Dissociation between the effects of damage to perirhinal cortex and area TE

Distribution of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-type glutamate receptor subunits (GluR2/3) along the ventral visual pathway in the monkey

By using immunohistochemical methods, we examined the distribution of cells expressing subunits of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-selective glutamate receptors (GluR2/3) in the cortical areas of the occipitotemporal pathway in monkeys. GluR2/3-immunoreactive (-ir) cells were primarily pyramidal cells; this category, however, also included large stellate cells in layer IVB of the striate cortex (V1) and fusiform cells in layer VI of all the areas examined. GluR2/3 immunoreactivity differed among the areas in laminar distribution and intensity. In V1, GluR2/3-ir cells were identified mainly in layers II, III, IVB, and VI. The prestriate areas V2 and V4 and the inferior temporal areas TEO and TE contained GluR2/3-ir cells in layers II, III, and VI. In the TE, GluR2/3-ir cells were also abundant in layer V. In area 36 of the perirhinal cortex, neurons in layers II, III, V, and VI were labeled in a similar manner to the TE labeling, but with greater staining intensity and numbers, especially in layer V. Thus, GluR2/3 immunoreactivity increased rostrally along the pathway. Within V1 and V2, cells strongly stained for GluR2/3 formed clusters that colocalized with cytochrome oxidase (CO)-rich regions. These distinct laminar and regional distribution patterns of GluR2/3 expression may contribute to the specific physiological properties of neurons within various visual areas and compartments.

Impairments in visual discrimination after perirhinal cortex lesions: testing 'declarative' vs. 'perceptual-mnemonic' views of perirhinal cortex function

Two experiments tested the predictions of 'declarative' vs. 'perceptual-mnemonic' views of perirhinal cortex function. The former view predicts that perirhinal cortex lesions should impair rapidly learned, but not more slowly learned, visual discriminations, whereas the latter view predicts that impairments should be related not to speed of learning but to perceptual factors. It was found that monkeys with perirhinal cortex lesions were impaired in the acquisition and performance of slowly learned, perceptually difficult greyscale picture discriminations, but were not impaired in the acquisition of rapidly learned, perceptually easier discriminations. In addition, these same monkeys were not impaired in the acquisition or performance of difficult colour or size discriminations, indicating that the observed pattern of impairments was not due to ceiling effects or difficulty per se. These findings, taken together, are consistent with the 'perceptual-mnemonic' view that the perirhinal cortex is involved in both perception and memory, but are not consistent with the 'declarative' view that the perirhinal cortex is important exclusively for declarative memory, having little or no role in perception. Moreover, the results are consistent with the more specific proposal that the perirhinal cortex contributes to the solution of complex visual discriminations with a high degree of 'feature ambiguity', a property of visual discrimination problems that can emerge when features of an object are rewarded when part of one object, but not when part of another. These and other recent findings suggest the need for a revision of prevailing views regarding the neural organization of perception and memory.

Recognition memory and the human hippocampus

The capacity for declarative memory depends on the hippocampal region and adjacent cortex within the medial temporal lobe. One of the most widely studied examples of declarative memory is the capacity to recognize recently encountered material as familiar, but uncertainty remains about whether intact recognition memory depends on the hippocampal region itself and, if so, what the nature of the hippocampal contribution might be. Seven patients with bilateral damage thought to be limited primarily to the hippocampal region were impaired on three standard tests of recognition memory. In addition, the patients were impaired to a similar extent at Remembering and Knowing, measures of the two processes thought to support recognition performance: the ability to remember the learning episode (episodic recollection) and the capacity for judging items as familiar (familiarity).

The contribution of visual areas to speech comprehension: a PET study in cochlear implants patients and normal-hearing subjects

Early visual cortex can be recruited by meaningful sounds in the absence of visual information. This occurs in particular in cochlear implant (CI) patients whose dependency on visual cues in speech comprehension is increased. Such cross-modal interaction mirrors the response of early auditory cortex to mouth movements (speech reading) and may reflect the natural expectancy of the visual counterpart of sounds, lip movements. Here we pursue the hypothesis that visual activations occur specifically in response to meaningful sounds. We performed PET in both CI patients and controls, while subjects listened either to their native language or to a completely unknown language. A recruitment of early visual cortex, the left posterior inferior temporal gyrus (ITG) and the left superior parietal cortex was observed in both groups. While no further activation occurred in the group of normal-hearing subjects, CI patients additionally recruited the right perirhinal/fusiform and mid-fusiform, the right temporo-occipito-parietal (TOP) junction and the left inferior prefrontal cortex (LIPF, Broca's area). This study confirms a participation of visual cortical areas in semantic processing of speech sounds. Observation of early visual activation in normal-hearing subjects shows that auditory-to-visual cross-modal effects can also be recruited under natural hearing conditions. In cochlear implant patients, speech activates the mid-fusiform gyrus in the vicinity of the so-called face area. This suggests that specific cross-modal interaction involving advanced stages in the visual processing hierarchy develops after cochlear implantation and may be the correlate of increased usage of lip-reading.

