Evolution of Organic Vision

It seems that vision at distance is useful only in conjunction with a brain capable of planing some future course of action

Only two basic types of eyes have come in widespread use in all the varieties of life forms as we know them:

• The single lens, camera like eye (mollusks and chordates)
• The multi lens, compound eye (arthropods)

These eyes differ mainly in resolution, sensitivity and geometric fidelity

The single lens eye can form a sharp image of objects located almost anywhere in its field of view, this implies sophisticated mechanisms to:

1. identify something of interest in the prefocused image
2. move or distort the lens to sharpen the boundary between the object of interest and the background

To take advantage of the sharp image it is needed:

• A high resolution sensor (retina)
• A computing capacity adequate to deal with the huge volume of data and make "real time" decisions

In addition to shape information, color is also an important attribute for identifying or classifying objects

A retinal cell sensitive to energy corresponding to a particular color must ignore energy associated to other colors; this implies a loss of sensitivity in dim light conditions, a price too high to many organisms

Many animals are color blind and of the mammals only man and some primates can see color

In the human eye, there are two distinct visual systems (based on two types of retinal cells)

1. scotopic vision  not color sensitive, more sensitive to light and mainly peripheral (rods)
2. photopic vision  color sensitive, less sensitive to light and mainly central (cones)

It appears that the main functions of these systems are:

1. detect anomalies (e.g. movement) in the visual field (scotopic vision)
2. perform a detailed analysis of the region of interest (photopic vision)

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Last modified on April 05, 1999, G. Scott Owen, owen@siggraph.org