Neuronal activity dependent on anticipated and elapsed delay in macaque prefrontal cortex, frontal and supplementary...

In macaque monkeys performing a memory-guided saccade task for a reward of variable size, neuronal activity in several areas of frontal cortex is stronger when the monkey anticipates a larger reward. This effect might depend on either the size or the value of the reward. To distinguish between these possibilities, we recorded from neurons in frontal cortex while controlling value through a manipulation of time rather than amount. A cue presented at the beginning of each trial, predicted the length of the delay during which the monkey would have to maintain fixation before performing a saccade and receiving a reward of fixed size. Predicting a short delay had effects closely similar to those of predicting a large reward: 1) monkeys were more motivated when working for a reward at short delay, 2) neurons tended to fire more strongly before a short delay, 3) individual neurons firing more strongly before a short delay tended also to fire more strongly before a large reward, and 4) the tendency to fire more strongly before a short delay was far more pronounced in premotor areas caudal to the arcuate sulcus than in association areas rostral to it. The association areas, in contrast, were marked by a tendency for neurons to fire more strongly at the end of the long delay. We conclude that predicting a short delay, like predicting a large reward, induces an enhancement of neuronal activity related to motivational modulation of the monkey's preparatory state.