The intraparietal sulcus (IPS) has been implicated in a broad range of perceptual, cognitive and motor functions. Within IPS, at least 7 visuotopically-organized areas have been described and characterized. However, it is unclear how attention affects the spatial profile of responses in these areas. We therefore used fMRI and population receptive field (pRF) mapping to investigate the relationship between spatial attention and topographic organization in parietal cortex. Ten subjects (4 left-handed) performed 6 runs each of an attend-to-fixation and an attend-to-stimulus condition. In the attend-to-fixation condition, subjects detected a luminance change at fixation while a bar containing a field of coherently moving dots traversed the visual field. In the attend-to-stimulus condition, subjects covertly attended the bar and detected a change in direction of the coherent motion within the bar. Eight stimulus blocks and 4 fixation blocks per run were used to determine the center and size of the pRF for each voxel, with the bar traversing the visual field in a different direction for each stimulus block.
To estimate the degree of lateralization for each pRF, we computed a lateralization index based on the pRF’s preferred location and size in each condition. Visuotopic parietal cortex demonstrated a hemisphere-dependent decrease in lateralization in the attend-to-stimulus condition relative to the attend-to-fixation condition. The decrease in lateralization was a result of increased pRF size in the hemisphere ipsilateral to the subjects’ dominant hand (right hemisphere for right-handed subjects and in the left hemisphere for left-handed subjects). While attending to the stimulus also increased pRF size in the hemisphere contralateral to the subjects’ dominant hand, representations in the contralateral hemifield shifted towards the periphery, resulting in a preservation of lateralization. These results demonstrate that spatial attention shifts visual field representations in visuotopic parietal cortex in a hemisphere- and handedness-dependent manner.
Meeting abstract presented at VSS 2012