Eye-Movement Responses to Step and Pulse-Step Stimuli*

A spot of light is presented to an observer who tracks its movement visually, doing so as quickly and accurately as possible. The positions of the eye are continuously recorded so that direction and magnitude of eye movements as a function of time can be assessed. Without warning, the target spot steps from its resting position, moving 6° horizontally to one side, followed after a time W by a 12° step in the opposite direction. The result is a pulse-step pattern of target motion with the time interval W msec defining the pulse duration. The directions of the pulse and step are always opposite but otherwise are unpredictable. Trials consisting of pulses followed by steps are intermixed randomly with a larger number of trials consisting of 6° steps alone. The experiments demonstrate that the visual system is sometimes able to cancel an eye-movement response to a pulse, on the basis of information contained in the subsequent step, to which it responds instead. As the step is delayed by progressively longer pulses, the probability increases that a response to the pulse will occur. If a response does occur in the direction of the step, it begins about 325 msec after the beginning of the step. This latency is independent of pulse time W and is about 40 msec longer than the latency of responses to steps presented alone. It is concluded that the visual system utilizes this 40 msec to operate upon a latent response to a pulse, and thereby to cancel its overt manifestation (eye movement) before initiating a response to the second, incompatible stimulus.