Species-typical IRMs in CA1-3 hippocampi and habituation memory in area dentata

Abstract

Sector CA3c hippocampi contains the genetically preprogrammed innate releaser mechanisms (IRMs) for species-typical threats; CA3b a the genetically preprogrammed innate releaser mechanisms for species-typical nutrients, playmates and surroundings. Activation of these mechanisms, whilst triggering species-appropriate motor responses via the fornix and the mammillo-tegmental tract, teaches the meaningfulness of the recognized patterns, via the mammillo-thalamic tract, into the neocortex which is unprogrammed genetically but passively files all percepts transmitted via the thalamic sensory relay nuclei. Sector CA1 contains the genetically preprogrammed innate releaser mechanisms for mating and parenting, but becomes available only after hormonal activation of its circuitry at puberty. Area dentata, with boosting from the medial septal theta system, automatically filters-out from transmission into CA most once-attended patterns which CA3 has not recognized. The filtered patterns are filed within area dentata, ie. area dentata automatically builds a memory bank of percepts not requiring motor reaction, including a conceptual map of the habitat. Desynchronization of the theta system by reward or punishment signals from the brainstem, in response to some once-attended novel patterns, halts the filter-file habituation mechanism, permitting some early-life learning of non-innate releaser patterns to be added to CA3c and CA3b a . In adulthood the relatively redundant ammonshorn may continue to be activated in parallel with the immensely more versatile neocortex, and the fornices may be bilaterally transected with relative impunity.