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Karl Gegenfurtner, Alexander Schütz, Max Schneider; Saccadic gain adaptation follows perceived position. Journal of Vision 2008;8(6):917. doi: https://doi.org/10.1167/8.6.917.
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The saccadic system maintains high accuracy by adaptation. In the laboratory, the amplitudes of saccades can be adapted by changing the position of the saccadic target during the saccade, using the double-step paradigm. Previous studies have shown that saccadic adaptation is mainly driven by the retinal position error and not by corrective saccades. Here we investigate if the physical position or the perceived position influences adaptation.
It is known that the position of a drifting sine-wave grating within a stationary Gaussian envelope is misperceived in the direction of motion. We made use of this visual illusion and a modified double-step paradigm to manipulate the perceived position of a saccade target without affecting its physical position. In each trial, subjects initially fixated the center of a stationary Gabor patch. The Gabor was composed by a 1 cpd, vertically oriented sine wave grating and a Gaussian envelope with a standard deviation of 0.5 deg. After a randomized delay, the Gabor patch stepped 5 deg rightwards. At the same time the sinusoid began to drift rightwards at a rate of 4 Hz. As soon as the subjects initiated a saccade to the new position of the Gabor, the sinusoid changed its motion direction and began to drift leftwards. This procedure corresponds to a backward step in a double-step paradigm, but influences solely the perceived position.
The results show a significant decrease of saccadic gain. This indicates that not only physical position errors but also perceived position errors can trigger saccadic adaptation. Furthermore it emphasizes that the perceptual illusion of misperceived position does affect the motor system.
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