DYNAMICS OF HIPPOCAMPAL MAP-SWITCHING IN SLOWLY MORPHING ENVIRONMENTS

The hippocampus forms spatially modulated activity patterns that are considered physiological substrates of spatial memories. Like some memories, these maps behave in an attractor-like manner and can be reactivated instantaneously by their corresponding cues. Here, we investigate the dynamics of map (re)activation during real-time, slow morphing between familiar environments.
Rats with tetrodes implanted in CA3 were trained to maintain separate hippocampal maps for two different lighting conditions in a square arena. After recording baselines in stable light cues, lights matching one map were slowly faded out, while those for the other were simultaneously faded in. Data was binned according to local theta into population vectors based on templates for each stable session. Neuronal timescales, based on spiking autocorrelation and theta frequencies, were assessed. All animals were capable of map switching in an instant teleportation paradigm and showed clear remapping between stable conditions.
Surprisingly, morphing resulted in global switching between hippocampal representations in some rats, but not others. Rats that switched between representations showed a gradual increase resp. decrease in cue-consistent cycles during morphing, resulting in a highly mixed transition period starting at 30-60% of the morphing duration where rapid switching between maps occurred. The temporal dynamics of this transition were highly individual and not entirely proportional to the mixture of cues. Rats that did not switch, by contrast, maintained a predominant active map matching initial light cues throughout multiple morphs. Switching dynamics seem related to individual neural timescales and associated temporal integration of hippocampal inputs.