Eye movements in Daphnia magna : regions of the eye are specialized for different behaviors

Three types of behavior of the compound eye of Daphnia magna are characterized: 'flick', a transient rotation elicited by a brief flash of light; 'fixation', a maintained eye orientation in response to a stationary light stimulus of long-duration; 'tracking', the smooth pursuit of a moving stimulus. The magnitudes of the flick and fixation responses vary with stimulus position and are generally proportional to stimulus intensity, although at high intensities there is an attenuation of both behaviors. When the stimulus is placed at a position approximately 80 degrees dorsal to the eye axis, there is no response; this area is called the null region. For stationary stimuli in other positions, the direction of the response is such as to bring the stimulus closer to the null region. During tracking, the relative positions of the eye and stimulus change; the eye velocity is approximately half that of the moving stimulus. The regions of the eye in which these behaviors may be induced are different, being largest for flick and smallest for tracking. It is proposed that flick and fixation responses are a means for rotating the eye so that the stimulus is within the area surrounding the null region which is used for tracking.