Each trial in the retinotopic adaptation session started with the fixation point presented at the center of the screen. Then, the fixation point started to slowly oscillate horizontally on the screen at a constant speed of 3 deg/s. The adaptor was yoked to the position of the fixation point, at 6 degrees to the left or to the right of the fixation, above or below the horizontal meridian, thereby adapting a single retinotopic area but multiple world locations. The upper panel of
Figure 5B shows the condition in which the adaptor was presented to the left of the fixation, in the upper visual field. In addition, another adaptor was presented in the part of the visual field opposite to that of the yoked adaptor (
Figure 5B, the adaptor is shown below the fixation). This adaptor started moving in the opposite direction relative to the fixation point, at twice the speed (6 deg/s), and the direction of movement reversed on reaching the edge of the screen. Thus, this adaptor covered the same area of screen but at twice the rate of the yoked adaptor. We introduced this adaptor in order to balance attention between the two parts of the visual field and to ensure that at each tested location, the adaptor was presented for approximately the same amount of time in world-centered coordinates (i.e., equalizing the world-centered component of the adaptation), while uncoupling the relative positions of the adapters. The speed and initial direction of motion of the second lattice were chosen as a compromise between the purpose of including the second lattice and the experimental setup available. In particular, we wanted the second lattice to cover a similar amount of screen, for a similar amount of time, as the adaptor yoked to the fixation point position and, in the same time, not to adapt a single retinal location. Given the size of the screen, speed of the fixation point, and the location of the adaptor lattice yoked to the position of the fixation point (6 degrees of visual angle), if moving in the opposite direction with the same speed, the second lattice would reach the border of the screen at a different time to that of the adaptor, whose position was yoked to the position of the fixation point, resulting in the second lattice having a fixed retinal location after the reversal. Therefore, we chose the speed of the second lattice to be twice the speed of the fixation point. The second lattice started moving in the direction opposite to the initial direction of the fixation point, but note that since it was moving at twice the speed, it would reverse its direction sooner than the fixation point, and therefore, for a certain amount of time, the direction of motion of both would be the same. In the first trial of each block, adaptation lasted 60 s, during which the fixation point oscillated along the horizontal meridian (10-s top-ups). The fixation spot changed luminance 300 ms before the end of the adaptation period, to signal the change of the display. To test the degree of adaptation, the standard pair was presented 500 ms after the end of the adaptation period at the adapted retinotopic location, and the comparison pairs were presented either above or below the standard location dependent on the position of the adapter in that block, in the same hemifield (see
Figure 5E). This location of the comparison pair corresponded to a retinotopic location that was not adapted during the adaptation phase (since the position of the adaptor was yoked to the oscillating fixation point location). Note that the locations of both the standard and test pairs were equally adapted in the world-centered reference frame. The control condition involved testing at two other locations (see
Figure 5E, the two locations below the fixation point) that were equally adapted in both retinotopic and world-centered reference frames. In different blocks, the fixation point began moving either to the right or to the left, and the adaptor was presented at one of the four possible locations relative to fixation and the horizontal meridian (to the left and above or below and, similarly, to the right and above or below).