A Model of Hippocampal Ca3 Circuitry

Abstract

It is postulated that one main function of the cerebral fornices during a mammal's behavior-formative period is to signal, from CA3, decisions on species-typical acts (such as run, freeze, climb, swim) within gross modes of behavior (such as flee, mate, hunt, eat) already determined by the core of the reticular formation. The gross mode is signalled over temporo-ammonic paths into distal apical dendrites of CA3 pyramids. CA3 responds to (all-modality) sensory input patterns to its pyramids' proximal apical dendrites, received over the mossy fiber system after the sensa have been habituation-filtered through the fascia dentata. By means of septo-fornical reinforcement signals, CA3 innate responses are sharpened, functionally and anatomically, from diffuse patterns which originally involved most of CA3′s neural circuitry. This releases most of CA3′s circuitry for conditioned learning of part-novel response patterns.

Various functional interpretations of CA3′s anatomy, for innate-response sharpening and conditioned learning, are being investigated by computer simulation utilizing different permutations of pyramidal and basket cell synaptic polarities, various cross-correlative memory-element specifications, and different reinforcement methods. The pilot model described here is concerned with tonic equilibria in CA3′s core of basket cell-pyramidal cell circuitry. It employs 7 pyramids and 5 baskets interconnected according to orthodox PSP polarities. Its behavior is governed by a system of first-order nonlinear differential equations that include several timing parameters and a trainable nonlinear component in each pyramidal output function. It has now been satisfactorily-programmed to hold good steady states, and is next going to be modified for training to sharpen innate responses to mossy fiber inputs whilst under the guidance of septo-fornical reinforcements. Part-novel-response learning will come later.