Is there behavioral relevance for the topographical organization of mammalian hippocampal formation?

grantor: University of TorontoThe idea that memory is constituted of a number of fundamentally different processes has been investigated using the concept of memory systems. Memory subsystems are hierarchically subordinate components distinguished on two criteria: kinds of information they process, and neuroanatomical substrates. It is generally accepted that the hippocampal formation is the site of a memory system dedicated to learning relationships among cues of various kinds. Anatomical, neurochemical, and physiological organization of this structure suggests that it may be subdivided into several memory subsystems. The work presently described investigated this possibility by studying the function of septal vs. temporal hippocampal areas in spatial learning in the water task, context conditioning, and conditioned place preference. Results indicated that both dorsal and ventral hippocampus are involved in spatial navigation, although the former is a better substrate for this process. In contrast, these networks were equally involved in comparing contexts with different affective valence. Third, dorsal hippocampal lesions impaired, while ventral hippocampal lesions enhanced performance in conditioned place preference. Taken together, these data demonstrate a dissociation between the functions of dorsal and ventral hippocampal areas. This distinction is made according to behavioral paradigms rather than kinds of information processed. Acquisition of spatial information, stimulus-affective response associations, and comparison among cues connect the three tasks employed, but the dorsal/ventral hippocampal lesions affected them differentially. This suggests that a more useful criterion for defining memory subsystems may be task requirements. From this perspective, the hippocampal formation may include three different memory subsystems. The first, important for spatial navigation, is based on activity of dorsal hippocampal area, and is connected to medial entorhinal cortex, septum and lateral nucleus accumbens. The second, involved in modulation of interaction with amygdala memory system, is based on activity of ventral hippocampal and is connected to medial nucleus accumbens. The third is relevant for comparison among cues presented simultaneously, utilizes the hippocampal longitudinal network, and is likely connected to lateral entorhinal cortex.Ph.D