Varying the sign and gain of optomotor feedback provides insights into mechanisms of course control in walking land crabs, Cardisoma guanhumi

Abstract

This study used a walking compensator consisting of a hollow styrofoam ball supported on a cushion of air. The crab, held above the ball, is unable to rotate or translate, but instead moves the ball under its feet. Visual stimuli, mimicking rotational optic flow, are generated by computers and projected onto two screens that cover 240° of the crab's visual field. Optical computer mice monitor movements of the ball about all three axes, and the signals from ball rotation about a vertical axis are fed back to the computers generating visual stimuli. In this way, the visual consequences of turns can be fed back to the visual system, with experimenter control of gain and sign. Eye movements are recorded using a capacitative position-sensing device. This apparatus was used to analyse the control systems that govern both optomotor body turns, used in course control, and compensatory eye movements. Results demonstrate that the optomotor system has a high gain and long time constant. Thus, responses to sinusoidal visual inputs with positive rather than negative feedback do not, as might be expected, simply generate body turns in the opposite direction to the visual input. Instead, they are dominated by the direction of turning that the crab initially makes, so that the turning movement is unidirectional and no longer follows the sinusoidal component of the visual input. Modelling such responses using Simulink (a Matlab package) allows us to test different models of the control system