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Lecture 2 - The eyes (Sensation & perception UU)
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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