Hippocampal coding of conspecific position.

Many previous studies have shown that hippocampal place cells respond to the spatial position of the animal itself. Several recent studies have shown that place cells in an observer animal can also encode the location of a conspecific. The interpretation of these previous studies is, however, compromised by the fact that the observer animal was required to complete a movement that was either a duplication of the others trajectory, or a modification of it. This raises the possibility that the observed representation of the other, may have instead been a plan for the self. To test for a representation of a conspecific in a task where immediate behaviour was not immediately required of the observer, Sprague-Dawley rats were trained to run the length of a shuttle box for a food reward. They then observed a second animal (the runner) performing the same task. Positional data was obtained from the runner, while hippocampal single unit data was collected from the observer. Hippocampal single units were observed to have only limited, low resolution, firing rate-modulated representations of the runner animal. There was also evidence of a weak relationship between place cell spatial firing representations of the self and other. Some above-chance evidence of phase-coding of the runner's position was also observed in the observer animals, with an observer-centred reference frame. These results indicate that hippocampal place cells encode some limited spatial information about others when the observer's subsequent behaviour is not dependent on that of the observed.