... light senitivity comes from a photosensitive pigment called iodopsin or visual purple. When a photon of light triggers the iodopsin an enzyme is released in the cone that alters the membrane of the cell, such that the normal flow of sodium ions into the cell is halted. This causes the negative charge that normally exists across the cell membrane to increase from -40mv to -70mv. In its resting state a cone will trigger a constant stream of neurotransmitters, but when the negative charge across the membrane is increased, this stream slows or ceases, which in turn causes the bipolar cells to fire. The bipolar cells then trigger the ganglion cells via a complex network of horizontal and amacrine cells, the ganglion cells then send the resulting neural signals to the primary visual cortex. It has long been recognised that colour perception would require a number of different types of sensor, ...

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