Memory Representation within the Parahippocampal Region

Abstract

The activity of 378 single neurons was recorded from areas of the parahippocampal region (PHR), including the perirhinal and lateral entorhinal cortex, as well as the subiculum, in rats performing an odor-guided delayed nonmatching-to-sample task. Nearly every neuron fired in association with some trial event, and every identifiable trial event or behavior was encoded by neuronal activity in the PHR. The greatest proportion of cells was active during odor sampling, and for many cells, activity during this period was odor selective. In addition, odor memory coding was reflected in two general ways. First, a substantial proportion of cells showed odorselective activity throughout or at the end of the memory delay period. Second, odor-responsive cells showed odor-selective enhancement or suppression of activity during stimulus repetition in the recognition phase of the task. These data, combined with evidence that the PHR is critical for maintaining odor memories in animals performing the same task, indicate that this cortical region mediates the encoding of specific memory cues, maintains stimulus representations, and supports specific match-nonmatch judgments critical to recognition memory. By contrast, hippocampal neurons do not demonstrate evoked or maintained stimulusspecific codings, and hippocampal damage results in little if any decrement in performance on this task. Thus it becomes increasingly clear that the parahippocampal cortex can support recognition memory independent of the distinct memory functions of the hippocampus itself. Key words: entorhinal cortex; perirhinal cortex; subiculum; hippocampus; recognition memory; delayed nonmatching; single unitsThe parahippocampal region (PHR) has become a focus for anatomical, behavioral, and electrophysiological studies of the medial temporal lobe memory system (Eichenbaum et al., 1994;Brown, 1996;Murray, 1996;Suzuki, 1996). This cortical area surrounds the hippocampus and amygdala and is composed of several distinct subdivisions, including the perirhinal, entorhinal, and parahippocampal (in monkeys) or postrhinal (in rats) cortices (Witter et al., 1989;Burwell et al., 1995). The PHR receives inputs from widespread secondary or "association" cortical regions and provides the major conduit for hippocampal outputs to the same cortical association areas. This anatomical evidence indicates that the PHR occupies a pivotal position for mediating memory functions of the hippocampal region.Neuropsychological findings indicate that the PHR indeed plays a critical role in recognition memory, independent of its role as an intermediary for cortical-hippocampal interactions (Eichenbaum et al., 1994;Murray, 1996). This evidence comes mainly from experiments examining the effects of damage to the hippocampal region on performance in a simple recognition memory test known as delayed nonmatching to sample (DNMS) (Eichenbaum et al., 1994). In the standard version of this task, originally devised by Gaffan (1974), animals are presented with a novel "sample" cue and then after a...

The human brain representation of odor identification

“…This discrepancy probably stems from a combination of MR technical (Kaza et al 2011;Ojemann 1997Ojemann , 2010 issues combined with local anatomic-physiologic characteristics of odor processing in the entorhinal cortex. In rats entorhinal neurons display odor selectivity (Young et al 1997), and thus both inter-and intraindividual activity to the identified odors may differ, thus contributing to a limited area of activation in the third level group analysis, although activity for each individual was high, as shown in the functional and anatomic entorhinal ROI analyses. These findings provide direct evidence for the proposed connection between entorhinal pathol- The model for spontaneous identified vs. nonidentified odors was event-related.…”

A locus and mechanism of action for associative morphine tolerance

“…Although we did not establish a contribution of hippocampal CCK, the Fos analysis suggests that CA1 neurons contribute to the expression of associative morphine tolerance. The hippocampus seems to serve as a locus for memory consolidation and to perform the match-mismatch discriminations necessary for memory recognition 46 . Moreover, there are direct, reciprocal connections between the hippocampus and L/BL amygdala 47,48 .…”

The rat perirhinal cortex: A review of anatomy, physiology, plasticity, and function

“…Rats with perirhinal-entorhinal lesions were found to be impaired in an olfactory recognition memory tasks (Kaut and Bunsey, 2001;Kaut et al, 2003). Electrophysiological recordings during delayed non-matching tasks also show a role for the perirhinal cortex (Young et al, 1997) and prefrontal cortex (Ramus and Eichenbaum, 2000) in olfactory discrimination. Bearing in mind the anatomical connections between the perirhinal cortex and olfactory regions (see Section 3.2) and that many object recognition studies in rats rely on nose contact as a measure of exploration, it is surprising that more attention has not been focussed on the role of the perirhinal cortex in olfactory recognition memory.…”

“…3a; Swanson and Cowan, 1977;Wyss, 1981;Kosel et al, 1982Kosel et al, , 1983Deacon et al, 1983;Kö hler, 1988;Van Groen and Wyss, 1990;Insausti et al, 1997;Burwell and Amaral, 1998a,b;Shi and Cassell, 1999;Kloosterman et al, 2003b). Based on anatomical (Witter et al, 2000b;Witter, 2002), electrophysiological (Young et al, 1997;Ivanco and Racine, 2000;Naber et al, 1997Naber et al, , 1999Naber et al, , 2001a and functional evidence (Bussey et al, 2000;Wan et al, 2001;Burwell et al, 2004a,b;Jenkins et al, 2004;Amin et al, 2006;Albasser et al, 2010;Romero-Granados et al, 2010), the hippocampal-parahippocampal region of the brain has been implicated in several aspects of learning and memory. The role of the perirhinal cortex in this hippocampal-parahippocampal network has been traditionally seen as a gateway for sensory information into the hippocampus via the entorhinal cortex (Witter et al, 2000a,b) but, as discussed below, the current evidence does not fully support this model but points to a more complex relationship between the hippocampal-parahippocampal regions.…”