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Presented at the Vision Science Society Annual Meeting 2015

Neurons in the lateral prefrontal cortex (LPFC) are active when monkeys decide whether two stimuli, S1 and S2, separated by a delay, move in the same or in different directions. Their responses show direction selectivity reminiscent of activity in motion processing area MT, and during S2, their responses are modulated by the remembered direction. A similar modulation, termed comparison effect (CE), has also been observed in area MT. These parallels between the two areas are consistent with their connectivity, although the nature of this connectivity suggests a possibility of asymmetries in the way contralateral and ipsilateral motion is represented in the LPFC during the motion tasks. Specifically, while signals about the contralateral motion reach LPFC directly from MT of the same hemisphere, ipsilateral motion processed by MT in the other hemisphere can only reach the LPFC indirectly via callosal connections from the opposite LPFC.

We explored the role of direct and indirect motion signals in the generation of the comparison effects by examining responses of LPFC to contralateral and ipsilateral stimuli during S2. The CE was measured by comparing response to identical S2 stimuli on trials when S2 was preceded by S1 moving in the same direction (S-trials) with trials when it was preceded by S1 moving in a different direction (D-trials). ROC analysis revealed two distinct groups of neurons preferring either S-trials or D-trials. Although these CE effects were equally likely to occur for ipsilateral and contralateral stimuli, the signals for the contralateral hemifield appeared 100ms earlier, a likely reflection of direct connectivity with area MT. These results demonstrate that the comparison between the current and the remembered stimulus can be carried out in the LPFC even in the absence of direct inputs from area MT.