Light Compare and contrast photopic and scotopic vision.

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Presentation transcript:

Light Compare and contrast photopic and scotopic vision. Why do we have both rods and cones? (N) Discuss the significance of convergence and divergence in the retinal processing of rod and cone signals. (M) Outline the visual processes that allow our vision to operate over a large luminance range. (N) How is the visual system able to function at widely varying levels of light intensity? (N,V) In what ways can one quantify the performance of the eye? What factors limit its performance? (M) What are believed to be the mechanisms of colour vision? How can defects in it be explained? How may visual acuity be tested? What factors influence it? (M) Discuss factors affecting the quality of the optical image on the retina. To what extent does the quality limit visual acuity? How does the visual system cope with the conflicting demands of high absolute sensitivity and high spatial acuity? (M) What structural features of the retina help to explain visual acuity? (M) What factors limit visual acuity at different light levels? What factors determine whether a visual stimulus will be seen? Write a concise account of any two of the following: colour vision; visual acuity; dark adaptation; eye movements; pupillary reflexes. (V)

Outline the visual processes that allow our vision to operate over a large luminance range Pupillary reflex Rods vs. Cones Pooling (spatial) Temporal summation Automatic gain control (mediated by fall in Calcium) Increases cGMP synthesis (GCAP) Closes cGMP activated channel (Calmodulin) Increases recoverin binding to Rh* Horizontal cells (synaptic inhibition) Bleaching (fewer receptors available)

Lamb model of photoreceptor

Compare and contrast photopic and scotopic vision Rods - just rhodopsin B&W More in peripheries Convergence 1000:1 Rod bipolar, AII, gc Sensitivity 10-4 cd/m2 Acuity low Slower, temporal summation Cones - 3 kinds of opsins Colour Fovea (and rim) Convergence small On/off bipolar, gc Mesopic and photopic only Acuity 2.5mm Fast ‘on’ and ‘off’

Contrast sensitivity function Decreasing contrast Frequency