Vision and Visual Systems

Vision

Vision is the process by which light is perceived. When the rod and cone cells of the retina are exposed to light, ranging in wavelengths from violet, about 4000 angstrom units, to nearly 7500 angstrom units, a chemical reaction takes place, causing discharge of a nervous impulse. This impulse reaches the brain, and creates in the conscious mind, the sensation of light.

Stereoscopic vision

Humans and other animals that are able to focus both eyes on a single object are capable of stereoscopic vision, which is fundamental to depth perception. The principle to stereoscopic vision can be described in terms of the vision process involved in the use of a stereoscope, which presents an image from two slightly different angles so that the eyes can merge them into a single image in three dimensions. A type of stereoscopic picture called an auto stereogram, not requiring a special viewing instrument, has been made possible by the advent of computer graphics. Two computer-generated pictures, each representing the view seen by one eye, are overlaid, as in an anaglyph. Viewed in the ordinary way the combined image looks like an abstract pattern, or else some scene unrelated to the hidden stereoscopic view. The latter is revealed when the viewer's eyes are focused on some point that is nearer or farther away than the auto stereogram.

Visual Systems

In human eyes and those of many other species, there are 2 light perceptive systems- one functioning best at low intensities and the other at high. The visual cells for the first are rods, for the second the cones. Energy falling upon an object must be absorbed in order to produce the chemical reaction that initiates the nerve impulse. A light-absorbing pigment, closely related to Vitamin A, has been found in the retina. Named 'visual purple' or 'rhodopsin', it is broken down under the influence of light into a yellowish compound (retinine & a protein). These compounds, maybe, in part, resynthesised into rhodopsin, but most of them decompose further to form Vitamin A. If one passes from a brilliantly lighted place to a dim illumination, as in entering a theatre at midday, the interior appears very dark. After some minutes, this impression passes and one sees distinctly. In this period of dark adaptation, the eye becomes almost entirely dependent on the rods for vision, since they operate best at low intensities of light. Since the rods do not distinguish colors, vision at low illumination is almost colorless (achromatic).

When the eye is suddenly exposed to bright light, vision is poor for a short period of light adaptation in which the cones become the essential visual elements. In good illumination, one can distinguish colors. Colour perception is a function of the cone-shaped visual cells.

The Physiology Of Vision