Posterior parietal cortex and long-term memory: some data from laboratory animals

Abstract

The posterior parietal cortex (PPC) was long viewed as just involved in the perception of spatial relationships between the body and its surroundings and of movements related to them. In recent years the PPC has been shown to participate in many other cognitive processes, among which working memory and the consolidation and retrieval of episodic memory. The neurotransmitter and other molecular processes involved have been determined to a degree in rodents. More research will no doubt determine the extent to which these findings can be extrapolated to primates, including humans. In these there appears to be a paradox: imaging studies strongly suggest an important participation of the PPC in episodic memory, whereas lesion studies are much less suggestive, let alone conclusive. The data on the participation of the PPC in episodic memory so far do not permit any conclusion as to what aspect of consolidation and retrieval it handles in addition to those dealt with by the hippocampus and basolateral amygdala, if any.

Fear Memory

“…These areas include hippocampus, BLA, medial septum, entorhinal, perirhinal, anterior cingulate, retrosplenial, prefrontal and posterior parietal cortex (19,52,53,84,265,273,277,365,444,445,649,678) (FIGURES 1 AND 5). Actually, in the making and in the recall of the memory of many or most tasks, no matter how simple, several brain areas are involved and the wide majority of memories do not depend on just one punctiform brain site.…”

“…The steps that follow after the LTP that are linked with cellular consolidation have been studied more extensively in the LA in Pavlovian conditioning (380, 475, 554, 564, 606 -608) and are shown in the diagram of FIGURE 4. The corresponding steps for IA processing subsequent to, and beyond, the encoding hippocampal LTP are largely unknown, except for a late participation first of the entorhinal cortex (169) and then of the posterior parietal cortex (277,445), and for rather assorted evidence in favor of a role of those two and several other cortical areas in systems or otherwise late consolidation of the task (169,277,558).…”

“…I, V, and VIII). Several others brain structures linked to the hippocampus are also involved and play a parallel and related role in that process (BLA, entorhinal and posterior parietal cortex) (FIGURES 1A AND 5) (19,84,265,272,365,445) and in the retrieval of the IA (52, 53). There is agreement that the extinction of IA, like that of other tasks, relies on hippocampal LTP depotentiation followed by LTD (123,226,244,637).…”

“…265, 272, 657 for its biochemical correlates) has been studied more deeply and in more detail than the LA LTP underlying classically conditioned fear (328,411,448,538). On the other hand, the integration of hippocampal LTP into the IA circuit has not been specifically and systematically studied, mainly because the rest of the circuit is basically unknown, aside from the BLA (401, 404) and enthorhinal and posterior parietal cortex (30,169,279,291,365,445), and so is less understood than that of the LA LTP in the circuit of Pavlovian fear, which is well-established (58, 98). In the latter, the connections and function of the events in LA with those in CE, the output nucleus of the amygdala, and with PL and IL, has been quite unequivocally ascertained (58, 98, 475), including the reversal of the role of the latter in extinction (598).…”

“…624). The lack of a precise procedure to investigate the engram at the cellular level motivated researchers to search under the lamppost for years: the hippocampus, the neighboring entorhinal (265), the distant posterior parietal cortex (FIGURE 5C) (445), the secondary sensory neocortex (559), and eventually the amygdala, the nucleus interpositus of the cerebellum, and other sites were at various times cited as home for the engrams (352, 518, 574; see Ref. 569).…”