Contribution of multiple sensory information to place field stability in hippocampal place cells.

Hippocampal place cells in rats display spatially selective firing in relation to both external and internal cues. In the present study, we assessed the effects of removing visual and/or olfactory cues on place field stability. Place cell activity was recorded as rats searched for randomly scattered food in a cylinder. During an initial recording session, the lights were on and the only available cue was a single white cue card. Following this session, three sessions were run in a row with the cue card removed. In addition, the lights were either turned off or left on and the floor was either cleaned or left unchanged, thus creating four conditions: dark/cleaning, dark/no cleaning, light/cleaning, and light/no cleaning. A fifth session was run with the cue card back on the cylinder wall and the lights turned on. The rat remained in the cylinder during all sessions without being removed at any time. In the dark/cleaning and light/cleaning conditions, most place fields were not stable (i.e., abruptly shifted position). In addition, half of the cells stopped firing in the dark/cleaning condition. In contrast, in the dark/no cleaning and light/no cleaning conditions, most place fields remained stable across sessions. These results suggest that 1) rats are not able to rely on only movement-related information to maintain a stable place representation, 2) visual input is necessary for the firing of a large number of cells, and 3) olfactory information can be used to compensate for the lack of visuospatial information.