Our lens:

• Makes sure that light converges to a single point in our retina
• Does this using accommodation
  
  • Near target: thick lens
  • Far target: broad lens

Accommodation is not enough: you also need convergence.

Retina

• We use degrees to measure sizes of things we see  visual angle
• Retinal layers:

• In our fovea: there are only cones.

When light hits the rod (or cone) the rhodopsin is broken down and cell hyperpolarizes. So less activity when there is light.

• We have 3 types of cones and 1 rod.
• Rods are more sensitive to cones.
• We don’t see color with rods.

Purkinje shift: the sensitivity to higher wavelengths is less in the dark because rods are insensitive to red (long wavelengths).

Cones are not sensitive to low amounts of light.

Temporal summation: photons reaching a rod add up over time, so fewer are necessary for a response.

What if one of the cone types is missing? Then you are color deficient.

• The L-cone and M-cone missing picture looks quite the same, cause they have overlap in their wavelengths.

There is always photon noise.

So:

• Light of the retina is noisy
• The density of receptors determines spatial acuity
• Type of receptor determines temporal acuity/sensitivity
• Stimulus-point on the retina has a spread with gradual slope towards un-stimulated area

We do not perceive light, we perceive changes in light (intensity) across space and/or time.

Horizontal cells:

• Create a sharper picture

Amacrine cells (not very important for exam)

Bipolar cells (important):

• On center (inverting)
• Off center (non-inverting)

Center-surround organisation

Three types of ganglion cells

• M cells (large receptive field)
  • Have on and off cells and different cells for different colors
  • Slow temporal response
• P cells (small receptive field)
  
  • Fast
• L cells
  
  • No center surround because we only have a few cones

Center-surround is important!

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