Figure 3.
Maps of local/global attention in comparison with maps of retinotopic eccentricity and spatial frequency peak tuning. Each panel shows flattened maps of the occipital cortex, including posterior portions of the temporal and parietal lobes as well. The right hemisphere is shown on the right side of each panel, and the left hemisphere is shown on the left of each panel. The gyral/sulcal topography in the original brain is also shown (light gray = gyri; darker gray = sulci) “beneath” the pseudocolor activity maps. (a) A scale bar for approximate cortical surface measurements (1 cm) is shown at the bottom right. (a–c) Separate activity maps from a single subject. Visual area borders for the subject shown in a–c are included in a. (a) A typical activity map of local versus global attention, produced by subtracting the average MR levels during local attention from the average MR levels during global attention. MR activity that was significantly higher during local attention is pseudocoded blue through cyan. Activity that was significantly higher during global attention is shown in red through yellow (see significance scale to the right of a: minimum = P < 10−2; maximum = P < 10−7). (b) A conventional phase-encoded map of retinotopic eccentricity. It was produced by presenting a ring of checks at systematically varied visual field eccentricities. As illustrated in the activity legend to the right of b, red pseudocolor reveals voxels that responded maximally when the stimulus was confined to foveal (central) eccentricities (center of red ≈1.7°). Blue and green indicate voxels responding maximally when the stimuli were confined to parafoveal and more peripheral eccentricities, respectively (center of blue ≈4.2°, center of green ≈11.3°). (c) A phase-encoded map of spatial frequency tuning. As illustrated in the activity legend to the right of c, red pseudocolor represents voxels that were maximally activated when the grating was of high spatial frequency (two cycles per degree), green represents voxels maximally activated when the grating was of low spatial frequency (≥0.05 cycle per degree), and blue represents voxels activated by intermediate spatial frequencies. (d) The group-averaged local/global attention maps. The averaged result is similar to the individual subject result (a), except that it is statistically more significant, indicating good across-subject reliability. For optimal comparability, the averaged activity map has been overlaid on the cortical surface from the subject shown in a–c. In addition, four numbered circles indicate the foci of maximal local/global attention activity, as taken from previous PET studies (see text).