The essential circuit for both item and associative stimulus recognition in any given sensory modality (or across modalities) consists of the relevant cortical sensory processing stream(s), the medial temporal periallocortex (i.e. parahippocampal, perirhinal, and entorhinal cortices), the ventromedial prefrontal cortex, and the magnocellular division of the medial dorsal nucleus of the thalamus. Associative recall, on the other hand, appears now to be organized hierarchically; thus, whereas context-free recall, or fact memory, also seems to depend primarily on the above basic memory circuit, context-rich recall, or event memory, seems to depend in addition on a higher-order circuit superimposed on the basic one and consisting of the hippocampus, mamillary body, anterior thalamic nuclei, and cingulate cortex. That item recognition at least does not depend on the higher-order memory circuit is supported by new evidence obtained in Jon (reported on initially in Science 277: 376-380, 1997) who has seemingly selective bilateral hippocampal pathology induced by a neonatal hypoxic/ischaemic insult. On standardized memory tests allowing quantitative assessment, Jon's recognition scores were at or above the 50th percentile, whereas his recall scores fell at or below the 1st percentile. That associative recognition also does not depend on the higher-order memory circuit (but does require the basic circuit) is supported by new evidence obtained in monkeys. These new results indicate that the ability to form one-trial object-place associations is unaffected by selective, excitotoxic damage to the hippocampus, and yet is severely impaired by ablation of the underlying parahippocampal cortex.
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NIH ExPorter