Spatially Distributed Encoding of Covert Attentional Shifts

16 17 Spatial attention modulates signal processing within visual nuclei of the thalamus but do other 18 nuclei govern the locus of attention in top-down mode? We examined fMRI data from three 19 subjects performing a task requiring covert attention to one of sixteen positions in a circular 20 array. Target position was cued after stimulus offset, requiring subjects to perform target 21 detection from iconic visual memory. We found positionally specific responses at multiple 22 thalamic sites, with individual voxels activating at more than one direction of attentional shift. 23 Voxel clusters at anatomically equivalent sites across subjects revealed a broad range of 24 directional tuning at each site, with little sign of contralateral bias. By reference to a thalamic 25 atlas, we identified the nuclear correspondence of the four most reliably activated sites across 26 subjects: mediodorsal/central-intralaminar (‘oculomotor’ thalamus), caudal 27 intralaminar/parafascicular, suprageniculate/limitans and medial pulvinar/lateral posterior. 28 Hence, the cortical network generating a top-down control signal for relocating attention acts 29 in concert with a spatially selective thalamic apparatus the set of active nuclei mirroring the 30 thalamic territory of cortical ‘eye-field’ areas, thus supporting theories which propose the 31 visuomotor origins of covert attentional selection. 32 33 34