Contributions of anterior perirhinal cortex to olfactory and contextual fear conditioning

The present experiments examined the effects of excitotoxic lesions of anterior perirhinal cortex (PRH) on the expression of fear conditioned to an explicit olfactory CS and to the context in which CS-US pairing took place. Animals with anterior PRH lesions exhibited an attenuation of fear conditioned to the explicit CS, but no attenuation of fear conditioned to the training context. These data replicate previous findings in our laboratory examining the effects of aspirative lesions of anterior PRH, and are consistent with the notion that this cortical region comprises a critically important component of the neural system mediating the acquisition and/or expression of associations between olfactory cues and footshock.     

Disruptive effects of posttraining perirhinal cortex lesions on conditioned fear: Contributions of contextual cues.

Lesions placed in the rostral perirhinal cortex (rPRh) after fear conditioning interfere with the expression of conditioned fear responses elicited by auditory and visual conditioned stimuli when these stimuli are presented in a context that differs from the conditioning context. The present study examined whether lesions of the rPRh have similar effects when animals are tested in the conditioning context. Two days after male rats received classical fear conditioning, involving the pairing of an auditory CS with footshock, bilateral electrolytic lesions were produced in the rPRh. Five days later conditioned freezing behavior was measured during a 60-s exposure to the CS in a novel context and then 1 hr later in the conditioning context. There were 3 major findings: rPRh-lesioned Ss froze significantly less than controls to the CS in the novel context, thus confirming previously reported findings. rPRh-lesioned Ss also froze less than controls to the CS in the conditioning context, but froze significantly more to the CS in the conditioning than in the novel context, suggesting that at least part of the deficit in the novel context is due to the absence of contextual cues. Ss with rPRh lesions froze significantly less than controls to the conditioning context itself.… (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Contextual fear discrimination is impaired by damage to the postrhinal or perirhinal cortex.

Postrhinal (POR) or perirhinal (PER) cortex damage impairs acquisition and expression of contextual fear, but the nature of the impairment remains unclear. This study used a contextual fear discrimination paradigm that biased subjects toward using a configural, rather than an elemental, strategy to distinguish between 2 contexts, 1 of which was paired with a mild footshock. Control rats discriminated between 2 contexts when a combination of several cues could be used (Exp 1), but not when individual sensory cues were manipulated (Exp 2). Rats with POR or PER lesions could not discriminate between the shock and no-shock contexts when multiple cues differentiated the contexts (Exp 3). The results indicate that both the POR and PER have a role in configural learning of contextual fear. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

A peculiar pattern of temporal involvement of rat perirhinal cortex in memory processing.

By means of the fully reversible tetrodotoxin inactivation technique, perirhinal cortex (PC) mnemonic function was investigated in rats trained to a passive avoidance response (PAR). It was shown that PC functional integrity is necessary during PAR acquisition, during late and very late consolidation (from 24 hr up to 192 hr after the training session), and during retrieval. An unexpected finding was that the PC was not involved in the early consolidation period. Thus the PC may play a relatively simple relay or connective role during acquisition, but its very late and very long consolidative involvement may indicate a peculiar function in consolidation and possibly in the storage of the PAR engram. The results are discussed in terms of the mnemonic characteristics of other neural sites (amygdala, hippocampus, and entorhinal cortex) involved in the same learning process. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Cortical afferents of the perirhinal, postrhinal, and entorhinal cortices of the rat

We have divided the cortical regions surrounding the rat hippocampus into three cytoarchitectonically discrete cortical regions, the perirhinal, the postrhinal, and the entorhinal cortices. These regions appear to be homologous to the monkey perirhinal, parahippocampal, and entorhinal cortices, respectively. The origin of cortical afferents to these regions is well-documented in the monkey but less is known about them in the rat. The present study investigated the origins of cortical input to the rat perirhinal (areas 35 and 36) and postrhinal cortices and the lateral and medial subdivisions of the entorhinal cortex (LEA and MEA) by placing injections of retrograde tracers at several locations within each region. For each experiment, the total numbers of retrogradely labeled cells (and cell densities) were estimated for 34 cortical regions. We found that the complement of cortical inputs differs for each of the five regions. Area 35 receives its heaviest input from entorhinal, piriform, and insular areas. Area 36 receives its heaviest projections from other temporal cortical regions such as ventral temporal association cortex. Area 36 also receives substantial input from insular and entorhinal areas. Whereas area 36 receives similar magnitudes of input from cortices subserving all sensory modalities, the heaviest projections to the postrhinal cortex originate in visual associational cortex and visuospatial areas such as the posterior parietal cortex. The cortical projections to the LEA are heavier than to the MEA and differ in origin. The LEA is primarily innervated by the perirhinal, insular, piriform, and postrhinal cortices. The MEA is primarily innervated by the piriform and postrhinal cortices, but also receives minor projections from retrosplenial, posterior parietal, and visual association areas.     

