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)     

Reaction of visual cortex neurons to cross-like figures during local inhibition blockade

Incidental finding of a primary malignant lymphoma of the ovary in a 20-year-old patient is presented. Two and a half years following ablative surgery and adjuvant chemotherapy, the patient is alive and disease free. Ovarian lymphoma is a disease of reportedly poor prognosis. However, many previously reported cases of ovarian lymphoma actually represented ovarian involvement by a more diffuse lymphomatous process. If stringent criteria are used for case selection, true primary ovarian lymphoma usually carries a favorable prognosis.     

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 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)     

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)     

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.     

The effects of neurotoxic lesions of the perirhinal cortex combined to fornix transection on object recognition memory in the rat

The effects of lesions centred in the perirhinal cortex region (Prh) or in both the perirhinal cortex region and the fornix (Prh + Fx) were assessed in two different working memory tasks, one spatial the other nonspatial. For the spatial task the rats were tested in an eight arm radial maze, using a standard procedure in which they were rewarded for avoiding previously visited arms. The Prh + Fx, but not the Prh, rats produced significantly more errors (re-entries) and these started significantly earlier in each session when compared with a surgical control group. The nonspatial task was a test of spontaneous object recognition in which rats were tested on their ability to discriminate between a familiar and a novel object. For the initial tests the Prh group failed to discriminate between the objects, but the Prh + Fx group showed a clear preference for the novel object. Observation of the test showed, however, that the Prh + Fx group were spending a greater length of time initially exploring the sample (familiar) object. When the amount of exposure to the sample object was limited to either 20 or 40 s (i.e. was the same for all three groups), the Prh + Fx group now failed to discriminate between the two objects. This change was especially evident for shorter sample duration (20 s). The Prh group did, however, show an amelioration of their deficit with this further testing. The present results support previous dissociation between spatial and nonspatial working memory, and indicate that there may be some recovery of function following perirhinal cortical damage.     

The effect of excitotoxic lesions centered on the hippocampus or perirhinal cortex in object recognition and spatial memory tasks.

Rats with bilateral ibotenic acid lesions centered on the hippocampus (HPC) or perirhinal cortex (PRC) and sham-operated controls were tested in a series of object recognition and spatial memory tasks. Both HPC and PRC rats displayed reduced habituation in a novel environment and were impaired in an object-location task. HPC rats were severely impaired in both the reference and working-memory versions of the water maze and radial arm maze tasks. In contrast, although PRC rats displayed mild deficits in the reference memory version of the water maze and radial arm maze tasks, they were markedly impaired in the working-memory version of both the tasks. These findings demonstrate that under certain conditions both the HPC and PRC play a role in the processing of spatial memory. Further investigation of these conditions will provide important new insights into the role of these structures in memory processes. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

When is early visual cortex activated during visual mental imagery?

Although many neuroimaging studies of visual mental imagery have revealed activation in early visual cortex (Areas 17 or 18), many others have not. The authors review this literature and compare how well 3 models explain the disparate results. Each study was coded 1 or 0, indicating whether activation in early visual cortex was observed, and sets of variables associated with each model were fit to the observed results using logistic regression analysis. Three variables predicted all of the systematic differences in the probability of activation across studies. Two of these variables were identified with a perceptual anticipation theory, and the other was identified with a methodological factors theory. Thus, the variability in the literature is not random. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

The Von Restorff effect in visual object recognition memory in humans and monkeys. The role of frontal/perirhinal interaction

This study reports the development of a new, modified delayed matching to sample (DMS) visual recognition memory task that controls the relative novelty of test stimuli and can be used in human and nonhuman primates. We report findings from normal humans and unoperated monkeys, as well as three groups of operated monkeys. In the study phase of this modified paradigm, subjects studied lists of two-dimensional visual object stimuli. In the test phase each studied object was presented again, now paired with a new stimulus (a foil), and the subject had to choose the studied item. In some lists one study item (the novel or isolate item) and its associated foil differed from the others (the homogenous items) along one stimulus dimension (color). The critical experimental measure was the comparison of the visual object recognition error rates for isolate and homogenous test items. This task was initially administered to human subjects and unoperated monkeys. Error rates for both groups were reliably lower for isolate than for homogenous stimuli in the same list position (the von Restorff effect). The task was then administered to three groups of monkeys who had selective brain lesions. Monkeys with bilateral lesions of the amygdata and fornix, two structures that have been proposed to play a role in novelty and memory encoding, were similar to normal monkeys in their performance on this task. Two further groups--with disconnection lesions of the perirhinal cortex and either the prefrontal cortex or the magnocellular mediodorsal thalamus--showed no evidence of a von Restorff effect. These findings are not consistent with previous proposals that the hippocampus and amygdala constitute a general novelty processing network. Instead, the results support an interaction between the perirhinal and frontal cortices in the processing of certain kinds of novel information that support visual object recognition memory.     

