The eyes of the crab
Carcinus
follow the movement of a horizontally rotating striped drum with a constantly increasing lag. The relative movement or slip speed is the stimulus for the eye, which with increasing stimulus increases its speed of response over a range of slip speeds from 0.001°/s to 10.0°/s. The gain in the forward control system, i. e. the ratio of eye speed to slip speed, can be as great as 15. The slowest effective slip speed is very low, about a sixth of the speed of the sun across the sky. Whether a seeing eye is allowed to move or not, it will drive the other eye if the latter sees no contrasting objects. An eye can also be driven in this way after section of its optic tract or after painting the cornea. When an eye which is free to move is exposed to a stationary striped field it suppresses the optokinetic response of the other eye, but if the eye exposed to the stationary contrasting field is clamped to the carapace, it no longer suppresses the optokinetic response of the contralateral eye; i. e. clamping the eye has here the same effect as blinding it. The control of the eye movement appears to be unaffected statically or dynamically by proprioceptors of the eye region. The initiation of the rapid flick back in the optokinetic response also takes no account of proprioceptors, but seems to occur when the efferent impulses to eye muscles reach a threshold frequency, which is only slightly modified by blinding one eye. The flick back is synchronized on the two sides. A different reflex, protective retraction, is a fast movement of the eye back into its socket, for which the minimum stimulus can be a touch to one sensory hair alone. During the rapid return phase of the optokinetic response, and during the retraction reflex, the efferent optokinetic impulses are suppressed, and, in addition, peripheral neuromuscular inhibition of tonic motor activity occurs when protective retraction over-rides the optokinetic response. On extending again after retraction, the blinded eye of a unilaterally blinded crab returns to the point in the optokinetic response where it left off. There are spontaneous eye movements which, although of less than one degree, are adequate to excite visual receptors in a stationary visual field. This is demonstrated by the decrease of the tremor when the visual field is changed from a blank one to a contrasting one. In addition, a small scanning movement of a few degrees is initiated when a contrasting object is introduced into the visual field. Therefore there is every reason to suppose that stationary contrasting objects can be perceived by spontaneous and by scanning movements. Similarities of the control system with that in mammals are the lack of a positional proprioceptive effect, coupled with the importance of the motor outflow as a controlling factor. Although there is eye tremor, as in mammals, the main control of eye movements in the crab is by the movement and not by position of a striped field relative to the eye.
Optokinetic Responses, Visual Adaptation and Multisensory Control of Eye Movements in the Spiny Lobster, Palinurus Vulgaris
