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Junji Watanabe, Atsushi Noritake, Taro Maeda, Susumu Tachi, Shin'ya Nishida; Space constancy around the time of a saccade for intransient stimuli. Journal of Vision 2004;4(8):104. doi: 10.1167/4.8.104.
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The visual system compensates for the retinal displacement caused by eye movements and realizes perceptual space constancy. It has been reported that the compensation process doesn't function perfectly around the time of a saccade: a perisaccadic flash is systematically mislocalized. However, it is debatable whether the observations made with transient flash stimuli indicate a general pattern of the perisaccadic failure of the space constancy, since we are living in such an environment as every object continues to exist before and after eye movements. Additionally, transient stimuli are likely to be mislocalized by the presence of retinal motion. Thus, also in terms of a perisaccadic flash, the localization error possibly includes some bias specific to the transient stimulus. To investigate the mechanism of space constancy in the intransient environment, we examined the time course of localization for a perisaccadic flicker with systematically varying the onset timing, the offset timing and the duration. Depending on the temporal timing of eye movements, a flickering LED was perceived either as a single dot or a dotted line called “phantom array”. The observers were asked to point to the position of the dot or the left and right ends of the dotted line. If each flash in flicker is localized independently in the same way as a single flash, the apparent position and length of the flicker should be predicted from the time course of mislocalization of a perisaccadic flash. However, the results did not support this prediction in many aspects. For instance, a dotted line (of the half length of the retinal image) was perceived during a saccade, but not before and after the saccade, although a single flash was mislocalized in all the periods. This indicates dissociation of processes on perceiving shape (‘what’) and position (‘where’). In general, our results support a hypothesis that the shape is first generated based on the retinal image, then, localized in space.
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