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3.7.2.1 Sight – The Basic Visual System When light enters the eye it first passes through the cornea of the eye and is then focused onto the retina by the lens, which acts as an alterable convex lens. The retina is a thin multilayer membrane of specialised cells at the back of the eyeball. This layer contains about 137 million photoreceptor cells which are divided into rods and cones. The rods register shades of black and white light, and the cones perceive coloured light. When light strikes a photoreceptor or ganglion, it becomes stained with the colour and/or shade of that light. Once the ganglions have become stained a recovery period is needed. This is made possible by microscopic movements (nystagmus) of the eye (Rosenzweig et al, 2004). Figure 13: Cross Section of the Eye (TPC, 2004) The movement focuses light on other ganglia allowing the previous ganglia time to recover in order to receive another light or colour ‘stain’. The resulting neural signals from the rods and cones then undergo complex processing by other neurons of the retina, and are transformed into action potentials in retinal Ganglion cells whose axons form the optic nerve. Ganglions (photoreceptors) cells form 90 percent within the retina. Of these 10 percent are M (Magnocellular) cells, 80 percent are P (Parvocellular) cells and 10 percent are K (Koniocellular) cells. P cells are selective for form and colour (cones), have a slow conduction velocity, low contrast sensitivity and high resolution. The M cells, however, are colour blind (rod cells), have a high conduction velocity and contrast but low resolution. K cells have only recently been discovered and as yet not much is known about them. An axon, or "nerve fiber," is a long slender projection of a nerve cell (in this case a ganglion), or "neuron," which conducts electrical 65PDF Image | THE USE OF LIGHT AND COLOUR AS A HEALING MODALITY
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