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Convergent flash localization near saccades without equivalent 'compression' of perceptual separation

Visual space is sometimes said to be “compressed” before saccadic eye movements. The most central evidence for this hypothesis is a converging pattern of localization errors on single flashes presented close to saccade time under certain conditions. An intuitive version of the compression hypothesis predicts that the reported distance between simultaneous, spatially separated presaccadic flashes should contract in the same way as their individual locations. In our experiment we tested this prediction by having subjects perform one of two tasks on stimuli made up of two bars simultaneously flashed near saccade time: either localizing one of the bars, or judging the separation between the two. Localization judgments showed the previously observed converging pattern over the 50-100 ms before saccades. Contractions in perceived separation between the two bars were not accurately predicted by this pattern: they occurred mainly during saccades, and were much weaker than convergence in localization. We conclude that different forms of spatial information about flashed stimuli can be differentially modulated before, during and after saccades. Structural alterations in the perceptual field around saccades may explain these different effects, but alternative hypotheses based on decision-making under uncertainty and on the influence of other perisaccadic mechanisms are also consistent with this and other evidence.



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