Striatal activity in concept learning

Striatal learning systems have been implicated in learning relationships between visual stimuli and outcomes. In the present study, the activity of the striatum during visual concept learning in humans was examined by using functional magnetic resonance imaging (fMRI). Participants performed three concept-learning tasks and a baseline task. The participants were trained to criterion before fMRI scanning on two tasks, verbal and implicit. In the verbal task, classification could be performed on the basis of a simple verbal rule, but in the implicit task, there was no simple verbal rule. The novel-implicit learning task, in which an implicit structure was used, was not encountered by the participants before scanning. Across all three concept-learning tasks, there was significant activation in the striatum, in comparison with the baseline task. The striatum was recruited similarly in classification when the participants had different levels of expertise (novel-implicit vs. verbal and implicit) and were able to verbalize their learning to different degrees (verbal vs. implicit and novel-implicit). There was left lateral occipital activation when learning was implicit (implicit and novel-implicit), but not when learning was easily verbalized (verbal).

Learning of discriminations is impaired, but generalization to altered views is intact, in monkeys (Macaca mulatta) with perirhinal cortex removal

Rhesus monkeys (Macaca mulatta) were taught a large number of visual discriminations and then either received bilateral removal of the perirhinal cortex or were retained as unoperated controls. Operated monkeys were impaired in retention of the preoperatively learned problems. To test for generalization to novel views, the monkeys were required to discriminate, in probe trials, familiar pairs of images that were rotated, enlarged, shrunken, presented with color deleted, or degraded by masks. Although these manipulations reduced accuracy in both groups, the operated group was not differentially affected. In contrast, the same operated monkeys were impaired in reversal of familiar discriminations and in acquisition of new single-pair discriminations. These results indicate an important role for perirhinal cortex in visual learning, memory, or both, and show that under a variety of conditions, perirhinal cortex is not critical for the identification of stimuli.

Excitotoxic lesions of the rhinal cortex in the baboon differentially affect visual recognition memory, habit memory and spatial executive functions

To specify the functional role of the rhinal cortex, baboons with bilateral excitotoxic lesions of the rhinal cortex (RH group) were tested on a series of computerized memory and learning tasks. Preoperatively, they were trained to and then tested on a delayed nonmatching-to-sample (DNMS) task with trial-unique stimuli. Postoperatively, this visual recognition memory task was given twice. As compared to a sham-operated group, the RH group showed good retention of rule learning and were unimpaired on the Delay memory subtest. Performance on the List Length memory subtest was, however, severely impaired at both postoperative evaluations, with a significant negative correlation between cognitive performance and neuronal loss in rhinal areas. Visual habit memory and spatial working memory were assessed postoperatively only, using a concurrent discrimination learning task and both a delayed-response task (with a two- and four-location choice) and a delayed alternation task, respectively. The RH group was unimpaired on the first two tasks and was even faster than the controls in learning the delayed-response task with four locations. Finally, most RH baboons failed to learn the delayed alternation task within the limits of testing. These results indicate that neuronal loss in the rhinal cortex is sufficient to impair visual recognition memory, and extend the implication of this area to spatial executive functions. Furthermore, the observation of impaired recognition memory and executive processes with preserved procedural memory and retrograde memory suggests that damage to the rhinal cortex probably participates in the cognitive deficits typical of the early stages of Alzheimer's disease.

Selective zif268 mRNA induction in the perirhinal cortex of macaque monkeys during formation of visual pair-association memory

To elucidate the molecular basis of cognitive memory formation in the primate, transcriptional activation during learning of a visual pair-association (PA) task was evaluated systematically along the occipito-temporo-hippocampal pathway in the macaque monkey brain. Split-brain monkeys were used for intra-animal comparison, which enables elimination of animal-to-animal variation in gene expression. We found that the expression of the mRNA of an immediate-early gene (IEG), zif268, was up-regulated selectively in the perirhinal cortex (area 36) during the formation of PA memory compared to that during learning of a visual control task. The mRNA expression levels of another IEG, c-jun, were not up-regulated during the PA learning in any cortical areas examined. We also showed that cells strongly expressing zif268 mRNA accumulated in patches in area 36 during learning of the PA task. As the zif268 gene encodes a transcription factor, these results suggest that the activation of zif268 mRNA in area 36 may function as a trigger of the cascade of gene activation that leads to cellular events underlying neuronal reorganization for visual long-term memory formation.

Verbal memory in left temporal lobe epilepsy: evidence for task-related localization

We explored the hypothesis that components of verbal memory are subserved by separate temporal lobe structures in patients with temporal lobe structures in patients with temporal lobe epilepsy [correction]. Uptake of 18F-fluorodeoxyglucose (FDG) measured by positron emission tomography, hippocampal volume, and memory for arbitrarily and semantically related verbal paired associates were examined in 27 patients with left temporary lobe epilepsy. Scores from memory tests performed outside the scanner were regressed against normalized resting FDG uptake at each voxel. Significant regression was seen in the left perirhinal cortex (Talaraich coordinates x, y, z: -29, 10, -34; p < 0.05) for arbitrarily related word pairs. For semantically related paired associates, significant regression was present in the left inferior temporal gyrus (x, y, z: -48, -18, -24; p < 0.05). In subsequent analyses, mean FDG uptake within a spherical region of interest centered on the perirhinal peak predicted performance on both tasks. Mean FDG uptake in a region of interest centered on the inferior temporal peak made an additional, independent contribution to memory for semantically related pairs. Hippocampal volumes did not explain any additional variance in memory scores. Our findings indicate that heterogeneity in the left temporal lobe structures mediating verbal memory function, and support the view that the perirhinal cortex is an important mnemonic substrate.

