Magnitude of the object recognition deficit associated with perirhinal cortex damage in rats: Effects of varying the lesion extent and the duration of the sample period.

The present study examines 2 factors that might moderate the object-recognition deficit seen after perirhinal cortex damage. Object recognition by normal rats was improved by extending (from 4 to 8 min) the sample period during which an object was first explored. Furthermore, there was a significant positive correlation between time spent in close exploration of the sample object and degree of successful novelty discrimination. In contrast, rats with perirhinal cortex lesions failed to benefit from increased close exploration and did not discriminate the novel object after even the longest sample period. Nevertheless, the lesions did not disrupt habituation across repeated exposure to the same object. The second factor was extent of perirhinal cortex damage. A significant correlation was found between total perirhinal cortex loss and degree of recognition impairment. Within the perirhinal cortex, only damage to the caudal perirhinal cortex correlated significantly with recognition memory deficits. This study highlights the critical importance of the perirhinal cortex within the temporal lobe for recognition memory and shows that the lesion-induced deficit occurs despite seemingly normal levels of close object exploration. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Lesions of the rat perirhinal cortex spare the acquisition of a complex configural visual discrimination yet impair object recognition.

Rats with perirhinal cortex lesions were sequentially trained in a rectangular water tank on a series of 3 visual discriminations, each between mirror-imaged stimuli. When these same discriminations were tested concurrently, the rats were forced to use a configural strategy to solve the problems effectively. There was no evidence that lesions of the perirhinal cortex disrupted the ability to learn the concurrent configural discrimination task, which required the rats to learn the precise combination of stimulus identity with stimulus placement (“structural” learning). The same rats with perirhinal cortex lesions were also unimpaired on a test of spatial working memory (reinforced T maze alternation), although they were markedly impaired on a new test of spontaneous object recognition. For the recognition test, rats received multiple trials within a single session in which on every trial, they were allowed to explore 2 objects, 1 familiar, the other novel. On the basis of their differential exploration times, rats with perirhinal cortex lesions showed very poor discrimination of the novel objects, thereby confirming the effectiveness of the surgery. The discovery that bilateral lesions of the perirhinal cortex can leave configural (structural) learning seemingly unaffected points to a need to refine those models of perirhinal cortex function that emphasize its role in representing conjunctions of stimulus features. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Object and place memory in the macaque entorhinal cortex

Lesions of the entorhinal cortex in humans, monkeys, and rats impair memory for a variety of kinds of information, including memory for objects and places. To begin to understand the contribution of entorhinal cells to different forms of memory, responses of entorhinal cells were recorded as monkeys performed either an object or place memory task. The object memory task was a variation of delayed matching to sample. A sample picture was presented at the start of the trial, followed by a variable sequence of zero to four test pictures, ending with a repetition of the sample (i.e., a match). The place memory task was a variation of delayed matching to place. In this task, a cue stimulus was presented at a variable sequence of one to four "places" on a computer screen, ending with a repetition of one of the previously shown places (i.e., a match). For both tasks, the animals were rewarded for releasing a bar to the match. To solve these tasks, the monkey must 1) discriminate the stimuli, 2) maintain a memory of the appropriate stimuli during the course of the trial, and 3) evaluate whether a test stimulus matches previously presented stimuli. The responses of entorhinal cortex neurons were consistent with a role in all three of these processes in both tasks. We found that 47% and 55% of the visually responsive entorhinal cells responded selectively to the different objects or places presented during the object or place task, respectively. Similar to previous findings in prefrontal but not perirhinal cortex on the object task, some entorhinal cells had sample-specific delay activity that was maintained throughout all of the delay intervals in the sequence. For the place task, some cells had location-specific maintained activity in the delay immediately following a specific cue location. In addition, 59% and 22% of the visually responsive cells recorded during the object and place task, respectively, responded differently to the test stimuli according to whether they were matching or non-matching to the stimuli held in memory. Responses of some cells were enhanced to matching stimuli, whereas others were suppressed. This suppression or enhancement typically occurred well before the animals' behavioral response, suggesting that this information could be used to perform the task. These results indicate that entorhinal cells receive sensory information about both objects and spatial locations and that their activity carries information about objects and locations held in short-term memory.     

Perirhinal cortex ablation in rats selectively impairs object identification in a simultaneous visual comparison task.

