'Saccadic suppression'- no need for an active extra-retinal mechanism.

In their recent review article on intra-saccadic visual perception, Ross et al. 1 concluded that motion of the world during saccades is not perceived because motion sensitivity is selectively reduced by an extra-retinal signal – although they concede that visual masking might also play an important role. However, we believe that extant data favour a more parsimonious interpretation, namely that the phenomenon called 'saccadic suppression' 2 has no functional role and is not the result of an extra-retinal signal. Instead, we propose a new interpretation based only on visual information processing. The first general reason to doubt that the decrease in contrast sensitivity observed during saccades 2 has a function is its lack of effectiveness. Three aspects of experimental data are worth considering. First, it has long been recognized that the threshold elevations reported in the literature are too small to account for the lack of motion perception when the scene contains high contrasts 3. Second, 'saccadic suppression' does not seem very useful when considering its temporal course: it is maximal at the beginning of the saccade and becomes less-marked as the saccade proceeds 1. It seems unlikely that this interesting result is the consequence of a mechanism that prevents us from perceiving intra-saccadic motion. Ideally, we would expect such a mechanism to be efficient during the whole course of the saccade, especially for long amplitude saccades, whose durations outlast the reported duration of saccadic suppression. At least, if the postulated mechanism operates in a gradual way, it should be most efficient when retinal velocity is highest, that is, near the saccadic peak velocity rather than near the beginning of the saccade. Third, the most convincing evidence that 'saccadic suppression' does not have the function postulated by Ross et al. is that intra-saccadic motion can be easily experienced when visual factors are optimized. We recently showed that saccades made in the direction of rapidly moving gratings produced a vivid intra-saccadic motion percept 4. This occurred when the retinal speed of the grating around the peak saccadic velocity was optimal for motion detection. Ross et al. 1 note that our study 'highlights the important point that centrally driven saccadic suppression attenuates visual motion but does not eliminate it'. If we all agree that 'saccadic suppression' is so inefficient, then regarding this phenomenon as a functional mechanism seems a highly controversial hypothesis. Finally, if suppressing the contrast of intra-saccadic images was really …