Selective perceptual impairments after perirhinal cortex ablation

It has been suggested that the primate perirhinal cortex contributes exclusively to memory. However, recent studies in macaque monkeys have implied that the perirhinal cortex may also contribute to object perception. To investigate whether the perirhinal cortex does contribute to perception, we devised several perceptual oddity tasks in which monkeys had to choose which stimulus of several presented concurrently on a touch screen was different. Macaques with bilateral perirhinal cortex ablations were selectively impaired relative to controls at perceptually discriminating the odd stimulus when the odd stimulus was a different object and when the discrimination could not be done on the basis of simple differences in features between the stimuli. They remained unimpaired relative to controls on discriminating the odd stimulus when the odd stimulus was a different color, a different shape, or a different size even when these discriminations were extremely difficult. They were also impaired on human and monkey face oddity tasks and oddity tasks with scenes containing objects. Therefore, we reject the notion that the macaque perirhinal cortex has a role exclusive to memory and conclude that the macaque perirhinal cortex does contribute to perception. We argue that the perirhinal cortex is neither specialized for perception nor memory processes alone, but rather, is specialized for processing stimuli that require processing at a more abstract level such as at the level of an object and that the perirhinal cortex contributes to both memory and perception of such stimuli.

The organization of visual object representations: a connectionist model of effects of lesions in perirhinal cortex

We have developed a simple connectionist model based on the idea that perirhinal cortex has properties similar to other regions in the ventral visual stream, or 'what' pathway. The model is based on the assumption that representations in the ventral visual stream are organized hierarchically, such that representations of simple features of objects are stored in caudal regions of the ventral visual stream, and representations of the conjunctions of these features are stored in more rostral regions. We propose that a function of these feature conjunction representations is to help to resolve 'feature ambiguity', a property of visual discrimination problems that can emerge when features of an object predict a given outcome (e.g. reward) when part of one object, but predict a different outcome when part of another object. Several recently reported effects of lesions of perirhinal cortex in monkeys have provided key insights into the functions of this region. In the present study these effects were simulated by comparing the performance of connectionist networks before and after removal of a layer of units corresponding to perirhinal cortex. The results of these simulations suggest that effects of lesions in perirhinal cortex on visual discrimination may be due not to the impairment of a specific type of learning or memory, such as declarative or procedural, but to compromising the representations of visual stimuli. Furthermore, we propose that attempting to classify perirhinal cortex function as either 'perceptual' or 'mnemonic' may be misguided, as it seems unlikely that these broad constructs will map neatly onto anatomically defined regions of the brain.

Perirhinal cortex resolves feature ambiguity in complex visual discriminations

The present experiment tested predictions of a 'perceptual-mnemonic/feature conjunction' (PMFC) model of perirhinal cortex function. The model predicts that lesions of perirhinal cortex should disrupt complex visual discriminations with a high degree of 'feature ambiguity', a property of visual discrimination problems that can emerge when features of an object are rewarded when they are part of one object, but not when part of another. As feature ambiguity is thought to be the critical factor, such effects should be independent of the number of objects to be discriminated. This was tested directly, by assessing performance of control monkeys and monkeys with aspiration lesions of perirhinal cortex on a series of concurrent discriminations in which the number of object pairs was held constant, but the degree of feature ambiguity was varied systematically. Monkeys were tested in three conditions: Maximum Feature Ambiguity, in which all features were explicitly ambiguous (AB+, CD+, BC-, AD-; the biconditional problem); Minimum Feature Ambiguity, in which no features were explicitly ambiguous (AB+, CD+, EF-, GH-); and Intermediate Feature Ambiguity, in which half the features were explicitly ambiguous (AB+, CD+, CE-, AF-). The pattern of results closely matched that predicted by simulations using a connectionist network: monkeys with perirhinal cortex lesions were unimpaired in the Minimum Feature Ambiguity condition, mildly impaired in the Intermediate Feature Ambiguity condition and severely impaired in the Maximum Feature Ambiguity condition. These results confirm the predictions of the PMFC model, and force a reconsideration of prevailing views regarding perirhinal cortex function.

Selective perceptual impairments after perirhinal cortex ablation

It has been suggested that the primate perirhinal cortex contributes exclusively to memory. However, recent studies in macaque monkeys have implied that the perirhinal cortex may also contribute to object perception. To investigate whether the perirhinal cortex does contribute to perception, we devised several perceptual oddity tasks in which monkeys had to choose which stimulus of several presented concurrently on a touch screen was different. Macaques with bilateral perirhinal cortex ablations were selectively impaired relative to controls at perceptually discriminating the odd stimulus when the odd stimulus was a different object and when the discrimination could not be done on the basis of simple differences in features between the stimuli. They remained unimpaired relative to controls on discriminating the odd stimulus when the odd stimulus was a different color, a different shape, or a different size even when these discriminations were extremely difficult. They were also impaired on human and monkey face oddity tasks and oddity tasks with scenes containing objects. Therefore, we reject the notion that the macaque perirhinal cortex has a role exclusive to memory and conclude that the macaque perirhinal cortex does contribute to perception. We argue that the perirhinal cortex is neither specialized for perception nor memory processes alone, but rather, is specialized for processing stimuli that require processing at a more abstract level such as at the level of an object and that the perirhinal cortex contributes to both memory and perception of such stimuli.

