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Abstract

The yaw movements of both distal eyestalks of the shore crab Carcinus maenas in response to a sinusoidally oscillated striped pattern were recorded simultaneously. The control of eye movements under various experimental conditions is interpreted on the basis of a linear model of the optokinetic system (Fig. 1). The dynamics of the open-loop response, combined with the results of other authors, lead to a description of the motion-detecting mechanism of the crab (Fig. 9a, c): The crab processes movement in three velocity tuned channels in parallel. Each channel can be described by a correlation-model with adequate time constants. The channel tuned to rapid movements habituates very fast. The long time constants of the channel tuned to very slow movements constitute the “optokinetic memory”. The dynamical properties of eye coupling can be described by a parallel shunted lowpass-filter of the order 0.5 (Fig. 9b, Table 1b). With decreasing illumination the motion-detecting mechanism remains unchanged. The overall gain, however, decreases and the delays increase (Table 1a). Nonlinear properties of the system become apparent when it is stimulated with large amplitudes. The dynamics and the design of the optokinetic system of the crab are discussed as a compromise between the needs of optimal information exploitation and prevention of instabilities. The results are compared with known electrophysiological data.