Psychophysical evidence for spatiotopic processing in area MT in a short-term memory for motion task

The middle temporal (MT) area has long been established as a cortical area involved in the encoding of motion information and has been thought to do so in retinotopic coordinates. It was previously shown that memory for motion has a spatial component by demonstrating that subjects do significantly worse on a match-to-sample task when the stimuli to be compared were spatially separated. The distance at which performance deteriorated (the critical spatial separation) increased at increasing eccentricities, suggesting that area MT was involved in the process. In this study, we asked whether optimal performance occurred when the stimuli were in the same retinotopic or spatiotopic coordinates. We found that the performance was best when the stimuli appeared in the same location in space rather than the same retinal location, after an eye movement. We also found that the relationship between retinal eccentricity and the critical spatial separation approximated that of area MT, as found previously. We conclude that area MT plays an important role in the memory for motion process and that this is carried out in spatiotopic coordinates. This conclusion supports the hypothesis that MT processing may have a spatiotopic component.