Spatial attention facilitates assembly of the briefest percepts: Electrophysiological evidence from color fusion.

When two different color stimuli are presented in rapid succession, the resulting percept is sometimes that of a mixture of both colors, due to a perceptual process called color fusion. Although color fusion might seem to occur very early in the visual pathway, and only happens across the briefest of stimulus presentation intervals (< 50 ms), the present study showed that spatial attention can alter the fusion process. In a series of experiments, spatial cues were presented that either validly indicated the location of a pair of (different) color stimuli in successive stimulus arrays, or did not, pointing toward isoluminant gray distractors in the other visual hemifield. Increased color fusion was observed for valid cues across a range of stimulus durations, at the expense of individual color reports. By contrast, perception of repeated, same-color stimulus pairs did not change, suggesting that the enhancement was specific to fusion, not color discrimination per se. Electrophysiological measures furthermore showed that the amplitude of the N1, N2pc, and P3 components of the ERP were differentially modulated during the perception of individual and fused colors, as a function of cueing and stimulus duration. Fusion itself, collapsed across cueing conditions, was reflected uniquely in N1 amplitude. Overall, the results suggest that spatial attention enhances color fusion and decreases competition between stimuli, constituting an adaptive slowdown in service of temporal integration.