5. excess light absorbed by RPE melatonin
cf tapetum lucidum
retinitis pigmentosa
6. Acuity vs Aperture
7. Rod / cone density
acuity vs background
colour vs motion
8. Rod vs cone
amplification Gi cascade, single photon (100x sens)
dark current and gain control (Ca alters cGMP metabolism; micropipette studies)
Weber Fechner law (Cornsweet 1970)
bleaching (afterimages, dark and light adaptation graphs (cones first))
Dark light (receptor noise, eigengrau, Barlow 1958)
deuteranopia
location on retina (Stiles 1959); fovea fewer axons/vessels
colour vision only 1 deg.
9. On centre off centre - metabo/iono bipolars (Kuffler 1951)
reduces number of Optic n. fibres
first stage om computation in the process of perceiving things
contrast not luminance = constancy
dot/edge detection
albedo 3-90%
10. wiring
Horizontal cells - negative feedback / lateral inhibition = gain control
spatiotemporal integration vs differentiation
11. Colour opponency (S/ML, M/L)
reduces fibres
relative wavelengths are important in object identity, not just total light.
colour constancy from centre-surround effect
colour-edge detection
12. parvo (midget) vs magno (parasol) ganglion cells
M - fast adapting, phasic, transient, fast conduction, more contrast, larger RF
P - sustained tonic (+colour)
respond to changes, vs identify
first stage in motion perception
13. cat X (spatiotemporally linear) and Y (nonlinear, motion) - Barlow 1964
X=double colour opponent. slower conduction, tonic, central retina.
Y=no centre-surround chromaticity. some to colliculus. (parasol-like)
Linear in dim, nonlinear in bright?
12. intrinsically photosensitive ganglion cells
13. blind spot