Behavioral training and reward scheme for monkeys.

✓-Yes, P - Probe or subthreshold stimuli✗ - No, S - Suprathreshold stimuliU - UndefinedMonkeys were trained in three phases to report their percepts unbiased by reward. The spatial offsets used depended on the training phase. Spatial offsets with magnitude more than 1.5° are marked as ‘S’ (Suprathreshold) and offsets in the range within ±1.5° are marked as ‘P’ (Probe or subthreshold stimulus). In phase-I (not depicted here), only suprathreshold stimuli were used and every correct trial was rewarded. In training phase-II, the monkeys performed the same task as in phase-I, but without immediate reward for some trials, by requiring them to complete one or two correct trials (one-trial and two-trial reward blocks respectively) to receive reward. In the two-trial reward blocks, the monkeys received reward after completing two (not necessarily consecutive) correct trials. In training phase-III, the monkeys performed the same task as in phase-II except that the suprathreshold and subthreshold stimuli were randomly interleaved in the non-rewarded trial slot of the two-trial reward blocks. We counted the monkeys' response (labeled as U) to probe stimuli as correct. However this did not give any response feedback as the animal could interpret these trials as either being part of a two-trial or a one-trial reward block depending on his percept.