Sample Essay

Light perception depends partly on the sensitivity properties of the photoreceptor mosaic on the retina. The mosaic comprises of three cone-type (responsible for perceptions of color) cells and one rod-type cell which alter themselves depending on the prevailing light conditions. The rod type cells are responsible for allowing low light visibility, and take over gradually as the light drops. The photoreceptor cones are active in brightly lit areas, sharing their contribution with the rods as the light gradually gets dimmer, so that at night when the light intensity is at its lowest (such as in moonlight), the entire vision is managed by rods.

Cones and rods have their own distinct characteristics when it comes to dealing with light. Where cones permit the perception of color, they also allow the retina to capture more detail than the rods would allow. Their response time is swifter than rods and as a result, their reaction to changing images is faster as well (Kandel, Schwartz, & Jessell, 2000). The transition from cone mediated vision to rod mediated vision is a particularly interesting phenomenon, and can be observed very easily by holding a torch in the dark and pointing it straight into the eye in front of a mirror. The pupil shrinks instantly, and the uncomfortable source of light (from the torch) is immediately detected. Contrast this reaction to when the torch goes off immediately afterwards, a faint bright spot is still visible after the source of light has stopped as the rods take over and try to adjust to the lack of light sources. This is a good example of the difference in response times between the two photoreceptors and the constraints placed upon the system by the front end.

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