Figure 2. Behavioral task.

(A) Stimuli comprised four categories (rows). In each category, liquid reward value (symbolized by blue dots) was proportional to either the distance between limbs or the curvature at limb junctions. These were the value-relevant cues, shown on the left. In each category, the other cue varied but was irrelevant, as shown on the right. (B) Task sequence. After the appearance of a 0.2° diameter fixation point, the monkey was required to fixate within a 1° radius window throughout the trial. After initiation of fixation, the task stages were: stimulus 1 presentation (600 ms), delay (600 ms), stimulus 2 presentation (600 ms), delay (600 ms), simultaneous offset of fixation point and onset of saccade targets. The monkey was required to saccade to a target within 1 s to receive the liquid reward amount indicated by either stimulus 1 (blue target) or stimulus 2 (yellow target). Stimuli were generated randomly but balanced for category and reward level. Targets appeared on opposite sides of the fixation point at a randomized angle. (C) Behavioral performance. Probability of choosing the reward indicated by stimulus 1 (see color scale) was averaged across 90 training sessions for the first monkey and 75 training sessions for the second monkey. Reward values are shown in arbitrary units, which equated to a variable amount across sessions, ranging from 15 to 20 μl. The range of values for stimulus 2 was actually larger (0–12 units, data not shown) to prevent early decisions based on maximum or minimum values for stimulus 1 (3–9 units). Thus, the maximum reward ranged from 0.18 to 0.24 ml. See also Figure S1.