Lesions of the perirhinal cortex do not impair integration of visual and geometric information in rats.

Rats with lesions of the perirhinal cortex and a control group were required to find a platform in 1 corner of a white rectangle and in the reflection of this corner in a black rectangle. Test trials revealed that these groups were able to integrate information regarding the shape of the pool and the color of its walls (black or white) to identify the correct location of the platform. A clear effect of the perirhinal cortex lesions was, however, revealed using an object recognition task that involved the spontaneous exploration of novel objects. The results challenge the view that the perirhinal cortex enables rats to solve discriminations involving feature ambiguity. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Functionally dissociating aspects of event memory: the effects of combined perirhinal and postrhinal cortex lesions on object and place memory in the rat

Reciprocal interactions between the hippocampus and the perirhinal and parahippocampal cortices form core components of a proposed temporal lobe memory system. For this reason, the involvement of the hippocampus in event memory is thought to depend on its connections with these cortical areas. Contrary to these predictions, we found that NMDA-induced lesions of the putative rat homologs of these cortical areas (perirhinal plus postrhinal cortices) did not impair performance on two allocentric spatial tasks highly sensitive to hippocampal dysfunction. Remarkably, for one of the tasks there was evidence of a facilitation of performance. The same cortical lesions did, however, disrupt spontaneous object recognition and object discrimination reversal learning but spared initial acquisition of the discrimination. This pattern of results reveals important dissociations between different aspects of memory within the temporal lobe. Furthermore, it shows that the perirhinal-postrhinal cortex is not a necessary route for spatial information reaching the hippocampus and that object familiarity-novelty detection depends on different neural substrates than do other aspects of event memory.     

Impairment of visual object-discrimination learning after perirhinal cortex ablation.

Eight cynomolgus monkeys learned preoperatively 20 concurrent visual discriminations between pairs of colored shapes presented on a touch screen with 24-hr intertrial intervals. Three then received bilateral perirhinal cortex ablation, and 5 remained controls. The ablated monkeys were severely impaired in reacquiring the preoperatively acquired set, whereas postoperative learning of 20 new discriminations was not significantly affected. The task was then made more difficult. First, the number of foils from which the stimulus had to be selected was increased to 2, 4, 7, and then 14. Second, larger sets of 40, 80, and 160 problems were presented. Both manipulations revealed some significant but relatively mild impairments in the monkeys with ablations. It is suggested that perirhinal cortex ablation impairs the monkey's capacity to identify individual objects, which leads to deficits in both visual-object recognition memory and discrimination learning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Positional firing properties of perirhinal cortex neurons

Neuronal activity in the perirhinal cortex was recorded while rats performed a spatial task on a four arm radial maze. The maze was defined by proximal multisensory cues on the arm surfaces and distal complex visual cues at the surround. During each recording session, rats were run in three conditions: baseline, a condition in which proximal and distal cues were manipulated, and a second baseline. Compared with the activity of hippocampal neurons in the same paradigm, a much smaller proportion of perirhinal neurons exhibited spatial selectivity and perirhinal place fields were larger than hippocampal place fields. Although perirhinal place fields exhibited a high degree of spatial tuning and reliability within a condition, they were not stable across conditions.     

Contributions of the retrosplenial and posterior parietal cortices to cue-specific and contextual fear conditioning.

The retrosplenial cortex (RSP) and the posterior parietal cortex (PPC) are the primary sources of cortical sensory input to the postrhinal cortex (POR) in rodents. Together, these areas compose a major corticohippocampal circuit that is involved in processing visuospatial information. The POR has been implicated in contextual learning and memory, consistent with the type of information presumably being processed by this region. By comparison, little is known about the role of the RSP or the PPC in contextual learning. In the present study, rats were trained either before or after surgery in a standard signaled fear conditioning task in which an auditory cue was paired with foot shock. Contextual fear and tone-specific fear were assessed in subsequent test sessions. In Experiment 1, electrolytic damage to the RSP either before or immediately after training impaired the expression of contextual fear but not tone-specific fear. In contrast, electrolytic damage to the PPC had no effect on conditional fear to the context or the tone in Experiment 2. These findings indicate that the RSP, but not the PPC, contributes to the processing of contextual information by the POR corticohippocampal processing stream. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Place memory is intact in rats with perirhinal cortex lesions.

