Figure 1. The 2AFC and 4AFC contrast discrimination tasks. a, In the 2AFC task, subjects determined whether the inner or the outer segment of the target stimulus contained a contrast increment. b, In the 4AFC task, subjects determined which of the 4 segments of the target stimulus contained a contrast increment. Across these two choice tasks, each trial started with either a focused or a divided attention cue. The focused attention cue pointed directly to the target stimulus, whereas the divided attention cue pointed to all stimuli (1 target and 3 nontarget stimuli). Feedback, which indicated the exact position of the contrast increment, was presented in blue for correct responses and in red for incorrect responses. The pedestal contrast of target and nontarget stimuli was pseudo-randomly and independently chosen from 0%, 3.75%, 7.5%, 15%, 30%, and 60% Michelson contrasts. Percentage values indicate pedestal contrast values of individual target and nontarget stimuli in example trials. Task difficulty was equated at ∼76% across choice tasks, attention conditions, and contrast levels. To simultaneously measure SSVEPs evoked by individual stimuli, the visual stimuli on the upper left, upper right, lower left, and lower right quadrants were flickered at 30, 17.1, 20, and 24 Hz, respectively.