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Jonas Knöll, Frank Bremmer; Spatial topography of saccade induced chronostasis. Journal of Vision 2011;11(11):1223. doi: 10.1167/11.11.1223.
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Saccade induced chronostasis (“The stopped clock illusion”) is defined as the subjective lengthening of the duration of a stimulus. Chronostasis has typically been described at the saccade endpoint. We recently showed that chronostasis can also be found at the starting point of a saccade (Knöll & Bremmer, VSS 2009). This led us to suggest that chronostasis occurs globally, but may differ in its strength and time course across space. In our current experiment, we hence measured the spatial topography of saccade-induced chronostasis. To this end, we measured temporal perception for stimuli shown perisaccadically either at the start, midpoint or end of the saccade trajectory. Human observers performed a saccade task with stimuli displayed on a CRT screen at 57 cm distance. Eye movements were recorded at 500 Hz with an infrared eye tracker (Eye-Link 2, SR-Research). A white square (40 cdm−2) was shown constantly for the duration of each trial on a gray background (20 cdm−2) at either of three positions: −10° (initial fixation), 0° (saccadic midway) or 10° (saccade target). 500 to 1000 ms after trial onset, subjects were cued to perform a saccade. At ±200 ms around the expected onset of the saccade the white stimulus turned dark (10 cdm−2) either for a variable duration or 500 ms. After an ISI of 750 ms an otherwise identical comparison stimulus was shown for 500 ms or a variable duration, respectively. Points of subjective equality were obtained at all three locations by asking subjects to indicate whether the first stimulus lasted longer than the second. We confirm previous findings of chronostasis at the start and end of the saccade. In addition, we find clear evidence of chronostasis at the midpoint of the saccade trajectory. Hence, chronostasis like other perisaccadic perceptual phenomena (saccadic suppression, spatial perception) occurs globally. The neural basis of this perceptual effect remains to be determined.
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