How can the eye turn both bright and dim light into electrical signals? What optical factors determine acuity?

Sanjay Manohar (Oxford 2015)

scotopic          photopic
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
    Photopigment concetration
  temporal summation
    long time constant
  spatial summation
    multiple discs
    Gprotein - PDE - cGMP - cation channels - hyperpol
    Baylor 1987 patch clamp
    Dark adaptation
      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
    Light adaptation
      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!
  Refractive error
    myopia, hypermetropia, presbyopia, astigmatism
    spherical aberration
    Chromatic aberration (dispersion)
  Eye factors
    Pupil diameter - lens aberration vs diffraction
    Hecht 1928 - luminance --> recruitment of cones --> higher acuity
    Diffusion/scattering/glare: cataract, corneal scars, vitreous abnormalities
  Retinal factors
    Receptor density; retinal location.
    Macular degeneration
    Stiles Crawford effect
  Non-optical factors: Hyperacuity, spatial summation, lateral inhibition

The eye trades off acuity in dark