Hippocampal place fields: A neural code for episodic memory?

For over a half of a century, neuroscientists speculated about the neurophysiological mechanisms that support hippocampal-dependent learning and memory. About 35 years ago, O’Keefe and Dostrovsky (1971) published a report that ultimately resulted in a paradigm shift (Kuhn, 1970) in this field. Their report showed that hippocampal pyramidal neurons selectively discharge as animals traverse a circumscribed location within its environment. This was an astounding finding not only because of the surprising clarity of the neural signal from a brain structure thought to be involved in complex forms of learning and memory, but the methodology represented by this finding launched the field into a new direction that was filled with exciting new possibilities. The ability to monitor a single neuron’s activity in freely behaving animals could allow scientists to \see" for the first time learning and memory \on-line." Very soon afterwards it was revealed that the localized firing (or place field) of these \place cells" reflected not only the external sensory conditions but also the behavioral or movement state of the animal. That is, properties of an animal’s movement (e.g. speed) induce changes in place fields, providing clear evidence that space per se is not the sole determinant of place cell discharge. These findings gave rise to the highly influential and strong hypothesis that the hippocampus generates a spatial cognitive map that incorporates not only the sensory qualities of an environment but also the spatial relationship among these features, and information that is relevant to an animal’s behavior at different locations (O’Keefe and Nadel, 1978).