Dentate gyrus and CA1 ensemble activity during spatial reference frame shifts in the presence and absence of visual input

In rats shuttling between a variably placed landmark of origin and a fixed goal, place fields of hippocampal CA1 cells encode location In two spatial reference frames. On the Initial part of the outbound journey, place fields encode location with respect to the origin while on the final segment, pla ce fields are aligned with the goal (Gothard et al., 1996b). An abrupt swit ch of reference frame can be induced experimentally by shortening the dista nce between the origin and the goal. Two linked hypotheses concerning this effect were addressed: (1) that the persistent, landmark-referenced firing results from some internal dynamic process (e.g., path integration or "mome ntum") and is not a result of maintained sensory input from the landmark of origin; and (2) that this hypothetical process is generated by connections either within CA3 or between CA3 and CA1, in which case the effect might b e absent from the dentate gyrus. Neuronal ensemble recordings were made sim ultaneously from CA1 and the dentate gyrus as rats shuttled on a linear tra ck between a variably located box and a goal, under light or dark condition s. The box-referenced firing persisted significantly longer in the dark in both hippocampal subfields, suggesting a competitive interaction between an internal dynamic process and external sensory cues. The similarity between reference frame transitions in the dentate gyrus and the CA1 region sugges ts that this process probably occurs before CA3, possibly in the entorhinal cortex or subiculum.