The influence of display characteristics on active pursuit and passively induced eye movements.

A series of experiments has been conducted on human subjects to examine the effect of the movement of small targets located in the peripheral visual field on oculomotor response. Subjects were presented with either a single centrally positioned target or a pair of targets displaced at angles of +/- 5 degrees, +/- 10 degrees and +/- 20 degrees from centre. Target movement was in the horizontal plane, the paired targets always moving in unison. The stimulus waveform consisted of either a sinusoidal or random target motion encompassing a frequency range from 0.1 to 4 Hz with an angular displacement of +/- 3.5 degrees. Subjects made two types of response. First they were instructed to follow the single target or the centre point of the paired targets. In this 'active' pursuit condition the gain of slow-phase eye velocity progressively decreased as the moving targets were moved from the central position to the most peripheral location (+/- 20 degrees). Secondly, subjects were required passively to ignore the target movement by staring blankly ahead. During this 'passive' response nystagmic eye movements were induced for which the slow-phase eye velocity also decreased with increasing target eccentricity, but the gains were always less than those induced during 'active' pursuit. The frequency characteristics of the 'passive' response were very similar to those of the 'active' response, breaking down at frequencies beyond 1 Hz. The ability to suppress the 'passive' response was also investigated by the presentation of a tachistoscopically illuminated earth-fixed target. The response was found to decline as the interval between presentations of the fixation target was decreased from 3000 ms to 100 ms. It is suggested that the 'passive' response originates from a basic velocity drive to the oculomotor system resulting from image movement across the retina. This velocity drive may be cancelled with adequate fixation but must be enhanced to accomplish desired eye velocity during active pursuit.