Imaging plasticity in cochlear implant patients

Auditory re-afferentation by cochlear implants (CI) offers a unique opportunity to study directly from within the auditory modality plastic changes taking place at organisational levels up to the supra- or polymodal level. These plastic changes resulting from deafness and chronic electrical stimulation can be studied using modern neuroimaging techniques. In this paper, we review the available techniques and the experimental approaches to human studies of plasticity, we discuss the different forms of plasticity that are associated with cochlear implantation and we point to the interest of imaging studies for providing a prognosis of functional outcome after implantation.

NMDA and muscarinic blockade in the perirhinal cortex impairs object discrimination in rats

To determine the possible involvement of NMDA and muscarinic activation of the perirhinal cortex in object discrimination, an NMDA antagonist, D,L-2-amino-5-phosphonopentanoic acid (AP5), and a muscarinic antagonist, scopolamine (SCP) were injected into the perirhinal cortex of rats. Each drug at the higher dose (AP5 60 mM, SCP 80 mM) significantly decreased correct choices on the retention test of object discrimination. SCP, but not AP5, also significantly increased response latency, but this increase was not necessarily related to the time spent for a choice. These results suggest that activation of both NMDA and muscarinic receptors contributes to object discrimination.

Neuronal representations of stimulus associations develop in the temporal lobe during learning

Visual stimuli that are frequently seen together become associated in long-term memory, such that the sight of one stimulus readily brings to mind the thought or image of the other. It has been hypothesized that acquisition of such long-term associative memories proceeds via the strengthening of connections between neurons representing the associated stimuli, such that a neuron initially responding only to one stimulus of an associated pair eventually comes to respond to both. Consistent with this hypothesis, studies have demonstrated that individual neurons in the primate inferior temporal cortex tend to exhibit similar responses to pairs of visual stimuli that have become behaviorally associated. In the present study, we investigated the role of these areas in the formation of conditional visual associations by monitoring the responses of individual neurons during the learning of new stimulus pairs. We found that many neurons in both area TE and perirhinal cortex came to elicit more similar neuronal responses to paired stimuli as learning proceeded. Moreover, these neuronal response changes were learning-dependent and proceeded with an average time course that paralleled learning. This experience-dependent plasticity of sensory representations in the cerebral cortex may underlie the learning of associations between objects.

Neither perirhinal/entorhinal nor hippocampal lesions impair short-term auditory recognition memory in dogs

Visual, tactile, and olfactory recognition memory in animals is mediated in part by the perirhinal/entorhinal (or rhinal) cortices and, possibly, the hippocampus. To examine the role of these structures in auditory memory, we performed rhinal, hippocampal, and combined lesions in groups of dogs trained in auditory delayed matching-to-sample with trial-unique sounds. The sample sound was presented through a central speaker and, after a delay, the sample sound and a different sound were played alternately through speakers placed on either side of the animal; the animal was rewarded for responding to the side emitting the sample sound. None of the lesion groups showed significant impairment in comparison either to their own preoperative performance or to the performance of intact control dogs. This was the case both for relearning the delayed matching rule at a delay of 1.5 s and for task performance at variable delays ranging from 10 to 90 s. From these findings we suggest that the tissue critical for auditory recognition memory is located outside both the perirhinal/entorhinal cortices and the hippocampus.

Brain activity during the encoding, retention, and retrieval of stimulus representations

Studies of delayed nonmatching-to-sample (DNMS) performance following lesions of the monkey cortex have revealed a critical circuit of brain regions involved in forming memories and retaining and retrieving stimulus representations. Using event-related functional magnetic resonance imaging (fMRI), we measured brain activity in 10 healthy human participants during performance of a trial-unique visual DNMS task using novel barcode stimuli. The event-related design enabled the identification of activity during the different phases of the task (encoding, retention, and retrieval). Several brain regions identified by monkey studies as being important for successful DNMS performance showed selective activity during the different phases, including the mediodorsal thalamic nucleus (encoding), ventrolateral prefrontal cortex (retention), and perirhinal cortex (retrieval). Regions showing sustained activity within trials included the ventromedial and dorsal prefrontal cortices and occipital cortex. The present study shows the utility of investigating performance on tasks derived from animal models to assist in the identification of brain regions involved in human recognition memory.

Impaired auditory recognition memory in amnesic patients with medial temporal lobe lesions

Two tests of auditory recognition memory were given to four patients with bilateral hippocampal damage (H+) and three patients with large medial temporal lobe lesions and additional variable damage to lateral temporal cortex (MTL+). When single stimuli were presented, performance was normal across delays as long as 30 sec, presumably because information could be maintained in working memory through rehearsal. When lists of 10 stimuli were presented, performance was impaired after a 5-min delay. Patients with MTL+ lesions performed marginally worse than patients with H+ lesions, consistent with findings for recognition memory in other modalities. The findings show that auditory recognition, like recognition memory in other sensory modalities, is dependent on the medial temporal lobe.

