How can the eye turn both bright and dim light into electrical signals? What optical factors determine acuity?
Sanjay Manohar (Oxford 2015)
3x10^-6 to 9x10^8 cd/m^2
pupil x16 only!
receptors generate a "dark current", suppressed by light
Rods vs Cones
evidence from purkinje shift
number: rod 90 million (cone 40 million) (130 million total)
1000 times more sensitive
Shape of discs
long time constant
Gprotein - PDE - cGMP - cation channels - hyperpol
Baylor 1987 patch clamp
Hecht 1936 - two-stage recovery from bleaching
- 10 min x 100
- 20 min x 1000 again
Hecht 1942 - single flash threshold = 5 photons = 1 photon per rod
Calcium channels close - low Ca - reduced cGMP binding to channels
- channel opening - depol -> reduced sensitivity
- ca increases cGMP synthesis
- ca alters PDE activity
Contrast sensitivity function!
myopia, hypermetropia, presbyopia, astigmatism
Chromatic aberration (dispersion)
Pupil diameter - lens aberration vs diffraction
Hecht 1928 - luminance --> recruitment of cones --> higher acuity
Diffusion/scattering/glare: cataract, corneal scars, vitreous abnormalities
Receptor density; retinal location.
Stiles Crawford effect
Non-optical factors: Hyperacuity, spatial summation, lateral inhibition
The eye trades off acuity in dark