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Holger Awater, Markus Lappe; Compression of visual space under steady fixation. Journal of Vision 2003;3(9):484. doi: 10.1167/3.9.484.
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© ARVO (1962-2015); The Authors (2016-present)
Under certain circumstances human perception exhibits a compression of visual space for perisaccadically flashed objects. Previously, we showed that compression depends on the presence of visual references. Furthermore we demonstrated that also the saccade target itself is mislocalized if its position is not determined by visual references. From these results we hypothesized a relative visual localization process between the saccadic target and the flashed object. In the present work we ask whether perisaccadic compression relies on the execution of the saccade or whether compression effects may also occur in the absence of a saccade.
We chose a paradigm that was identical to the saccade paradigm in which we previously obtained compression. In complete darkness, each trial started with the appearance of a small dot, that observers had to fixate during the whole trial. After a while the dot disappeared and a second dot (which in previous experiments served as the saccadic target and will therefore be named ‘target’) appeared for 50 ms 12.8 to the right of the fixation position. Subsequently a vertical bar was flashed for 8 ms at one of four possible positions. Observers had to localize, in blocked conditions, either the position of the bar or that of the target. Both condition were tested with and without visual references.
When indicating the bar position, observers showed systematic mislocalizations towards the fovea,irrespective of the target and irrespective of visual references. Perceived location of the target, however, was influenced by the position of the bar. If the target position was not indicated by visual references it appeared shifted towards the bar. When we calculated the apparent distance between target and bar we found compression that was similar in strength to that observed in saccade experiments. These results confirm our hypothesis compression of space before saccades results from a visual process of relative localization between the two objects.
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