Global visual processing for saccadic eye movements

Four experiments are reported in which saccadic eye movements are examined when the eye moves to targets in peripheral vision which consist of two discrete stimuli. It is found that under a variety of conditions, the saccade amplitude is such that the saccade lands at an intermediate position between the stimuli. This result has been termed the global effect and is interpreted as an influence of the global target configuration on the saccade amplitude. It is suggested that this phenomenon may be explicable in terms of activity in an ensemble of cells with large receptive fields. The experiments demonstrate the global effect in the situations of rapid automatic tracking, scanning for target detail and comparison of target configurations. The effect depends in a systematic quantitative manner on the properties of the visual stimuli. This may be loosely described by saying the saccade is directed to the centre of gravity of the target configuration. The saccades are however in general directed closer to the near target than predicted by the geometric centre of gravity. Although the effect appears in a similar form in all the conditions tested, minor differences do occur. It is also shown that the effect shows a dependence on the latency of the saccade, being most pronounced for saccades with short latencies. It is suggested that this may be a consequence of the dynamics of visual information processing.