LACK OF SEQUENCE DISAMBIGUATION IN RAT MEDIAL PREFRONTAL CORTEX ENSEMBLES

D.R. Euston*; R.P. Roop; B.L. McNaughton

NSMA, Univ Arizona, Tucson, AZ, USA

Learning sequences with repeated elements requires differential neural activity to disambiguate sequential contexts of the repeated elements, but where in the brain the critical distinctions are made is unknown. Rat hippocampal cells can disambiguate sequential context, but hippocampal disambiguation is not necessary for task performance (Bower et al., 2002). Another candidate for sequence disambiguation is the medial prefrontal cortex (mPFC), a region implicated in working memory. A rat was trained to run to a sequence of goal locations around the perimeter of a 1.5m arena using medial forebrain bundle stimulation as reward. The sequence, 8 segments long, contained two repeated segments (e.g., 5-2-7-1-5-2-3). During cued trials, a flashing LED indicated the target. In contrast, non-cued trials forced the rat to locate the target without the LED (i.e., from memory). Cued and non-cued runs alternated in blocks of 3 complete sequences. The rat acquired non-cued sequences within 3, 90-minute sessions and made few errors even at locations requiring a context-dependent choice of direction. Typically, a single session included 120 traversals of the full sequence. 40-80 cells per session were simultaneously recorded from either the dorsal anterior cingulate (ACd) or the prelimbic cortex (PL). Of 222 cells, 48% showed elevated firing specific to the run beginning, the run midpoint, or the destination approach. The majority also showed differential firing on different physical segments of the sequence, but firing rates and spatial distributions were essentially identical on all repeated segments, even though the turns following these repeated segments differed (i.e., there was no predictive activity within the mPFC indicative of sequence disambiguation). This was equally true of both cued trials and memory driven, non-cued trials. It can be concluded that neither the PL or ACd is the locus of the short-term memory processes required for sequence performance.

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