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Review
, 95 (3), 781-7

Event-related Brain Potentials in the Study of Visual Selective Attention

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Review

Event-related Brain Potentials in the Study of Visual Selective Attention

S A Hillyard et al. Proc Natl Acad Sci U S A.

Abstract

Event-related brain potentials (ERPs) provide high-resolution measures of the time course of neuronal activity patterns associated with perceptual and cognitive processes. New techniques for ERP source analysis and comparisons with data from blood-flow neuroimaging studies enable improved localization of cortical activity during visual selective attention. ERP modulations during spatial attention point toward a mechanism of gain control over information flow in extrastriate visual cortical pathways, starting about 80 ms after stimulus onset. Paying attention to nonspatial features such as color, motion, or shape is manifested by qualitatively different ERP patterns in multiple cortical areas that begin with latencies of 100-150 ms. The processing of nonspatial features seems to be contingent upon the prior selection of location, consistent with early selection theories of attention and with the hypothesis that spatial attention is "special."

Figures

Figure 1
Figure 1
Grand average visual ERPs over 17 subjects recorded from four scalp sites in response to small circular checkerboard stimuli in a spatial attention task. Stimuli were flashed in a rapid, randomized sequence to the left and right visual fields while subjects attended to one visual field at a time. ERPs shown are in response to left field flashes, with waveforms superimposed for attend-left (solid lines) and attend-right (dotted lines) conditions. Note that attending to the stimulus location produces an increased amplitude of the P1 components (80–130 ms) over the contra- and ipsilateral occipital scalp, as well as of multiple N1 components (120–200 ms) over frontal (front), parietal (par), and occipital (occ) scalp areas. In contrast, the earlier C1 component (50–90 ms), which was localized to primary visual cortex, did not change as a function of attention. Abscissa, time base in milliseconds. Reproduced with permission from Clark and Hillyard (32) (Copyright 1996, by MIT Press).
Figure 2
Figure 2
(A) ERP waveforms and associated scalp current distributions of the P1 component (80–130 ms) in a spatial attention task (7) in which subjects attended to either the right or left halves of flashed symbol arrays (Top). Spatial attention increased the P1 amplitude over the occipital scalp contralateral to the attended hemifield in relation to a neutral (passive) condition. (B) Comparison of PET activation foci during the same task (in a separate session) with the positions of model dipoles calculated (using BESA) to best fit the P1 attention effect. To eliminate any effects of general arousal, difference images were formed by subtracting the attend-right from the attend-left condition. The dipoles in the right and left hemispheres have opposite polarities due to their being calculated from the subtracted (attend-left minus attend-right) scalp distributions. (C) Anatomical localizations of the best-fit P1 dipole and center of PET activation (red circle) mapped onto brain sections from the Talairach and Tournoux (44) atlas. For simplicity, only right hemisphere data are shown. Dashed circle surrounding dipole indicates the zone where changes in dipole position had minimal (<2%) effects on residual model variance. The center of PET activation in the posterior fusiform gyrus is included within the range of error of the P1 dipole.
Figure 3
Figure 3
(A) Grand average ERP difference waves in a color task (43) in which red and blue flashed checkerboards were presented in a rapid, randomized sequence at the center of the screen. Stimulus durations were 100 ms, and stimulus onset asynchronies ranged from 150 to 450 ms. Either the red or blue checks were attended on separate runs. Difference waves were formed by subtracting ERPs to the unattended color from those to the attended color, collapsed over red and blue stimuli. Attention-related components include a posterior selection negativity (SN), an anterior selection positivity (SP), and an early positive difference (PD130). (B) Scalp voltage distributions of attention-related ERP difference components at different latency ranges.
Figure 4
Figure 4
(A) Set of dipoles (Right) and source waveforms (Left) in the best-fit BESA model of the attention-related ERP difference components shown in Fig. 3. Dipolar sources were constrained to be mirror-symmetrical in location across the hemispheres (left hemisphere, solid line; right hemisphere, dashed line). Note that dipoles 1 and 2 provided a best fit to the PD130 component, with activity beginning at 100 ms. The principal SN dipoles (3 and 4) and source waveforms are shown in red. The remaining dipoles account for subsequent, overlapping phases of the SN–SP complex, with onset times as indicated for each dipole pair. (B) Examples of how calculated dipole locations were coregistered with anatomical MRI sections. In this instance, dipoles 3 and 4 were localized to the fusiform gyrus of the occipital cortex. (C) Projections of mean dipole positions in the coordinate system of the Talairach and Tournoux (44) atlas, superimposed upon coordinates of PET or fMRI activations in previous studies of color-selective processing [solid squares, Corbetta et al. (17); outlined squares, Clark et al. (56); solid circles, Gulyas et al. (57); solid triangles, Martin et al. (58); outlined circles, Zeki et al. (59); outlined triangles, Sakai et al. (60)]. Activations shown in green lie close to dipoles 1 and 2; in red, to dipoles 3 and 4; in orange, to dipoles 5 and 6; and, in blue, to dipoles 7 and 8.
Figure 5
Figure 5
(A) Sample stimuli from study by Anllo-Vento and Hillyard (31). Stimuli were presented to the left and the right visual field (LVF and RVF, respectively) in random order. Each stimulus consisted of a pair of briefly flashed adjacent squares separated by a short time interval (50 or 150 ms), which was perceived as a square moving in the direction of the arrow. Infrequent target pairs (150-ms separation) seem to move more slowly than standard pairs (50-ms separation). Stimulus color (red, blue), field of presentation (left, right) and movement direction (vertical, horizontal) were all randomized. (B) Subtracted ERP difference waves reflecting the hierarchical selection of location, feature, and target under attend-color (Left) and attend-motion (Right) conditions. Selection of the relevant visual field location (Top) was reflected in the difference wave formed by subtracting the ERP to the unattended field from the ERP to the attended field; difference ERPs shown are from contralateral occipito-temporal sites and show attentional modulation of the P1 and N1 components starting at 80 ms. Selection by feature (Middle) was indexed by the SN component beginning at about 150 ms and was seen in the difference waves formed by subtracting the ERP to the unattended feature value from the ERP to the attended feature value for stimuli at the attended location (thick line), but not at the unattended location (thin line). Selection of targets (movement speed) was reflected in N2 and LPC components beginning at 250–300 ms and was apparent in the difference waves formed by subtracting the ERPs to the nontarget (fast-moving) stimuli from the target (slow-moving) stimuli presented at the attended location and having the attended feature value. Mean motor reaction time (RT) is shown on time base for each condition.

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