Have you ever wondered how we are able to see in the dark or how we see such a variety of colours throughout our everyday lives? These activities are possible due to receptor cells in the retina called rods and cones which each have a specialized function. The rods are involved mainly in low-light and peripheral vision, whereas the cones are mainly involved in colour and central vision. For both rods and cones, the outer segments of the receptor contain photopigment, which when exposed to light activates the cell.
Rods are densely packed in the periphery of the retina and are organized so that multiple rods are connected to a single projector cell. This allows for a high sensitivity to light (important for night vision) because dim light can hit any one of these rods to be noticed. The drawback to this set-up is that the spatial resolution is low, therefore it is hard to tell exactly where the light is coming from.
There are 3 types of cones (S, M, and L) required for perceiving colours. Each cone responds differently to various wavelengths of light, and the differences in the responses allows our brain to interpret colour. Cones are densely packed in the central retina and are organized so that only a few cones are connected to a single projector cell. This allows for high spatial resolution to light so that the point in space where the light is coming from can be identified. The drawback to this set-up is that the sensitivity to light is low, therefore cones need a higher intensity of light to be activated compared to rods.
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