Perception of edges versus areas by the crab Carcinus.

The previous paper (Horridge, 1966) showed that a crab gives a directional eyestalk response to a striped drum which has been moved during a dark period. The crab's ability to do this shows that the former position of the stripes is remembered in some way for many minutes in the dark and is correlated with the new position on reillumination. In that paper two aspects were omitted for simplicity. It was assumed that the crab perceives the stripes much as we do as areas of black and white, and that eye tremor can be ignored. In the present account it will be shown that neither of these assumptions is justified, and that both must be modified by one new fact, that eye tremor accentuates the perception of edges.