Abstract
We employed steady-state visual evoked potentials (SSVEPs) to measure effects of sustained spatial attention on visual responses and their modulation by orientation-specific surround suppression. Displays contained a circular sinusoidal grating consisting of separate annulus and surround regions. Subjects continuously maintained fixation on a central square for 4.8 s while the contrast of the surround and annulus regions of the display reversed at 12.5 Hz and 16.67 Hz, respectively. In separate blocks, we manipulated spatial attention by instructing participants to detect a contrast decrement either within the target annulus or within the fixation square. Surround suppression of contrast discrimination performance was stronger when the annulus and surround had the same orientation compared to when they were orthogonally oriented. Fourier decomposition was used to separately measure response amplitudes at the surround and annulus frequencies. This analysis revealed distinct scalp topographies for annulus and surround portions of the stimuli, with maximal responses to the annulus at lateral occipital recording sites and maximal surround responses at posterior midline sites. Based on these topographies, sites of interest corresponding to annulus and surround responses were defined. We examined effects of both surround processing and spatial attention. First, orientation-specific modulation of SSVEP responses to the surround was positively correlated with behavioral orientation-specific surround suppression. This finding links neural responses to the surround with surround suppression of discriminability of the annulus. Second, sustained spatial attention exhibited different effects at annulus and surround sites. For annulus sites, attending to the annulus enhanced annulus responses. Furthermore, this enhancement was only observed when the annulus and surround shared the same orientation, corresponding to the condition in which behavioral surround suppression is strongest. For surround sites, attending to the annulus decreased surround responses. These results demonstrate attentional enhancement of responses to a task-relevant stimulus and reduction of responses to an ignored surround.