Hierarchical coding for sequential task events in the monkey prefrontal cortex.

The frontal lobes play a key role in sequential organization of behavior. Little is known, however, of the way frontal neurons code successive phases of a structured task plan. Using correlational analysis, we asked how a population of frontal cells represents the multiple events of a complex sequential task. Monkeys performed a conventional cue-target association task, with distinct cue, delay, and target phases. Across the population of recorded cells, we examined patterns of activity for different task phases, and in the same phase, for different stimulus objects. The results show hierarchical representation of task events. For different task phases, there were different, approximately orthogonal patterns of activity across the population of neurons. Modulations of each basic pattern encoded stimulus information within each phase. By orthogonal coding, the frontal lobe may control transitions between the discrete steps of a mental program; by correlated coding within each step, similar operations may be applied to different stimulus content.