Our senses are fighting a constant battle against too much information. Because of their great sensitivity and the very broad range of intensity over which they work, sense organs and the nerves that transmit their messages to the brain have developed tricks of detection and coding, detecting changes and differences rather than the absolute value of the physical stimulus. The retina, for example, transmits little information about uniform areas of light; most messages from the eye are concerned with changes in the intensity of light at the edges of objects, and in signals that change with time. This explains why eyes are constantly in motion, why moving objects are usually easier to see than stationary ones, and why some animals are virtually blind if nothing around them moves. However, interpreting the meaning of messages about difference and change depends on memory of the preceding signals; and in other instances, senses dealing with stimuli that don’t often change must relay absolute and not just relative information. (60 minutes)