Differences in task-difficulty encoding during logical decision-making in prefrontal and premotor cortical activity from non-human primates

Prefrontal (PFC) and dorsal Premotor Cortices (PMd) have been reported to play a major role in Perceptual Decision-making. However, their contribution in selecting the target item between elements encoded within a memory scheme is still unexplored. Transitive Inference (TI) is one such task, where the subject needs to construct a mental schema comprising a ranked series of items (e.g. A>B>C>D>E>F) and consequently identify the higher ranking item at one of the two spatial locations. The subject’s ability to infer the rank of each item suggests a mapping of the items in a mental line, with a partial overlapping of adjacent items, described as Symbolic Distance Effect (SDE). The SDE gives rise to a logical complexity resulting in slower and less accurate decisions for selection between closely located items. To study the neural correlates underpinning these decisions, we recorded the extracellular activity from PFC and PMd of three macaque monkeys while they performed a 6-item TI task. The performance and the reaction times of all the monkeys exhibited a SDE. Easier comparisons elicited a higher neuronal response involving greater number of neuron contributing to spatial selectivity of the target item. Furthermore, the latencies of this selectivity were found to be lower in both the PFC and the PMd for easier pair comparisons. An inter-area comparison between the two areas revealed that this modulation occurred earlier in PFC than in PMd.