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Anatomy and Physiology
It is well known that efferents from frontal, parietal-occipital, and temporal ACs all converge upon the entorhinal cortex and adjacent regions of the parahippocampal gyrus (Jones & Powell, 1970; Seltzer & Pandya, 1976; Van Hoesen & Pandya, 1975; Van Hoesen, Pandya & Butters, 1975). These projections are both direct and through limbic neocortex (cingulate cortex, posterior parahippocampal gyrus, and orbito-frontal cortex). The limbic neocortex projects to the subiculum (Van Hoesen, Rosene, & Mesulam, 1979). The entorhinal cortex is the major source of afferents to the hippocampus, whose main outflow is to subicular cortex. The subicular cortex projects directly to the limbic neocortex (Rosene & Van Hoesen, 1977), which in turn projects back to AC. Of greatest importance, Van Hoesen (1980) has recently shown that the posterior parahippocampal gyrus projects to every region of AC. Potentially, therefore, these fibers could form multiple positive feedback loops between AC and hippocampal formation, with links in limbic neocortex and subiculum. The synapses in the hippocampal segment of this loop are excitatory and highly plastic. A single tetanic activation of several hippocampal synapses leads to long lasting increases in their efficacy—long-term enhancement (LTE; see Bliss & Gardner-Medwin, 1973; McNaughton, 1983). Reciprocal connections between AC and the amygdala along similar routes could also be traced (Price, 1981). These synapses also may show long-lasting changes if repeatedly activated (Goddard, Mclntyre, & Leech, 1969).
Our model assumes that the projections from AC onto MTL neurons are (1) cognitive, (2) randomly convergent-divergent, and (3) reciprocal. First, we assume that successive levels of AC contain neurons that respond to progressively more complex features of the environment (Konorski, 1967). At the highest level, these features can no longer be thought of as passively reflecting patterns of sensory inputs. Rather, the highest features are better thought of as endogenous expectations and inferences: They are the elements that compose our cognitive frames. The AC neurons that project to the MTL are assumed to correspond to these cognitive elements. Within a cognitive frame (in the AC), these elements are associated and related in lawful ways corresponding to the underlying structure of our cognitive world. However, we assume that these relations are largely lost in the AC projection to the MTL, where each AC neuron projects to many MTL neurons. This divergence results in the convergence upon individual MTL neurons of excitation from (cortically) unrelated elements in a more or less random fashion. Finally, we assume that if a particular AC neuron excites a particular MTL neuron, then excitation of that MTL neuron will reciprocally excite that same AC neuron.
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