Printed in Great Britain OPTOKINETIC MEMORY IN THE CRAB, CARCINUS

For many lower animals the optokinetic response of the eye or head to a movement of the whole visual field is a useful indication that the. movement has been seen. The lower velocity limit of the optokinetic response is extraordinarily low in some animals, In the dog, ter Braak (1936) reports responses to drum movements at 6-4°/hr., which is comparable to the lower limit in the crab. The question arises as to whether a memory of successive positions is involved. It has been shown (Horridge & Sandeman, 1964) that the eye of the crab Carcinus responds to movements of a brightly illuminated drum as low as o-ooi7°/sec.; the eye then responds with a movement of about o-ooi°/sec. so that the average movement of the stimulus across the eye is a slip speed of o-ooo7°/sec. This corresponds to an angular movement of 0-17 rev./24 hr. period. At low stimulus speeds in the range o>ooo8-o-02°/sec. the percentage following of the drum by the eye can be 90-95 %, so long as no stationary objects remain in the visual field. It must be stressed however that these are average movements, for the eye is free to move and it makes continual tremor movements of an amplitude of 0-04-0-2 ° at a frequency of around i/sec