Opposite relationship of hippocampal and rhinal cortex damage to delayed nonmatching-to-sample deficits in monkeys

Three recent studies in macaque monkeys that examined the effects on memory of restricted hippocampal lesions (Murray and Mishkin, J Neurosci 1998;18:6568-6582; Beason-Held et al., Hippocampus 1999;9:562-574; Zola et al., J Neurosci 2000;20:451-463) differed in their conclusions about the involvement of the hippocampus in recognition memory. Because these experiments used a common behavioral procedure, trial-unique visual delayed nonmatching-to-sample (DNMS), a quantitative synthesis ("meta-analysis") was performed to determine whether hippocampal lesions produced a reliable net impairment in DNMS performance, and whether this impairment was related to the magnitude of hippocampal damage. A similar analysis was performed on data from monkeys with perirhinal or rhinal cortex damage (Meunier et al., J Neurosci 1993;13:5418-5432; Buffalo et al., Learn Mem 1999;6:572-599). DNMS performance scores were transformed to d' values to permit comparisons across studies, and a loss in d' score, a measure of the magnitude of the recognition deficit relative to the control group, was calculated for each operated monkey. Two main findings emerged. First, the loss in d' following hippocampal damage was reliably larger than zero, but was smaller than that found after lesions limited to the perirhinal cortex. Second, the correlation of loss in d' with extent of hippocampal damage was large and negative, indicating that greater impairments were associated with smaller hippocampal lesions. This relationship was opposite to that between loss in d' and rhinal cortex damage, for which larger lesions were associated with greater impairment. These findings indicate that damage to the hippocampus and to the rhinal cortex affects recognition memory in different ways. Furthermore, they provide a framework for understanding the seemingly disparate effects of hippocampal damage on recognition memory in monkeys, and by extension, for interpreting the conflicting reports on the effects of such damage on recognition memory abilities in amnesic humans.

Multiple brain-memory systems: the whole does not equal the sum of its parts

Most contemporary theories of memory are based on the assumption that memory can be divided into multiple psychological systems that are subserved by different neural substrates and that contribute to performance in a relatively independent manner. Although the study of individual memory systems has proved to be enormously useful, recent data increasingly point towards complex interactions between memory systems during performance of any given memory task. Three basic classes of interactions between different memory systems (competition, synergism and independence) are presented that appear to be congruent with the findings of many behavioral studies. Consideration of interactions among multiple memory systems will enhance our current understanding of memory by encouraging the view that memory systems are dynamic interactive units, rather than independent modules that act in isolation.

Role of perirhinal cortex in object perception, memory, and associations

The perirhinal cortex plays a key role in acquiring knowledge about objects. It contributes to at least four cognitive functions, and recent findings provide new insights into how the perirhinal cortex contributes to each: first, it contributes to recognition memory in an automatic fashion; second, it probably contributes to perception as well as memory; third, it helps identify objects by associating together the different sensory features of an object; and fourth, it associates objects with other objects and with abstractions.

Visual habit formation in monkeys with neurotoxic lesions of the ventrocaudal neostriatum

Visual habit formation in monkeys, assessed by concurrent visual discrimination learning with 24-h intertrial intervals (ITI), was found earlier to be impaired by removal of the inferior temporal visual area (TE) but not by removal of either the medial temporal lobe or inferior prefrontal convexity, two of TE's major projection targets. To assess the role in this form of learning of another pair of structures to which TE projects, namely the rostral portion of the tail of the caudate nucleus and the overlying ventrocaudal putamen, we injected a neurotoxin into this neostriatal region of several monkeys and tested them on the 24-h ITI task as well as on a test of visual recognition memory. Compared with unoperated monkeys, the experimental animals were unaffected on the recognition test but showed an impairment on the 24-h ITI task that was highly correlated with the extent of their neostriatal damage. The findings suggest that TE and its projection areas in the ventrocaudal neostriatum form part of a circuit that selectively mediates visual habit formation.

Detection and explanation of sentence ambiguity are unaffected by hippocampal lesions but are impaired by larger temporal lobe lesions

We address the recent suggestion that the "hippocampal system" is important for understanding ambiguities in language (MacKay et al., J Cogn Neurosci 1998;10:377-394). Seven amnesic patients and 11 controls first decided whether a sentence was ambiguous and then tried to explain the ambiguity. Three amnesic patients with damage limited to the hippocampal formation and one amnesic patient with primarily diencephalic damage performed like the controls in all respects. Thus, the ability to comprehend ambiguity is independent of the hippocampal formation. By contrast, three patients with larger temporal lobe lesions, which extended beyond the medial temporal lobe, were impaired to about the same degree as the noted amnesic patient H.M. (as reported by Lackner, Neuropsychologia 1974;12:199-207; MacKay et al., J Cogn Neurosci 1998;10:377-394). Patient H.M., like our 3 impaired patients, has some damage outside the medial temporal lobe. However, patient H.M. also had additional difficulties on these and other language tests that the patients with larger temporal lobe lesions did not exhibit. Accordingly, it is possible that H.M.'s impairment has a different basis.

