Hippocampus as a memory map: synaptic plasticity and memory encoding by hippocampal neurons

Hippocampal cells contribute to memory by rapidly encoding information about the perceptual and behavioral structure of experience. This paper describes two complementary experimental approaches that illustrate two important mechanisms that confer these properties to hippocampal cells: (1) Enduring spatial memory and stable place fields each depend upon synaptic plasticity mechanisms that normally rely on the same NMDA-receptor mediated metabolic events as long-term potentiation (LTP). Thus, hippocampal cells "learn" to encode information about the perceptual and behavioral structure of experiences. (2) Hippocampal cells encode the structure of experience and respond in a manner inconsistent with a spatial representation. Place fields are distributed heterogeneously in space, their locations are determined by non-geometric information, the population of active cells can indicate more than one location in space, and hippocampal cells encode discriminative stimuli independent of their spatial location. To the extent that the hippocampus encodes a map, it is more simply described as a memory map than a spatial map. Rather than computing spatial locations, the space it encodes is better described as a life or a problem space that encodes the history of experience into the relational structure of episodes.