Abstract
Visual attention is a function that selectively picks up particular information to process from large amounts of visual information projected on the retina. We studied the spatial extent of visual attention by using a component of visual evoked potential called SSVEP (Steady-State Visual Evoked Potential). SSVEP is a sinusoidal modulation in the evoked potential induced by continuously flickering stimulus, which has the same temporal frequency as that of the stimulus. It has been reported that attending to one of the flickering stimuli modulates amplitude of the SSVEP of corresponding temporal frequency. We measured changes in the SSVEP amplitude at various distances from the focus of attention. We used eight stimuli arranged equidistantly along a circle (diameter = 10.0 deg), centered at the fixation point. The stimuli flickered at different temporal frequencies, and changes in the amplitude of the SSVEP were measured. Subjects conducted a detection task at one of the eight stimulus locations while directing attention to a direction designated by a cue. We found clear peaks of SSVEPs corresponding to the temporal frequency of each stimulus. We also found modulations of SSVEP amplitudes by attention. The SSVEP amplitude declined gradually with the distance from the attention focus. These SSVEP measurements succeed in measuring the spatial extent of visual attention across the visual field with an objective measure.