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Colloquium on Neuroimaging of Human Brain Function
FIG. 4. Range of activity produced by stimuli of systematically varied extent in the ipsilateral visual field. Stimuli such as that in Fig. 1A were presented within a range of sector sizes (shown in Fig. 1B) in one representative ipsilateral hemisphere (subject JM). The stimulus was displaced from the vertical meridian by 40° of polar angle in A, 20° in B, 10° in C, and 5° in D (see logos). Visual cortical area borders, revealed in the same hemisphere by tests of contralateral retinotopy, are indicated for comparison. Representations of the contralateral horizontal meridian are indicated by solid lines, and representations of the vertical meridia are indicated by dashed lines. The pseudocolor activity scale bar is as described above, except that MR decreases are not shown, for simplicity. In general, cortical activity increased as the stimulus encroaches progressively closer to the vertical meridian. Within early retinotopic areas, such as the border between V1 and V2 (especially C and D), activity appeared first at the representation of the vertical meridian. Ipsilateral activity was correspondingly lacking at area borders corresponding to the contralateral horizontal meridian, such as the borders between V2/V3 and V2/VP. There were also distinct differences between areas in the degree of ipsilateral activation: across a significant range of stimulus extent (5–20° in this example), areas V3A and V4v showed more widespread ipsilateral activation than immediately adjacent areas V3 and VP. Furthermore, the activity in V3A and V4v extended well beyond the vertical meridian representations of these areas, even though these areas show clear contralateral retinotopy in other tests. The differences in ipsilateral fMRI topography between lower (e.g., V1, V2, and V3/VP) vs. presumably higher-tier (e.g., V3A and V4v) retinotopic areas is consistent with the presence of larger receptive fields in the latter. This evidence for larger receptive fields in V3A/V4v is also consistent with other human fMRI (24) and macaque electrophysiology (40).
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