Abstract
Cai & Schlag (VSS 2001) found that the location of an odd-colored flashed element appearing within a motion sequence of a different color is perceived to be shifted along the motion trajectory. To gain insight into the neural mechanisms underlying this perceptual shift, we quantified it in humans and then presented the same sequences to monkeys while recording neuronal responses in Area V4, a color-selective, retinotopically-ordered area. On each trial the flashed element occurred at one of eight locations spanning the cell's receptive field (RF) border. Each location was tested both with apparent motion sequences moving outward from near the RF center and inward from outside the RF. For 50/59 cells tested (85%) motion direction changed the number of positions where the flash elicited a response (i.e., shifted the receptive field border). Flashes appearing on the inward trajectory often elicited responses at locations where flashes in the outward trajectory failed to elicit a response (36/50 cells, 72%). The shift was significant across the population (59 cells, t-test: p<0.0001). Thus, the retinotopic locus of activity evoked by the flashed element was shifted in the direction of the motion sequence. This shift is consistent with the V4 neurons responding to the perceived, not veridical, location of the flash. The shift was apparent in the first spikes elicited by the flashed element and was similar in magnitude to the mislocalization perceived by human subjects. Motion alone was not sufficient to cause the shift: when tested with identical motion sequences composed entirely of a single color, a shift was not induced (t-test: p=0.14). Nor was motion following the flash necessary: when tested with motion sequences that terminated with the flash, the response to the flash was still shifted (t-test: p<0.0001).
Funded by NSF graduate research fellowship (KAS), R01 EY13802-01 (MF, JHR)