FIGURE 4. Uncertainty effects persist with individual visual stimuli.

a, Comparison of the response of a single neuron for trials in which the target selected was always the same one of the eight possible targets (blocked trials) or varied among the eight possible targets (mixed target trials). Each spike density function is an average of 30 trials. The alignment is the same as in Fig. 3 . In this task, the selected target and the fixation point offset occurred simultaneously. The neuron in this example had very little visual activity in the eight-target condition. b, Same as in a except this neuron had a more prominent visual response. Each spike density function is an average of 50 trials. c, Mean per cent difference (mean response blocked - mean response mixed/mean response mixed 100) in activity of 32 neurons from two monkeys in the blocked and mixed target trials. Bars are 1 standard error of the mean. The intervals over which we quantified were the same as in Fig. 3 . Because the target dimming and the fixation point turning off occurred simultaneously in this task the two periods of selection largely overlap. Note also that the differences in the neural activity during the different phases of uncertainty were variable across our sample of neurons (compare a and b), resulting in a large range of difference values . E is a schematic of eye position.