Properties of the optokinetic motor fibres in the rock lobster: build-up, flipback, afterdischarge and memory, shown by their firing patterns.

Motor fibres which cause eye movement in the horizontal plane in response to optokinetic stimuli have been demonstrated in the crab, Carcinus (Burrows & Horridge, 1968; Wiersma & Fiore, 1971 a, b) and in the crayfish, Procambarus (Wiersma & Oberjat, 1968). Such optokinetic movements are limited to an angle of up to about 15°. Periodically the eyes flick back toward their starting point and resume their slow motion. Motor fibres responsible for horizontal eye movements which have input from the statocysts, either solely or in combination with visual input, are known only in Carcinus (Wiersma & Fiore, 1971 b). Here, one large phasic fibre shows only statocyst input in that it responds to rotation of the animal in the horizontal plane in one direction, and to the stopping of rotation in the other, in either light or darkness. A second fibre responds similarly to rotation in the dark, but with a lower threshold, as well as to moving objects travelling in the other direction over the eye's visual field, whereas the third responds solely to visual input. In the crayfish rotation in the dark is without influence on any of its optokinetic fibres. This study on analogous motor fibres in the rock lobster shows that, as in the crayfish, these units have no statocyst input. However, in addition to visual influences, antenna! movement has a weak effect on the fibres. The possible significance of the latter input in the antenna! 'pointing' reactions of the lobster will be discussed. In addition, neurophysiological events in these units will be described which appear to underlie an 'optokinetic memory' similar to that reported in the crab, as shown by eye-position measurements, by Horridge (1966a, b; 1968). The presence of such a memory is demonstrated in the lobster by the fact that about the same firing rate caused by standing contrasting stripes in the visual field was resumed after a short period of darkness and, even more compelling, adjusted to the new situation when the stripes had been moved clockwise or anticlockwise by a few degrees. From the behaviour of the lobster motor neurones it is obvious that the strength of this memory is very variable from animal to animal.