Associative Memory Models of Hippocampal Areas CA1 and CA3

The hippocampal regions CA3 and CA1 have long been proposed to be auto and heteroassociative memories, respectively (Marr, 1971; McNaughton and Morris, 1987; Treves and Rolls, 1994), for the storage of declarative information. An autoassociative memory is formed when a set of neurons are recurrently connected by modifiable synapses, whereas a heteroassociative memory is formed through modifiable connections from an input layer of neurons to an output layer. Associative memory storage simply requires a Hebbian strengthening of connections between neurons that are coactive (Amit, 1989; Hopfield, 1982; Willshaw et al., 1969). Recall proceeds from a cue activity pattern across neurons that is a partial or noisy version of a previously stored pattern. A suitable firing threshold on each neuron that receives input from already active neurons ensures that neural activity evolves towards the stored pattern. This may happen with only one or two updates of each neuron’s activity.