Suppression to visual, auditory, and gustatory stimuli habituates normally in rats with excitotoxic lesions of the perirhinal cortex.

In 3 habituation experiments, rats with excitotoxic lesions of the perirhinal cortex were found to be indistinguishable from control rats. Two of the habituation experiments examined the habituation of suppression of responding on an appetitive, instrumental baseline. One of those experiments used stimuli selected from the visual modality (lights), the other used auditory stimuli. The third experiment examined habituation of suppression of novel-flavored water consumption. In contrast to the null results on the habituation experiments, the perirhinal lesions disrupted transfer performance on a configural, visual discrimination, indicating the behavioral effectiveness of the lesions. Implications for comparator theories of habituation are considered, and it is concluded that others’ demonstrations of the sensitivity of object recognition to perirhinal cortex damage is not the result of standard habituation. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Human primary visual cortex and lateral geniculate nucleus activation during visual imagery

The functional magnetic resonance (fMRI) technique can be robustly used to map functional activation of the visual pathway including the primary visual cortex (V1), the lateral geniculate nucleus (LGN), and other nuclei of humans during visual perception stimulation. One of the major controversies in visual neuroscience is whether lower-order visual areas involve the visual imagery process. This issue was examined using fMRI at high magnetic field. It was demonstrated for the first time that the LGN was activated during visual imagery process in the human brain together with V1 and other activation. There was a tight coupling of the activation between V1 and the LGN during visual imagery.     

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)     

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)     

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.     

Contributions of postrhinal and perirhinal cortex to contextual information processing.

The role of the postrhinal cortex (POR) and the perirhinal cortex (PER) in processing relational or contextual information was examined with Pavlovian fear conditioning. Rats with electrolytic or neurotoxic lesions of the POR or PER were tested in 2 contextual fear conditioning paradigms. In Experiment 1, electrolytic lesions of the POR or PER produced impairments in contextual fear conditioning but not in conditioning to a phasic auditory conditioned stimulus. Neurotoxic lesions of the POR or PER likewise resulted in anterograde (Experiment 2) and retrograde (Experiment 3) deficits in fear conditioning to the training context in an unsignaled shock paradigm. The results suggest that operations performed on sensory information by the POR and PER are necessary to support contextual learning. (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Variability in visual cortex activation during prolonged functional magnetic resonance imaging

OBJECTIVE: To review the EEG features of ring 20 syndrome in two patients and determine the characteristic pattern of this syndrome. The features of our cases and 24 patients reported in the literature will be discussed. SUBJECTS AND METHODS: Report of two patients and review of literature. RESULTS: The two patients had intractable epilepsy since childhood. Their clinical seizures were mostly complex partial seizures. Often the patients seizures were of prolonged duration. Ictal EEG revealed characteristic slow waves, and sharp waves. The slow waves were (1) usually synchronous high-voltage slow waves with or without a spike component predominantly in the frontal and frontopolar areas, (2) sometimes showed a change in frequency every several seconds, (3) continued for a long period, and (4) easily spread diffusely. The sharp waves were 5-6 Hz irregular and diffuse discontinuous sharp waves, and sometimes appeared predominantly in the centroparietal area. The clinical seizure pattern and EEG findings were similar in the 24 published cases. CONCLUSIONS: These EEG findings may be a characteristic feature of ring 20 syndrome and thus may be useful as a diagnostic clue.     

Effects of orthographic neighborhood in visual word recognition: cross-task comparisons

Effects of orthographic neighborhood in visual word recognition in Spanish were examined in 5 paradigms: progressive demasking, standard lexical decision, lexical decision with blocking of neighborhood density, naming, and semantic categorization. The results showed inhibitory effects of neighborhood frequency in the progressive-demasking task, in both lexical-decision tasks, as well as for low-density words in the naming task, and for high-density words in the semantic-categorization task. Higher levels of neighborhood density produced an inhibitory trend in the progressive-demasking task, facilitation in lexical decision (significant only when neighborhood density was blocked), and a robust facilitation effect in naming (only for words with higher frequency neighbors). A global analysis across tasks and one simulation study helped outline some of the underlying task-specific and task-independent mechanisms.     

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)     

Attention constraints of semantic activation during visual word recognition.

The reduction of semantic priming following letter search of the prime suggests that semantic activation can be blocked if attention is allocated to the letter level during word processing. Is this true even for the very fast-acting component of semantic activation? To test this, the authors explored semantic priming of lexical decision at stimulus onset asynchronies (SOAs) of either 200 or 1000 ms. Following semantic prime processing, priming occurred at both SOAs. In contrast, no priming occurred at the long SOA following letter-level processing. Of greatest interest, at the short SOA there was priming following the less demanding consonant/vowel task but not following the more attention-demanding letter search task. Hence, semantic activation can occur even when attention is directed to the letter level, provided there are sufficient resources to support this activation. The authors conclude that the default setting during word recognition is for fast-acting activation of the semantic system. (PsycINFO Database Record (c) 2010 APA, all rights reserved)