Pre-saccade target color influences the perception of its post-saccade counterpart
Ziad M. Hafed & James J. Clark McGill University, Canada, McGill University, Canada
When a saccade is generated towards an object in space, the representation of this object's spatial location in the brain is updated in anticipation of saccade end. We investigated whether a similar updating occurs for the object's features as well. We designed a psychophysical task in which subjects looked at a central square that disappeared 200 msec after trial onset. After a gap of 200 msec, a peripheral square appeared 17 degrees to the right or left of fixation. Subjects were instructed to saccade to this square and report whether it had the same color as the central square or not. The peripheral square was extinguished 50-150 msec after saccade detection, which happened midway through saccades. A saccade-contingent change in the color of the peripheral square occurred in approximately 50% of the trials. In particular, we had four possible color variations, described by the acronyms: XXX, XYY, XYX, and XXY. In XYX trials, the central square had a color X (first letter in the acronym), the pre-saccade color of the peripheral square was Y (second letter), and the post-saccade color of the peripheral square was X (third letter). The remaining acronyms described square colors in a similar fashion. X was red, yellow, green, or blue, and for each X, the color Y was chosen such that subjects detected a color difference in approximately 70% of all XYY trials. We found that there were significantly more "SAME" responses in XXY trials than in XYY trials, and the effect was strongest approximately 50 msec after saccade end, after which it decayed. The effect was much weaker in XYX trials (subject performance was similar to that in XXX trials) perhaps because of priming of the color X as a result of task requirements. We hypothesize that saccade target properties are enhanced in anticipation of the retinal consequences of a saccadic eye movement and that this effect mirrors dynamic remapping of spatial location in areas such as the lateral intraparietal area (LIP).