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Cerebral blood flow PET scans and high-density event-related potentials (ERPs) were recorded (separate sessions) while subjects viewed rapidly-presented, lower-visual-field, bilateral stimuli. Active attention to a designated side of the stimuli (relative to passive-viewing conditions) resulted in an enhanced ERP positivity (P1 effect) from 80-150 msec over occipital scalp areas contralateral to the direction of attention. In PET scans, active attention vs. passive showed strong activation in the contralateral dorsal occipital cortex, thus following the retinotopic organization of the early extrastriate visual sensory areas, with some weaker activation in the contralateral fusiform. Dipole modeling seeded by the dorsal occipital PET foci yielded an excellent fit for the P1 attention effect. In contrast, dipoles constrained to the fusiform foci fit the P1 effect poorly, and, when the location constraints were released, moved upward to the dorsal occipital locations during iterative dipole fitting. These results argue that the early ERP P1 attention effects for lower-visual-field stimuli arise mainly from these dorsal occipital areas and thus also follow the retinotopic organization of the visual sensory input pathways. These combined PET/ERP data therefore provide strong evidence that sustained visual spatial attention results in a preset, top-down biasing of the early sensory input channels in a retinotopically organized way.
Woldorff et al. (Wed,) studied this question.