In Experiment 1, rats discriminated among computer-generated visual displays (scenes) comprising 3 different shapes (objects). One constant scene (unrewarded) appeared on every trial together with a trial-unique variable scene (rewarded). Four types of variable scene were intermingled: (a) unfamiliar objects in different positions from the constant; (b) unfamiliar objects in same positions as the constant; (c) same objects as the constant in different positions; (d) same objects and positions, recombined. Aspiration lesions of perirhinal cortex impaired performance with type (b) only. Experiment 2 tested spatial delayed nonmatching-to-sample. The perirhinal group were impaired nonsignificantly, and less than fornix-transected rats in an earlier study. Rats' perirhinal cortex, like monkeys', subserves object identification in the absence of memory requirement but does not contribute substantially to hippocampal system spatial memory function. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The effect of excitotoxic lesions centered on the perirhinal cortex in two versions of the radial arm maze task.

Rats with bilateral ibotenic acid lesions centered on the perirhinal cortex and sham-operated controls were tested in 2 versions of a spatially guided radial arm maze task. Lesioned rats made significantly more errors and required more sessions to reach criterion relative to controls in the standard radial maze task. When they were tested in a delayed nonmatch to sample version of this task, lesioned rats made more errors during the predelay phase and at both the 30-s and 10-min delays of the postdelay phase. These findings provide further support for the hypothesis that the role of the perirhinal cortex in object recognition memory may include reference to some spatial aspect of the environment. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

“I've seen it all before”: Explaining age-related impairments in object recognition. Theoretical comment on Burke et al. (2010).

Animal models are useful in elucidating the neural basis of age-related impairments in cognition. Burke, Wallace, Nematollahi, Uprety, and Barnes (2010) tested young and aged rats in several different protocols to measure object recognition memory and found that object recognition deficits in aged rats were consistent with these rats behaving as if novel objects were familiar, rather than familiar objects being treated as novel (that is, forgotten). A similar pattern of behavior has been observed in young rats with perirhinal cortex lesions. Moreover, age-related impairments in object recognition were uncorrelated with deficits in spatial learning in the water maze, a task that requires the integrity of the hippocampus and is also reliably impaired in aged rats. Taken together, these findings support functional specialization of structures within the medial temporal lobe “memory system,” as well as the independence of age-related deficits in different cognitive domains. They also potentially form a foundation for neurobiological study of age-related impairments in perirhinal cortex function. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Perirhinal cortex and place–object conditional learning in the rat.

The present study examined whether excitotoxic lesions of the perirhinal cortex can affect acquisition of a place–object conditional task in which object and spatial information must be integrated. Testing was carried-out in a double Y-maze apparatus, in which rats learned a conditional rule of the type, "In Place X, choose Object A, not Object B (A+ vs B–); in Place Y, choose Object B, not Object A (A– vs B+)." Perirhinal cortex lesions significantly impaired acquisition of this task while sparing performance of an allocentric spatial memory task performed in a radial arm maze. Perirhinal cortex lesions also had no apparent effect on a 1-pair object discrimination task performed in the double Y-maze or on retention and acquisition of 4-pair concurrent discrimination problems performed in a computer-automated touch screen testing apparatus. The results suggest that, although the perirhinal cortex and hippocampus can be functionally dissociated, their normal mode of operation includes the integration of object and spatial information. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Memory impairment on a delayed non-matching-to-position task after lesions of the perirhinal cortex in the rat.

Previous research conducted in monkeys and rats has established that the perirhinal cortex is critically involved in object- or stimulus-recognition memory, whereas other research suggests this region may contribute to memory for object discriminations. These findings do not rule out the possibility that the perirhinal cortex plays a more general role in memory. The present experiment addressed whether selective lesions of the perirhinal cortex would result in a delay-dependent deficit on a test of memory that did not involve stimulus recognition or object memory. Rats with bilateral perirhinal lesions were tested on a delayed non-matching-to-position task. Lesions of the perirhinal cortex did not interfere with acquisition or performance at short (0–4 s)-delay intervals, but lesions did impair performance at longer delays. It is suggested that the perirhinal cortex is involved in maintaining representations of trial-specific information over time. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Interaction of frontal and perirhinal cortices in visual object recognition memory in monkeys

