Abstract
Subjects made left and right saccades between two fixation points separated by 20 deg on a front monitor. At the same time, a target moved left and right by 20 deg on a side monitor set to the right of the subject. While the subject fixated the right point on the front monitor, a target was presented on the side monitor at a location determined to be the rightmost edge of the subject's visual field. This target was extinguished and the subject made a 20 deg saccade to the left point on the front monitor. The target was then presented 20 deg to the left of its first location on the side monitor but, because both eyes and target shifted by 20 deg, there was little or no shift between the target's two positions on the retina. In this condition, subjects reported leftward motion corresponding to the spatiotopic displacement (20 deg) not the retinotopic displacement (0%). Wurtz (2008) has described how spatiotopic responses may be simulated by purely retinotopic representations in saccade related cortical areas (LIP, FEF). Specifically, the saccade step can be subtracted from the target's current location to predict the retinotopic location it will have once the saccade lands. Using this efference copy to remap, the target's activity maintains a pointer to the target's location in space despite the image shift on the retina. In our stimuli, any mismatch between the remapped and actual target location is seen as apparent motion (see also Cavanagh & Szinte, VSS 2009). If this remapping explanation of spatiotopic apparent motion is correct, our results here require that a target's representation may even be remapped outside the visual field (by about 20 deg in our stimulus). This suggests that some retinotopic cortices may register locations that fall outside the limits of the retina.
This research was supported by a Chaire d'Excellence grant to PC.