A neural circuit analysis of visual recognition memory: role of perirhinal, medial, and lateral entorhinal cortex

Using a continuous recognition memory procedure for visual object information, we sequentially presented rats with eight novel objects and four repeated objects (chosen from the 8). These were selected from 120 different three-dimensional objects of varying sizes, shapes, textures, and degree of brightness. Repeated objects had lags ranging from 0 to 4 (from 0 to 4 different objects between the first and repeated presentation). An object was presented on one side of a long table divided in half by an opaque Plexiglas guillotine door, and the latency between opening the door and the rat moving the object was measured. The first presentation of an object resulted in reinforcement, but repeated presentations did not result in a reinforcement. After completion of acquisition training (significantly longer latencies for repeated presentation compared with the first presentation of an object), rats received lesions of the perirhinal, medial, or lateral entorhinal cortex or served as sham operated controls. On the basis of postsurgery testing and additional tests, the results indicated that rats with perirhinal cortex lesions had a sustained impairment in performing the task. There were no sustained deficits with medial or lateral entorhinal cortex lesions. The data suggest that recognition memory for visual object information is mediated primarily by the perirhinal cortex but not by the medial or lateral entorhinal cortex.

Impairments in visual discrimination learning and recognition memory produced by neurotoxic lesions of rhinal cortex in rhesus monkeys

Much work on the cognitive functions of the primate rhinal (i.e. entorhinal plus perirhinal) cortex has been based on aspiration lesions of this structure, which might disrupt fibres passing nearby and through the rhinal cortex in addition to removing the cell bodies of the rhinal cortex itself. To determine whether damage limited to the cell bodies of the rhinal cortex is sufficient to impair visual learning and memory, four rhesus monkeys (Macaca mulatta) were preoperatively trained on a battery of visual learning and memory tasks, including single-pair discrimination learning for primary reinforcement, single-pair discrimination reversals, concurrent discrimination learning and reversal, and delayed matching-to-sample. Following acquisition of these tasks and a preoperative performance test, ibotenic acid was injected bilaterally into the rhinal cortex, and the monkeys were retested. Consistent with the results of studies using aspiration lesions, the monkeys were impaired on single-pair discrimination learning as well as recognition memory performance postoperatively, although reliable reversal learning impairments were not observed. The magnitude of postoperative impairment in discrimination learning was not correlated with the magnitude of postoperative impairment in recognition memory, suggesting a possible dissociation between these functions within the rhinal cortex. The correspondence of behavioural deficits following aspiration and neurotoxic lesions of the rhinal cortex validates the attribution of various cognitive functions to this structure, based on the results of studies with aspiration lesions.

Neural correlates of olfactory recognition memory in the rat orbitofrontal cortex

The orbitofrontal cortex (OF) is strongly and reciprocally connected with the perirhinal (PR) and entorhinal areas of the medial temporal lobe and plays an important role in odor recognition memory. This study characterized firing patterns of single neurons in the OF of rats performing a continuous odor-guided delayed nonmatch to sample (DNMS) task. Most OF neurons fired in association with one or more task events, including the initiation of trials, the sampling of odor stimuli, and the consumption of rewards. OF neurons also exhibited sustained odor-selective activity during the memory delay, and a large proportion of OF cells had odor-specific enhanced or suppressed responses on stimulus repetition. Most OF neurons were activated during several task events, or associated with complex behavioral states. The incidence of cells that fired in association with the critical match/non-match judgement was increased as the DNMS rule was learned, and was higher in OF than in perirhinal and entorhinal cortex. Furthermore, the classification of match and nonmatch trials was correlated with accuracy in performance of that judgement. These findings are consistent with the view that OF is a high order association cortex that plays a role both in the memory representations for specific stimuli and in the acquisition and application of task rules.

Backward spreading of memory-retrieval signal in the primate temporal cortex

Bidirectional signaling between neocortex and limbic cortex has been hypothesized to contribute to the retrieval of long-term memory. We tested this hypothesis by comparing the time courses of perceptual and memory-retrieval signals in two neighboring areas in temporal cortex, area TE (TE) and perirhinal cortex (PRh), while monkeys were performing a visual pair-association task. Perceptual signal reached TE before PRh, confirming its forward propagation. In contrast, memory-retrieval signal appeared earlier in PRh, and TE neurons were then gradually recruited to represent the sought target. A reasonable interpretation of this finding is that the rich backward fiber projections from PRh to TE may underlie the activation of TE neurons that represent a visual object retrieved from long-term memory.

Clustering of perirhinal neurons with similar properties following visual experience in adult monkeys

The functional organization of early visual areas seems to be largely determined during development. However, the organization of areas important for learning and memory, such as perirhinal cortex, may be modifiable in adults. To test this hypothesis, we recorded from pairs of neurons in perirhinal cortex of macaques while they viewed multiple complex stimuli. For novel stimuli, neuronal response preferences for pairs of nearby neurons and far-apart neurons were uncorrelated. However, after one day of experience with the stimuli, response preferences of nearby neurons became more similar. We conclude that specific visual experience induces development of clusters of perirhinal neurons with similar stimulus preferences.

BDNF upregulation during declarative memory formation in monkey inferior temporal cortex

In primates, visual long-term memory of objects is presumably stored in the inferior temporal (IT) cortex. Because brain-derived neurotrophic factor (BDNF) is involved in activity-dependent neural reorganization, we tested the hypothesis that BDNF would be upregulated in IT cortex during formation of visual pair-association memory. To eliminate genetic and cognitive variations between individual animals, we used split-brain monkeys for intra-animal comparison in PCR-based mRNA quantitation. The monkeys learned a pair-association (PA) task using one hemisphere and a control visual task using the other, to balance the amount of visual input. We found that BDNF was upregulated selectively in area 36 of IT cortex during PA learning, but not in areas involved in earlier stages of visual processing. In situ hybridization showed that BDNF-expressing cells were localized in a patchlike cluster. The results suggest that BDNF contributes to reorganization of neural circuits for visual long-term memory formation in the primate.