Two experiments compared the effects of bilateral lesions of the hippocampal formation (HPC) or perirhinal cortex (PRh) on rats' performance of an allocentric spatial working memory task—delayed matching-to-place (DMTP) in a water maze. DMTP trials consisted of paired swims, and the hidden platform was moved to a new location on each trial. Performance was assessed with intervals between the first and second swim (i.e., retention delays) of 4, 30, 120, and 300 s. The rats received extensive presurgery training in Experiment 1 and no presurgery training in Experiment 2. In both experiments, rats with HPC lesions displayed DMTP deficits at all delays, taking longer and swimming farther to find the platform on the second swims than did sham-operated controls. By contrast, rats with PRh lesions displayed normal DMTP acquisition and performance. The results suggest that, unlike the functions of HPC, those of PRh are not critical for allocentric spatial working memory. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Lesions of rat perirhinal cortex exacerbate the memory deficit observed following damage to the fimbria-fornix.

Rats that had received bilateral lesions of the perirhinal cortex, fimbria-fornix, combined lesions of both these structures, or sham operations were tested on an object-guided delayed non-match-to-sample task. Perirhinal lesioned and fimbria-fornix lesioned rats were moderately impaired when delay intervals of 30 s or more were introduced between the sample and test phases of the experiment. Animals with combined lesions displayed a considerably greater impairment than animals with lesions of either structure alone. The combined lesioned animals were severely impaired in the initial acquisition of the task and displayed a profound memory deficit at delay intervals of greater than 4 s. These results emphasize the importance of the perirhinal cortex to memory function and suggest that the perirhinal cortex and the hippocampal formation may function interactively in the execution of memory processes. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Contributions of prefrontal cortex to recognition memory: Electrophysiological and behavioral evidence.

To clarify the involvement of prefrontal cortex in episodic memory, behavioral and event-related potential (ERP) measures of recognition were examined in patients with dorsolateral prefrontal lesions. In controls, recognition accuracy and the ERP old–new effect declined with increasing retention intervals. Although frontal patients showed a higher false-alarm rate to new words, their hit rate to old words and ERP old–new effect were intact, suggesting that recognition processes were not fundamentally altered by prefrontal damage. The opposite behavioral pattern was observed in patients with hippocampal lesions: a normal false-alarm rate and a precipitous decline in hit rate at long lags. The intact ERP effect and the change in response bias during recognition suggest that frontal patients exhibited a deficit in strategic processing or postretrieval monitoring, in contrast to the more purely mnemonic deficit shown by hippocampal patients. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Dissociating the role of the parietal cortex and dorsal hippocampus for spatial information processing.

Dorsal hippocampus, parietal cortex, and control lesioned rats were tested on both a metric and topological task. The metric task consisted of 2 different objects placed 68 cm apart on a cheese board. After habituation, the objects were moved to a separation of 38 cm on Day 1 and to a separation of 98 cm on Day 2. The topological task consisted of 4 different objects placed in a square orientation. After habituation, the first 2 objects were switched, and after the rats habituated to that change, the back 2 objects were switched. This was repeated on a different day with 4 new objects. The data suggest that the hippocampus is necessary for metric representations, whereas the parietal cortex is necessary for topological representations. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Role of the perirhinal cortex in rats' conditioned taste aversion response memorization.

The role of the perirhinal cortex (PC) in conditioned taste aversion (CTA) learning was investigated in Long-Evans rats. CTA was induced by the intraperitoneal administration of LiCl 60 min after saccharin-sweetened water drinking. The PC was reversibly inactivated by the stereotaxic administration of tetrodotoxin (TTX) 60 min before saccharin drinking, immediately after saccharin drinking (Experiment 1), 6 or 24 hr after LiCl administration (Experiment 2), and 60 min before CTA retrieval testing (Experiment 3). Only pre-saccharin drinking PC inactivation disrupted CTA. Thus, PC integrity is necessary only during the earliest phases of CTA mnemonic processing, that is, taste information acquisition and early gustatory memory elaboration. The results are discussed in relation to PC connectivity and PC temporal involvement in the memorization process of other aversive responses. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Contributions of specific cell information to judgments of interevent contingency.

College students considered the possible effect of an experimental drug on a skin rash. The information came from a 2?×?2 contingency table involving receipt or nonreceipt of the drug and improvement or nonimprovement of the rash: Cell A?=?receipt–improvement; Cell B?=?receipt–nonimprovement; Cell C?=?nonreceipt–improvement; Cell D?=?nonreceipt–nonimprovement. Without numerical information, Ss judged cells to be ordered A?>?B?>?C?>?D. The same order held when the contribution of each cell was derived from the contingency judgments of other Ss given numerical information. No such consistency was seen when one group of Ss made both judgments; whether individual Ss equally or unequally assessed the importance of the four cells, their contingency estimates showed cell use to be ordered A?>?B?>?C?>?D. These findings may result from strong biases that Ss harbor in processing contingency information. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Lesions of the dorsal hippocampus or parietal cortex differentially affect spatial information processing.

