Visualizing the perisaccadic shift of spatiotopic coordinates WAYNE

A point light source flickering on and off during a sac-cade projects a pattern onto the retina. The perceived pattern differs from the retinal pattern, with this difference reflecting the perisaccadic shift of spatiotopic coordinates. Hershberger (1987) characterized the perceptual pattern as a phantom array, fixed in space, with the sequential order of the flashes creating an impression of apparent motion within the array. Figure 1 illustrates the phantom array, as Hershberger first described it. Note that the phantom array is only about half as long as the retinal array (or, saccade) and is displaced in the direction of the saccade. Hershberger and Jordan (in press) found that theoretically naive observers see it this way as well: They tested 75 undergraduates who responded to 12 forced-choice questions regarding flashes they saw while saccading horizontally across a light-emitting diode (LED) flashing in the dark at a frequency of 200 Hz (a 1-msec flash every 5 msec); each flash had a luminance of about 50 cd/m 2. The questions were designed to determine in what direction the flashes appeared to move and/or be displaced. All but 2 of the 75 naive observers consistently saw a horizontal row of flashing dots whenever they sac-caded horizontally across the flashing LED. Furthermore, all but 5 of these 73 observers saw the individual dots as being stationary or as moving in a direction opposite to the eye movement's (this includes the phi produced by the sequential order of the flashes). Also, 63 of these 73 observers consistently saw the first flash in the array as being displaced toward the new direction of gaze. Finally, 57 of these 63 observers reported that the entire array appeared to be located to that side of the LED associated with the new direction of gaze. Irrespective of whether the minority of incongruent answers represent fundamental individual differences or merely experimental noise, it is clear that theoretical naiveté does not preclude an observer's experiencing the phantom array as Hersh-berger described it. The most remarkable feature of the phantom array is the discrete displacement of the array in the direction of the eye movement, with the first flash being displaced the most. Despite the fact that the angular displacement of the first flash is about half the size of the attendant saccade, this displacement develops fully during a single interflash interval. The size of the first flash's displacement, relative to the size …