Fig. 1.

Covert attention-mapping task used to generate maps of attention-related activity. Each trial began with an electronically synthesized spoken cue that corresponded to one of the segments in the annulus. After a variable delay period, a grating was presented with 50% probability, independently in each segment. An auditory click then signaled subjects to respond with a button press indicating whether a target was present or absent in the cued segment. After the response, the next trial began with another verbal cue that directed attention to one of the adjacent segments, either ascending in number (clockwise scans) or descending (counterclockwise scans). Total trial duration was 3 s. Subjects were instructed to maintain fixation throughout each scan and responded only to the cued segment on each trial. Only the cued segment varied systematically over time. Although there were visual stimuli presented on every trial, there was no systematic pattern of visual stimulation.