Any understanding of the function of the human eye serves as an insight into how machine vision might be solved. Indeed, it was some of the early work by Hubel and Wieselon the receptive fields in the retina that has led to the fundamental operation of spatial filtering that nowadays dominates so much of early image processing. There are many good references to the function of the eye, although Frisby gives an excellent overview with a computational flavour. The eye is considered by most neuroscientists as actually part of the brain. It consists of a small spherical globe of about 2cm in diameter, which is free to rotate under the control of 6 extrinsic muscles. Light enters the eye through the transparent cornea, passes through the aqueous humor, the lens, and the vitreous humor, where it finally forms an image on the retina. It is the muscular adjustment of the lens, known as accommodation, that focuses the image directly on the retina. If this adjustment is not correctly accomplished, the viewer suffers from either nearsightedness or farsightedness. Both conditions are easily corrected with optical lenses. The retina itself is a complex tiling of photoreceptors. These photoreceptors are known as rods and cones. When these photoreceptors are stimulated by light, they produce electrical signals that are transmitted to the brain via the optic nerve. The location of the optic nerve on the retina obviously prohibits the existence of photoreceptors at this point. This point is known as the blind spot and any light that falls upon it is not perceived by the viewer. Most people are unaware of their blind spot, although it is easy to demonstrate that it exists. And its existence has been known about for many years as it is reputed that the executioners in France during the revolution used to place their victim so that his or her head fell onto the blind spot, thus illiciting a pre-guillotine perception of the poor character without a head. The rods and cones do not have a continuous physical link to the optic nerve fibres. Rather, they communicate through three distinct layers of cells, via junctions known as synapses. These layers of cells connect the rods and cones to the ganglion cells, which respond to the photostimulus according to a certain receptive field. We can see from Figure 2 that the rods and cones are at the back of the retina. Thus the light passes through the various cell layers to these receptive fields, and is then transmitted via various synaptic junctions back towards the optic nerve fibre.
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