Abstract
Visual objects presented briefly at the time of saccade onset appear compressed toward the saccade target. Compression strength depends on the presentation of a visual saccade target signal and is strongly reduced during the second saccade of a double-step saccade sequence (Zimmermann et al., 2014b). Here, I tested whether perisaccadic compression is linked to saccade planning by contrasting two double-step paradigms. In the same-direction double-step paradigm, subjects were required to perform two rightward 10° saccades successively. At various times around execution of the saccade sequence a probe dot was briefly flashed. Subjects had to localize the position of the probe dot after they had completed both saccades. I found compression of visual space only at the time of the first but not at the time of the second saccade. In the reverse-direction paradigm, subjects performed first a rightward 10° saccade followed by a leftward 10° saccade back to initial fixation. In this paradigm compression was found in similar magnitude during both saccades. Analysis of the saccade parameters did not reveal indications of saccade sequence preplanning in this paradigm. I therefore conclude that saccade planning, rather than saccade execution factors, is involved in perisaccadic compression.
Highlights
Saccade eye movements displace the eyes with high-speed motion around three times per second, implying the need of a counteracting mechanism which stabilizes the perception of visual space
When saccade targets are presented long and the sensorimotor system has time to encode them in spatiotopic coordinates (Sharika et al, 2014; Zimmermann et al, 2014d), the visual saccade target signal which drives perisaccadic compression is available in both saccades again
Previous studies have found that the interval between end of the first and start of the second saccade in the double-step paradigm can be extremely short and suggested that the second saccade must therefor be preplanned before execution of the first (Becker and Jüergens, 1979)
Summary
Saccade eye movements displace the eyes with high-speed motion around three times per second, implying the need of a counteracting mechanism which stabilizes the perception of visual space. Perisaccadic compression has been tested in a double-step saccade paradigm (Lavergne et al, 2012; Zimmermann et al, 2014b) In this paradigm—which had been introduced by Westheimer (1954) and Hallett and Lightstone (1976)—two saccade targets are shown, to which subjects saccade sequentially. Both targets are switched off before the first saccade of the sequence is initiated so that the second saccade is guided by the memorized position of the targets (while the first can be programmed as a simple sensory-to-motor transformation). When saccade targets are presented long and the sensorimotor system has time to encode them in spatiotopic coordinates (Sharika et al, 2014; Zimmermann et al, 2014d), the visual saccade target signal which drives perisaccadic compression is available in both saccades again
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