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)
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