Cues that hippocampal place cells encode: dynamic and hierarchical representation of local and distal stimuli

Hippocampal place fields were recorded as rats explored a four-arm radial maze surrounded by curtains holding distal stimuli and with distinct local tactile, olfactory, and visual cues covering each arm. Systematic manipulations of the individual cues and their interrelationships showed that different hippocampal neurons encoded individual local and distal cues, relationships among cues within a stimulus set, and the relationship between the local and distal cues. Double rotation trials, which maintained stimulus relationships within distal and local cue sets, but altered the relationship between them, often changed the responses of the sampled neural population and produced new representations. After repeated double rotation trials, the incidence of new representations increased, and the likelihood of a simple rotation with one of the cue sets diminished. Cue scrambling trials, which altered the topological relationship within the local or distal stimulus set, showed that the cells that followed one set of controlled stimuli responded as often to a single cue as to the constellation. These cells followed the single cue when the stimulus constellation was scrambled, but often continued firing in the same place when the stimulus was removed or switched to respond to other cues. When the maze was surrounded by a new stimulus configuration, all of the cells either developed new place fields or stopped firing, showing that the controlled stimuli had persistent and profound influence over hippocampal neurons. Together, the results show that hippocampal neurons encode a hierarchical representation of environmental information.