The optokinetic responses of the crab, Carcinus Maenas
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The movements of the eyecups of the common shore crab, Carcinus maenas L., that occur in response to a variety of different visual stimuli, have been studied with a view to analysing the mechanisms of eye movement control in the crustaces. In most experiments, a light flag was glued to one of the crab's eyecups. The flag was orientated so that is partially occluded a beam of light that was focused on a pair of photocells, mounted in opposition to one another. Flag movements thus caused changes in the amount of light reaching the photocells, whose outputs were amplified and displayed on a pen recorder. That small eyecup movements occur in the absence of moving stimuli has been confirmed. These movements have been classified into four categories; tremor - oscillations of peak to peak amplitude 0.01° - 0.2° and predominant frequency 1-3 o.p.s., which could be modified to a small extent by external stimuli; drift - slow wondering movements that mainly occurred when there were no contracts in the visual field; saccades - spontaneous jumps whose frequency and amplitude were extremely variable; and scanning movements of amplitude 0.1° -2.0° peak to peak, and frequency 2-3 o.p.s.; which were always associated with periods of leg waving. All other experiments were directly concerned with the optokinetic responses of Carcinus, which were usually elicited by rotating a black and white striped drum around the crab. Optokinetic nystagnus consists of two phases, a slow forward phase during which the eyes move in the direction of rotation of the stripes, and a fast return phase in which the eyes are flicked back in the opposite direction. Although the evidence is by no means conclusive, it appeared that carcinus has neither a proprioceptive nor a coulometer feedback loop in its eye movement control system. It may thus be unable to distinguish apparent motion, induced by its own eye movement, from world motion. Optokinetic responses also occurred to the movements of a pinlight in an otherwise dark visual field. When recorded in two dimensions on an X-Y plotter, the responses to the movement of a pinlight in a circle were seen to be approximate ellipses, though stepwise movements frequently occurred instead of diagonal movements. The possibility, suggested by this observation, that carcinus resolved diagonal movements into their horizontal and vertical components was confirmed by the finding that the angles of the responses to diagonal movements of the pinlight depended upon the ration of the gains (gain equals response divided by stimulus) to horizontal and vertical pinlight movements. The possibility that carcinus uses this ability to resolve the sun or moon's motion cannot be excluded. When one pinlight was switched off and a similar one was switched on nearby, the crab responded optokinetically, the amplitude of this response being proportional to the stimulus amplitude for apparent movements of up to 3° -4 °. This suggests that movement correlation takes place primarily between closely spaced ommatidia. The responses to the movement or apparent movement of two or more lights were proportional to the shift in the centre of light intensity, an indication that the spatial resolution of the eyes is poor. Optokinetic memory responses, which occur following shifts in the drum position that are not seen by the crab, were also studied. By varying the length of time that the crab viewed the stripes before they were moved, memory was shown to build-up approximately linearly, reaching a plateau representing a gain of between 6 and 30 after 40-100 seconds. The retention of memories allowed to build-up for different periods of time was also examined. There was no evidence for the existence of more than one memory store.
Thesis, PhD Doctor of Philosophy
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