Perception of movement by the crab

Abstract

PERCEPTION of movement describes, in essence, a relation which is derived from two successive intensity changes at nearby points in the visual field. When a contrasting edge moves, a simple means of determining the movement would be to remember the old position and compare it with the new one in a system containing an absolute spatial representation of the retina. The following experiments on the crab Carcinus suggest that, in this animal, this may well be an important part of the mechanism for the perception of slow movement. The eyes of a crab are on stalks and normally they will follow the movement of a striped drum placed around the animal. Over a range of drum speeds the eye moves more slowly than the drum, leaving a residual slip speed which maintains the stimulus for the eye movement1. The percentage following, defined as the ratio of eye speed to drum speed multiplied by a hundred, varies with the speed of movement and is low for very slow or very fast shifts of the drum. With the crab out of water, the eye is capable of following drum movements down to about 0·002 degrees/sec, that is, several times slower than the hour hand of a clock, which is 0·0083° /sec.