Monkeys were trained preoperatively in visual object recognition memory. The task was delayed matching-to-sample with lists of trial-unique randomly generated visual stimuli in an automated apparatus, and the stimuli were 2D visual objects made from randomly generated coloured shapes. We then examined the effect of either: (i) disconnecting the frontal cortex in one hemisphere from the perirhinal cortex in the contralateral hemisphere by crossed unilateral ablations; (ii) disconnecting the magnocellular portion of the mediodorsal (MDmc) thalamic nucleus in one hemisphere from the perirhinal cortex in the contralateral hemisphere; or (iii) bilaterally ablating first the amygdala, then adding fornix transection, then finally perirhinal cortex ablation. We found that both frontal/perirhinal and MDmc/perirhinal disconnection had a large effect on visual object recognition memory, whereas both amygdalectomy and the addition of fornix transection had only a mild effect. We conclude that the frontal lobe needs to interact with the perirhinal cortex within the same hemisphere for visual object recognition memory, but that routes through the amygdala and hippocampus are not of primary importance.     

Lesions of the perirhinal cortex interfere with conditioned excitation but not with conditioned inhibition of fear.

Posttraining lesions of the perirhinal cortex (Prh) have been shown to interfere with the expression of fear. This study assessed whether Prh lesions would also disrupt the inhibition of fear as measured with conditioned inhibition of fear-potentiated startle. Following light + shock, noise→ light-no shock conditioned-inhibition training, rats were given Prh lesions. The lesions interfered with the expression of fear-potentiated startle to the light. To assess whether conditioned inhibition was affected, the rats were given light + retraining without additional noise→ light - training. The noise-conditioned inhibitor retained its ability to inhibit fear-potentiated startle to the retrained light. These results suggest that the areas of the Prh that are essential for the initial expression of conditioned fear are not important for the expression of conditioned inhibition of fear. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Perirhinal cortex ablation impairs visual object identification

Impairments in both recognition memory and concurrent discrimination learning have been shown to follow perirhinal cortex ablation in the monkey. The pattern of these impairments is consistent with the hypothesis that the perirhinal cortex has a role in the visual identification of objects. In this study we compared the performance of a group of three cynomolgus monkeys with bilateral perirhinal cortex ablation with that of a group of three normal controls in two tasks designed to test this hypothesis more directly. In experiment 1 the subjects relearned a set of 40 familiar concurrent discrimination problems; the stimuli in each trial were digitized images of real objects presented in one of three different views. After attaining criterion they were tested on the same problems using similar, but previously unseen, views of the objects. In experiment 2 the subjects were tested on their ability to perform 10 of these familiar discriminations with each problem presented in the unfamiliar context of a digitized image of a unique complex scene. The subjects with ablations were significantly impaired on both tasks. These results demonstrate that the role of the perirhinal cortex is not restricted to memory, and they support the hypothesis that the perirhinal cortex is involved in visual object identification. We suggest that the perirhinal cortex is crucially involved in processing coherent concepts of individual objects. A deficit of this nature could underlie the pattern of impairments that follow perirhinal cortex damage in both visual object recognition memory and visual associative memory.     

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)     

Medial septal lesions disrupt spatial, but not nonspatial, working memory in rats.

In Exp 1, rats with small medial septal lesions were less able than were control rats to remember the location of the arm of a Y maze they had been forced to enter on the preceding sample run. Moreover, as the retention interval between the sample and choice runs on this spatial delayed nonmatching-to-sample (DNMTS) task was increased to 1 and 2 min, the magnitude of the deficit increased. In contrast, these same lesioned rats were not deficient in Exp 2 in their ability to remember the object they had encountered in the straight alley on the sample run. In fact, when the retention interval was increased to 1 min on this nonspatial DNMTS task, the rats with medial septal lesions were more accurate than were the controls. This pattern of results did not appear to be due to task difficulty, recovery of function, or sequence of training. Rather, these results indicate that damage to the septohippocampal system disrupts spatial working memory more than it disrupts nonspatial working memory. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Object exploration and reactions to spatial and nonspatial changes in hooded rats following damage to parietal cortex or hippocampal formation.

Hooded rats with bilateral lesions of the anterior part of the hippocampal formation (HIP), anterior region of the posterior parietal cortex (APC), or posterior region of the posterior parietal cortex (PPC) were compared with controls for their exploration of 5 objects in an open field, habituation of locomotion and object investigation, and response to spatial and nonspatial change. First, all groups displayed habituation of both locomotor and exploratory activity. Second, controls selectively reexplored displaced objects, and APC-lesioned rats reexplored all objects, whereas PPC- and HIP-lesioned rats failed to react to the spatial change. Third, a novel object induced reexploration in all groups. Results are consistent with the roles of the HIP and PPC in spatial information processing. Moreover, the APC and PPC are involved in attentional effortful processing and visuospatial information processing necessary for spatial representation, respectively. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Spatial attention and implicit sequence learning: Evidence for independent learning of spatial and nonspatial sequences.