Neural correlates of olfactory recognition memory in the rat orbitofrontal cortex

The orbitofrontal cortex (OF) is strongly and reciprocally connected with the perirhinal (PR) and entorhinal areas of the medial temporal lobe and plays an important role in odor recognition memory. This study characterized firing patterns of single neurons in the OF of rats performing a continuous odor-guided delayed nonmatch to sample (DNMS) task. Most OF neurons fired in association with one or more task events, including the initiation of trials, the sampling of odor stimuli, and the consumption of rewards. OF neurons also exhibited sustained odor-selective activity during the memory delay, and a large proportion of OF cells had odor-specific enhanced or suppressed responses on stimulus repetition. Most OF neurons were activated during several task events, or associated with complex behavioral states. The incidence of cells that fired in association with the critical match/non-match judgement was increased as the DNMS rule was learned, and was higher in OF than in perirhinal and entorhinal cortex. Furthermore, the classification of match and nonmatch trials was correlated with accuracy in performance of that judgement. These findings are consistent with the view that OF is a high order association cortex that plays a role both in the memory representations for specific stimuli and in the acquisition and application of task rules.

Perception and Recognition Memory in Monkeys Following Lesions of Area TE and Perirhinal Cortex

Monkeys with lesions of perirhinal cortex (PR group) and monkeys with lesions of inferotemporal cortical area TE (TE group) were tested on a modified version of the delayed nonmatching to sample (DNMS) task that included very short delay intervals (0.5 sec) as well as longer delay intervals (1 min and 10 min). Lesions of the perirhinal cortex and lesions of area TE produced different patterns of impairment. The PR group learned the DNMS task as quickly as normal monkeys (N) when the delay between sample and choice was very short (0.5 sec). However, performance of the PR group, unlike that of the N group, fell to chance levels when the delay between sample and choice was lengthened to 10 min. In contrast to the PR group, the TE group was markedly impaired on the DNMS task even at the 0.5-sec delay, and three of four monkeys with TE lesions failed to acquire the task. The results provide support for the idea that perirhinal cortex is important not for perceptual processing, but for the formation and maintenance of long-term memory. Area TE is important for the perceptual processing of visual stimuli.

Perception and recognition memory in monkeys following lesions of area TE and perirhinal cortex

Monkeys with lesions of perirhinal cortex (PR group) and monkeys with lesions of inferotemporal cortical area TE (TE group) were tested on a modified version of the delayed nonmatching to sample (DNMS) task that included very short delay intervals (0.5 sec) as well as longer delay intervals (1 min and 10 min). Lesions of the perirhinal cortex and lesions of area TE produced different patterns of impairment. The PR group learned the DNMS task as quickly as normal monkeys (N) when the delay between sample and choice was very short (0.5 sec). However, performance of the PR group, unlike that of the N group, fell to chance levels when the delay between sample and choice was lengthened to 10 min. In contrast to the PR group, the TE group was markedly impaired on the DNMS task even at the 0.5-sec delay, and three of four monkeys with TE lesions failed to acquire the task. The results provide support for the idea that perirhinal cortex is important not for perceptual processing, but for the formation and maintenance of long-term memory. Area TE is important for the perceptual processing of visual stimuli.

A cortical-hippocampal system for declarative memory

Recent neurobiological studies have begun to reveal the cognitive and neural coding mechanisms that underlie declarative memory--our ability to recollect everyday events and factual knowledge. These studies indicate that the critical circuitry involves bidirectional connections between the neocortex, the parahippocampal region and the hippocampus. Each of these areas makes a unique contribution to memory processing. Widespread high-order neocortical areas provide dedicated processors for perceptual, motor or cognitive information that is influenced by other components of the system. The parahippocampal region mediates convergence of this information and extends the persistence of neocortical memory representations. The hippocampus encodes the sequences of places and events that compose episodic memories, and links them together through their common elements. Here I describe how these mechanisms work together to create and re-create fully networked representations of previous experiences and knowledge about the world.

Revisiting the maturation of medial temporal lobe memory functions in primates

In a review of the literature on the development of the medial temporal lobe region in humans, monkeys, and rodents, Bachevalier and Beauregard indicated that in primates, memory functions subserved by this neural system emerge early in life and increment gradually with further postnatal maturation. Furthermore, they stated that the late-developing memory functions of normal neonates was more likely owing to the slow maturation of the association areas of the cortex than to the slow maturation of the hippocampal formation. This conclusion was based on the limited knowledge concerning the development of hippocampal-dependent memory functions and the maturational events in the medial temporal lobe of monkeys. Over the last decade, however, more information has accumulated about the structural, functional, and behavioral changes occurring throughout ontogeny in monkeys that suggest a refinement of this view. Whereas there is still much to be discovered, we thought it timely to put into perspective the latest findings in hope of shedding light on memory development in general, and particularly, on the role of medial temporal lobe structures in infant and adult memory. [Note: Hippocampal formation refers to the hippocampus proper (Ammon's fields), dentate gyrus, and subicular complex. Hippocampal region refers to the hippocampal formation and the adjacent entorhinal, perirhinal, and parahippocampal cortex.]

