The human hippocampus has been implicated in memory, in particular episodic or declarative memory. In rats, hippocampal lesions cause selective spatial deficits, and hippocampal complex spike cells (place cells) exhibit spatially localized firing, suggesting a role in spatial memory, although broader functions have also been suggested. Here we report the identification of the environmental features controlling the location and shape of the receptive fields (place fields) of the place cells. This was done by recording from the same cell in four rectangular boxes that differed solely in the length of one or both sides. Most of our results are explained by a model in which the place field is formed by the summation of gaussian tuning curves, each oriented perpendicular to a box wall and peaked at a fixed distance from it.