FIRING CHARACTERISTICS OF DEEP LAYER NEURONS IN PREFRONTAL CORTEX IN RATS PERFORMING SPATIAL WORKING-MEMORY TASKS

Single cells were recorded with 'tetrodes' in regions of the rat media l prefrontal cortex, including those which are targets of hippocampal afferents, while rats were performing three different behavioral tasks : (i) an eight-arm radial maze, spatial working memory task, (ii) a fi gure-eight track, delayed spatial alternation task, and (iii) a random food search task in a square chamber. Among 187 recorded units, very few exhibited any evidence of place-specific firing on any of the beha vioral tasks, except to the extent that different spatial locations we re related to distinct phases of the task. Furthermore, no prefrontal unit showed unambiguous spatially dependent delay activity that might mediate working memory for spatial locations. Rather, the cells exhibi ted diverse correlates that were generally associated with the behavio ral requirements of performing the task. This included firing related to intertrial intervals, onset or end of trials, selection of specific arms on the eight-arm radial maze, delay periods, approach to or depa rture from goals, and selection of paths on the figure-eight track. Al though a small number of cells showed similar behavioral correlates ac ross tasks, the majority of cells showed no consistent correlate when recorded across multiple tasks. Furthermore, some units did not exhibi t altered firing patterns in any of the three tasks, while others show ed changes in firing that were not consistently related to specific be haviors or task components. These results are in agreement with previo us lesion and behavioral studies in rats that suggest a prefrontal cor tical role in encoding 'rules' (i.e. structural features) or behaviora l sequences within a task but not in encoding allocentric spatial info rmation. Given that the hippocampal projection to this cortical region is capable of undergoing LTP, our data lead to the hypothesis that th e role of this projection is not to impose spatial representations upo n prefrontal activity, but to provide a mechanism for learning the spa tial context in which particular behaviors are appropriate.