CORRELATES OF HIPPOCAMPAL COMPLEX-SPIKE CELL-ACTIVITY IN RATS PERFORMING A NONSPATIAL RADIAL MAZE TASK

The observation of hippocampal place cells forms a major line of evide nce supporting the view that the hippocampus is dedicated to spatial p rocessing. However, most studies demonstrating the spatial properties of hippocampal unit activity have employed tasks that emphasize spatia l cues but minimize nonspatial cues. In the present experiment we reco rded the activity of hippocampal complex-spike cells from rats perform ing a nonspatial radial maze task. Performance in this task was guided by local visual-tactile cues on the maze arms, while distal spatial c ues were minimized and made irrelevant. The influence of three variabl es on unit activity was examined: type of cue on an arm, spatial locat ion of an arm, and the relative position of the animal on an arm. Of t he units recorded, almost one-fifth were classified as ''cue cells'' i n that their activity was associated with cue type but not spatial loc ation. Conversely, a similar proportion of the units were classified a s ''place cells'' in that their activity was associated with location, but not cue type. In an additional similar proportion of units, firin g was influenced only by relative position and not by local cues or sp atial locations. For the majority of units, however, firing was relate d to combinations of these three variables, indicating that most hippo campal neurons encoded conjunctions or relations between spatial and l ocal cue information. This pattern of results indicates that when loca l rather than distal spatial cues are emphasized, hippocampal neural a ctivity is strongly influenced by salient nonspatial cues and shows no overwhelming predominance of place coding. These findings are at odds with the hypothesis that the hippocampus is selectively involved in s patial processing and, conversely, support the broader view that the h ippocampus encodes both spatial and nonspatial relations among importa nt experimental variables.