Voluntary selection of the target for smooth eye movement in the presence of superimposed, full-field stationary and moving stimuli

Prior work has shown that smooth eye movements in the presence of both stationary and moving stimuli are determined, at least in part, by the voluntary selection of either the stationary or the moving stimulus as the target for smooth eye movements. The effectiveness of voluntary selection in eliminating the influence of the stimuli not selected (i.e. backgrounds) on smooth eye movement is not known because prior studies used targets and backgrounds with different physical characteristics. Thus, effects of voluntary selection were confounded with the relative strength of target and background as stimuli for smooth eye movements. We measured eye movements (resolution 1') of two highly-experienced eye movement subjects with a target and background with the same physical characteristics: two, identical, full-field, superimposed patterns of randomly-positioned dots (1 dot/deg2 or 8 dots/deg2). One field was stationary and the other moved at 70.2 minarc/sec. The effect of the moving background on smooth eye movements when the stationary field was the target, and the effect of the stationary background on smooth eye movements when the moving field was the target was negligible (0-4% for one subject; 0-2% for the other). The influence of the background on smooth eye movements was affected by a six-fold reduction in the intensity of either the target or background, but effects of such intensity changes were small and different for each subject. Taken together, these results show that the effectiveness of voluntary selection in eliminating the influence of background stimuli on smooth eye movements can be virtually complete. Any observed influence of the background--however small--can be attributed to voluntary factors (e.g. subjects' failure to apply sufficient effort or attention) rather than to the operation of an involuntary mechanism that automatically integrates velocity information from target and background. The attention and effort required to ensure that voluntary selection is perfect may impair the accuracy of psychophysical judgments made about the background.