Intact visual perceptual discrimination in humans in the absence of perirhinal cortex

While the role of the perirhinal cortex in declarative memory has been well established, it has been unclear whether the perirhinal cortex might serve an additional nonmnemonic role in visual perception. Evidence that the perirhinal cortex might be important for visual perception comes from a recent report that monkeys with perirhinal cortical lesions are impaired on difficult (but not on simple) visual discrimination tasks. We administered these same tasks to nine amnesic patients, including three severely impaired patients with complete damage to perirhinal cortex bilaterally (E.P., G.P., and G.T.). The patients performed all tasks as well as controls. We suggest that the function of perirhinal cortex as well as antero-lateral temporal cortex may differ between humans and monkeys.

The parahippocampal region and object identification

The hippocampus has long been thought to be critical for memory, including memory for objects. However, recent neuropsychological studies in nonhuman primates have indicated that other regions within the medial temporal lobe, specifically, structures in the parahippocampal region, are primarily responsible for object recognition and object identification. This article reviews the behavioral effects of removal of structures within the parahippocampal region in monkeys, and cites relevant work in rodents as well. It is argued that the perirhinal cortex, in particular, contributes to object identification in at least two ways: (i) by serving as the final stage in the ventral visual cortical pathway that represents stimulus features, and (ii) by operating as part of a network for associating together sensory inputs within and across sensory modalities.

Contrasting effects on discrimination learning after hippocampal lesions and conjoint hippocampal-caudate lesions in monkeys

Eighteen monkeys with lesions of the hippocampal region (the hippocampus proper, the dentate gyrus, and the subiculum) made by an ischemic procedure, radio frequency, or ibotenic acid were tested on a simple, two-choice object discrimination learning task that has been shown to be sensitive to large lesions of the medial temporal lobe. The monkeys were also tested on two other discrimination tasks (pattern discrimination and eight-pair concurrent discrimination) that can be learned normally by monkeys with large medial temporal lobe lesions. All of the lesion groups were impaired at learning the simple object discrimination task. Seven of the monkeys who had sustained damage to the hippocampal region also sustained damage to the tail of the caudate nucleus. These seven monkeys, but not the other 11 monkeys with hippocampal lesions, were impaired on pattern discrimination and concurrent discrimination learning. The results suggest that the hippocampal region is important for learning easy, two-choice discriminations, whereas the caudate nucleus is necessary for the normal learning of more difficult, gradually acquired discrimination tasks. The findings support the distinction between declarative memory, which depends on the hippocampus and related medial temporal lobe structures, and habit learning, which depends on the caudate nucleus.

Topographic organization of cortical inputs to the lateral nucleus of the macaque monkey amygdala: a retrograde tracing study

The objective of this study was to identify cortical areas that project to the lateral nucleus of the macaque monkey amygdaloid complex. Discrete injections of the fluorescent retrograde tracers Fast blue and Diamidino yellow were placed into different locations within the lateral nucleus. Retrogradely labeled cells were mapped using a computer-aided digitizing system. In the frontal cortex, low numbers of retrogradely labeled cells were observed in medial and orbitofrontal regions (areas 10, 11, 12, 13, 13a, and 14). In the anterior cingulate cortex, low to moderate numbers of retrogradely labeled cells were located in areas 25, 24, and 32. In the insula, there were moderate to high numbers of retrogradely labeled cells in agranular and dysgranular regions. The parainsula cortex also demonstrated a moderate to high number of retrogradely labeled cells. In the temporal lobe, retrogradely labeled cells were most numerous in the rostral (polar) portion of the perirhinal cortex. Large numbers of labeled cells were also located throughout more caudal portions of the perirhinal regions as well as in the entorhinal cortex, area TE, and the superior temporal gyrus. Fewer retrogradely labeled cells were observed in the cortex along the dorsal bank of the superior temporal sulcus, in the parahippocampal cortex, and in area TEO. Although retrograde tracers can provide only limited evidence for topography, we nonetheless noted that the density of retrogradely labeled cells in a cortical area reliably depended on the location of the tracer injection in the lateral nucleus.

Contrasting effects on discrimination learning after hippocampal lesions and conjoint hippocampal-caudate lesions in monkeys

Eighteen monkeys with lesions of the hippocampal region (the hippocampus proper, the dentate gyrus, and the subiculum) made by an ischemic procedure, radio frequency, or ibotenic acid were tested on a simple, two-choice object discrimination learning task that has been shown to be sensitive to large lesions of the medial temporal lobe. The monkeys were also tested on two other discrimination tasks (pattern discrimination and eight-pair concurrent discrimination) that can be learned normally by monkeys with large medial temporal lobe lesions. All of the lesion groups were impaired at learning the simple object discrimination task. Seven of the monkeys who had sustained damage to the hippocampal region also sustained damage to the tail of the caudate nucleus. These seven monkeys, but not the other 11 monkeys with hippocampal lesions, were impaired on pattern discrimination and concurrent discrimination learning. The results suggest that the hippocampal region is important for learning easy, two-choice discriminations, whereas the caudate nucleus is necessary for the normal learning of more difficult, gradually acquired discrimination tasks. The findings support the distinction between declarative memory, which depends on the hippocampus and related medial temporal lobe structures, and habit learning, which depends on the caudate nucleus.