Mw. Jung et al., FIRING CHARACTERISTICS OF DEEP LAYER NEURONS IN PREFRONTAL CORTEX IN RATS PERFORMING SPATIAL WORKING-MEMORY TASKS, Cerebral cortex (New York, N.Y. 1991), 8(5), 1998, pp. 437-450
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.