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Wei Song Ong, Nina Hooshvar, James Bisley; Memory for motion is optimally represented in spatiotopic coordinates. Journal of Vision 2009;9(8):566. doi: 10.1167/9.8.566.
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The direction of stimulus motion is encoded by directionally selective neurons in intermediate visual areas, such as area MT. In studying the storage of this information, Zaksas et al (2001) demonstrated a sharp drop in performance of a memory for motion task as the spatial separation of the 2 stimuli to be compared reached a critical distance. This critical separation increased with retinal eccentricity in a way consistent with the receptive field sizes of neurons in cortical areas MT, V4 and LIP. To differentiate between these areas, we asked whether this critical separation is in retinotopic or spatiotopic coordinates. We measured the ability of human subjects to discriminate the directions of two coherently moving random-dot stimuli (sample and test) separated by a 1350 ms delay. The subjects first fixated on a spot and the sample stimulus appeared in the right or left upper hemifield for 500 ms. The subjects were then required to make a rightward or leftward saccade of 10 degrees and the test stimulus would then appear in the same spatiotopic or retinotopic location as the sample. The subjects then indicated if they thought the sample and test stimuli were moving in the same direction or different directions via a button press. The sample stimulus could move in any of the 8 cardinal directions, and the test stimulus differed from the sample by 3.8 to 90 degrees. We found that the direction difference thresholds were lower when the test stimulus appeared in the same spatiotopic location as the sample stimulus compared to when it appeared in the same retinotopic location. This suggests that memory for motion is likely stored in a region that spatially updates remembered information across saccades, such as area LIP, as opposed to a retinotopic area, such as MT or V4.
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