The present experiments used 2 versions of a modified Hebb-Williams maze to test the role of the dorsal hippocampus (dHip) and parietal cortex (PC) in processing allocentric and egocentric space during acquisition and retention. Bilateral lesions were made to either the dHip or PC before maze testing (acquisition) or after maze testing (retention). The results indicate that lesions of the dHip impair allocentric maze acquisition, whereas lesions of the PC impair egocentric maze acquisition. During retention, lesions of the PC produced a significant impairment on both maze versions, whereas lesions of the dHip produced short-lived, transient impairments on both maze versions. These results suggest that during acquisition, the hippocampus and PC process spatial information in parallel; however, long-term retention of spatial information requires the PC with the dHIP as necessary for retrieval and/or access but not necessarily storage. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Perirhinal and postrhinal cortices of the rat: interconnectivity and connections with the entorhinal cortex

The cortical regions dorsally adjacent to the posterior rhinal sulcus in the rat can be divided into a rostral region, the perirhinal cortex, which shares features of the monkey perirhinal cortex, and a caudal region, the postrhinal cortex, which has connectional attributes similar to the monkey parahippocampal cortex. We examined the connectivity among the rat perirhinal (areas 35 and 36), postrhinal, and entorhinal cortices by placing anterograde and retrograde tracers in all three regions. There is a dorsal-to-ventral cascade of connections in the perirhinal and entorhinal cortices. Dorsal area 36 projects strongly to ventral area 36, and ventral area 36 projects strongly to area 35. The return projections are substantially weaker. The cascade continues with the perirhinal to entorhinal connections. Area 35 is more strongly interconnected with the entorhinal cortex, ventral area 36 somewhat less strongly, and dorsal area 36 projects only weakly to the entorhinal cortex. The postrhinal-to-perirhinal connections also follow this general pattern. The postrhinal cortex is more heavily connected with dorsal area 36 than with ventral area 36 and is more heavily connected with area 36 than with area 35. The rostral portion of the postrhinal cortex has the strongest connections with the perirhinal cortex. Like in the monkey, the perirhinal and postrhinal cortices have different patterns of projections to the entorhinal cortex. The perirhinal cortex is preferentially connected with the rostrolateral portion of the entorhinal cortex. The postrhinal cortex projects to a part of this same region but is also connected to caudal and redial portions of the entorhinal cortex. The perirhinal and postrhinal projections to the entorhinal cortex originate in layers III and V and terminate preferentially in layers II and III.     

Learning and transfer of object—reward associations and the role of the perirhinal cortex.

Perirhinal cortex ablation has previously been shown only to impair new postoperative object discrimination learning with large stimulus set sizes (≥40 problems). In this study, 3 cynomolgus monkeys (Macaca fascicularis) with bilateral perirhinal cortex ablations were impaired relative to 3 normal controls on concurrent discrimination learning tasks with only 10 problems with the objects presented in different orientations in each trial to increase the demands placed on object identification. This supports the hypothesis that perirhinal cortex damage impairs the ability to identify multiple individual objects. Fewer errors were made to digitized images of objects than toward real objects. Both groups subsequently transferred specific object-reward associations from real objects to digitized images of the respective objects and vice versa, providing evidence that cynomolgus monkeys can recognize photographic representations of objects with experience. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Role of the posterior parietal association cortex in the processing of spatial event information.

Humans and monkeys with damage to the parietal association cortex (PC) exhibit deficits on visual-spatial tasks that are often dependent upon the attentional requirements of the task at hand. In order to test whether there is a correspondence between humans and rats in terms of the function of PC in the processing of spatial information, separate groups of rats were trained in two list-learning tasks, a win–stay task and a win–shift task, on an eight-arm radial maze. It is assumed that these two tasks vary in degree of required attention effort, because the win–shift rule is considerably easier for the rat to learn than the win–stay rule. One group of animals in each condition was given preoperative training before receiving bilateral aspiration lesions of PC. Another group in each condition received PC lesions prior to training. The results indicated that animals with PC lesions are unable to acquire either the win–stay or the win–shift rule. However, of the animals with preoperative training, only the win–stay trained animals were deleteriously affected by the brain lesion. The win–shift animals showed no postoperative decline in performance. Because rats with PC lesions are only impaired on the more demanding spatial task, these results are consistent with the human clinical observations. (PsycINFO Database Record (c) 2010 APA, all rights reserved)