This research investigated whether regular spatial orienting sequences can be learned implicitly and independently of response requirements. In a new version of a serial response task introduced by M. J. Nissen and P. Bullemer (1987) participants had to discriminate between objects that could occur at different locations. Independent sequences determined the succession of locations and objects. Even participants who were not aware of any regularities exhibited evidence for learning of both sequences (Experiment 1). Experiment 2 showed that the joint learning of spatial and object sequences was as efficient as learning of single sequences and that it even occurred when learning required memory for past sequence elements and attention was blocked through a secondary tone-counting task. Results are consistent with the idea that independent systems may exist for the implicit acquisition of spatial and nonspatial regularities. (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)     

Neurotoxic lesions of the rat perirhinal cortex fail to disrupt the acquisition of performance of tests of allocentric spatial memory.

Rats with neurotoxic lesions of the perirhinal cortex (n = 9) were compared with sham controls (n = 14) on a working memory task in the radial am maze. Rats were trained under varying levels of proactive interference and with different retention intervals. Finally, performance was assessed when the maze was switched to a novel room. None of these manipulations differentially impaired rats with perirhinal lesions. Rats were next trained on delayed matching-to-place in the water maze. Even with retention delays of 30 min, there was no evidence of a deficit. Although interactions between the perirhinal cortex and hippocampus may be important for integrating object-place information, the perirhinal cortex is often not necessary for tasks that selectively tax allocentric spatial memory. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Entorhinal-perirhinal lesions impair performance of rats on two versions of place learning in the Morris water maze.

The effects of entorhinal–perirhinal lesions in rats were studied with 2 versions of a place learning task in the Morris water maze. These lesions impaired performance on a multiple-trial task (3 days of 6 trials and a probe trial). This assessment was followed by a task in which rats were repeatedly trained to find novel locations with a variable delay (30 sec or 5 min) imposed between each sample trial and retention test. Entorhinal–perirhinal damage produced a delay-dependent deficit in spatial memory: Rats with lesions were impaired at the 5-min delay relative to the control group and to their own performance at 30 sec. These findings are discussed in relationship to memory impairment after entorhinal damage and spatial learning deficits observed after hippocampal damage. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Pattern separation deficits may contribute to age-associated recognition impairments.

Normal aging is associated with impairments in stimulus recognition. In the current investigation, object recognition was tested in adult and aged rats with the standard spontaneous object recognition (SOR) task or two variants of this task. On the standard SOR task, adult rats showed an exploratory preference for the novel object over delays up to 24 h, whereas the aged rats only showed significant novelty discrimination at the 2-min delay. This age difference appeared to be because of the old rats behaving as if the novel object was familiar. To test this hypothesis directly, rats participated in a variant of the SOR task that allowed the exploration times between the object familiarization and the test phases to be compared, and this experiment confirmed that aged rats falsely “recognize” the novel object. A final control examined whether or not aged rats exhibited reduced motivation to explore objects. In this experiment, when the environmental context changed between familiarization and test, young and old rats failed to show an exploratory preference because both age groups spent more time exploring the familiar object. Together these findings support the view that age-related impairments in object recognition arise from old animals behaving as if novel objects are familiar, which is reminiscent of behavioral impairments in young rats with perirhinal cortical lesions. The current experiments thus suggest that alterations in the perirhinal cortex may be responsible for reducing aged animals' ability to distinguish new stimuli from ones that have been encountered previously. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The effects of cytotoxic perirhinal cortex lesions on spatial learning by rats: A comparison of the dark Agouti and Sprague-Dawley strains.

The effects of perirhinal cortex lesions in rats on spatial memory might depend on the choice of strain. The present study, therefore, compared perirhinal lesions in Sprague-Dawley rats (associated with deficits) with Dark Agouti rats (associated with null effects). Tests of reference memory and working memory in the water maze failed to provide evidence that perirhinal lesions disrupt overall levels of performance (irrespective of strain) or that these lesions have differential effects on the rates of spatial learning in these 2 strains. Strain differences were, however, found, as the Dark Agouti strain was often superior. Furthermore, the perirhinal lesions did have differential effects in the 2 strains, but these did not appear to relate directly to changes in spatial learning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)