How is the visual system able to operate at widely varying levels of light intensity?

Sanjay Manohar, Cambridge 2001

Need

1. Dynamic range

2. Perceptual constancy (albedo not luminance)

3. Coding - bandwidth

4. Prevent retinal damage

Definitions: Luminance, Illuminance, Albedo

Mechanism

1. pupil

2. rod vs cone

                rod          sacrifice                 low spat res           colourless              low saturation      slow adapting

                                for                           low thresh

3. Amplification cascade

                automatic gain control?

4. field adaptation

                increment threshold

                Weber-Fechner as gain control => contrast

                Eigengrau

               

               

5. bleaching adaptation

                pigment is active

                                increased noise

                                decreased gain

                small bleaching => large gain reduction

               

6. Photoreceptor adaptation / Amplification cascade

                Ca dependent

                                inhibits GC

                                activates PDE

                                activates G prot

                                inhibits RK

7. Lateral inhibition

                centre-surround ganglion cell (1-200Hz)

8. Summation & inhibition

9. Central adaptation

               

?Peripheral retina x

Rod vs cone

	Rod	Cone
Spatial summation	Yes	allows more receptors to contribute
Colour		Yes
Adaptation	Slow	Fast
Temporal integration	long	short
Saturation luminance	low (scotopic)	high (photopic)
Threshold	low	high
Density	spread out	high : cones => acuity
Number	12m	8m
Distribution	periph	foveal

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The eye contains two types of receptors with very different transducing properties. Rods, having low thresholds and being more sensitive to low light intensities, are specialised for dim lighting (scotopic vision). They are found in the periphery of the retina, undergo much spatial summation, and adapt only slowly. But there's a price to pay -- they are saturated at relatively low light levels (100cd/m2), and have low spatial resolution. At levels higher than this, the cones take over. These receptors are of three subtypes with different peak wavelengths, and are thus facilitate colour vision. They are less sensitive to light, and are found in high densities at the fovea -- making them able to discriminate much finer detail than the rods.

Stiles crawford effect