The temporal dynamics of the effects in occipital cortex of visual-spatial selective attention.

The temporal dynamics of the effects of lateralized visual selective attention within the lower visual field were studied with the combined application of event-related potentials (ERPs) and positron emission tomography (15O PET). Bilateral stimuli were rapidly presented to the lower visual field while subjects either passively viewed them or covertly attended to a designated side to detect occasional targets. Lateralized attention resulted in strongly enhanced PET activity in contralateral dorsal occipital cortex, while ERPs showed an enhanced positivity (P1 effect, 80-160 ms) for all stimuli (both non-targets and targets) over contralateral occipital scalp. Dipole modeling seeded by the dorsal occipital PET foci yielded an excellent fit for the peak P1 attention effect. However, more detailed ERP modeling throughout the P1 latency window (90-160 ms) suggested a spatial-temporal movement of the attention-related enhancement that roughly paralleled the shape of the dorsal occipital PET attention-related activations-likely reflecting the sequential attention-related enhancement of early visual cortical areas. Lateralized spatial attention also resulted in a longer-latency contralateral enhanced negativity (N2 effect, 230-280 ms) with a highly similar distribution to the earlier P1 effect. Dipole modeling seeded by the same dorsal occipital PET foci also yielded an excellent fit. This pattern of results provides evidence for re-entrance of attention-enhanced activation to the same retinotopically organized region of dorsal extrastriate cortex. Finally, target stimuli in the attended location elicited an additional prolonged enhancement of the longer-latency negativity over contralateral occipital cortex. The combination of PET activation and dipole modeling suggested contribution from a ventral-occipital generator to this target-related activity.

Retinotopic organization of early visual spatial attention effects as revealed by PET and ERPs.

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.