Two functional components of the hippocampal memory system

Imaging recollection and familiarity in the medial temporal lobe: a three-component model

The medial temporal lobe (MTL) plays a crucial role in supporting memory for events, but the functional organization of regions in the MTL remains controversial, especially regarding the extent to which different subregions support recognition based on familiarity or recollection. Here we review results from functional neuroimaging studies showing that, whereas activity in the hippocampus and posterior parahippocampal gyrus is disproportionately associated with recollection, activity in the anterior parahippocampal gyrus is disproportionately associated with familiarity. The results are consistent with the idea that the parahippocampal cortex (located in the posterior parahippocampal gyrus) supports recollection by encoding and retrieving contextual information, whereas the hippocampus supports recollection by associating item and context information. By contrast, perirhinal cortex (located in the anterior parahippocampal gyrus) supports familiarity by encoding and retrieving specific item information. We discuss the implications of a 'binding of item and context' (BIC) model for studies of recognition memory. This model argues that there is no simple mapping between MTL regions and recollection and familiarity, but rather that the involvement of MTL regions in these processes depends on the specific demands of the task and the type of information involved. We highlight several predictions for future imaging studies that follow from the BIC model.

Activating the Medial Temporal Lobe during Oddity Judgment for Faces and Scenes

Impairments in visual discrimination beyond long-term declarative memory have been found in amnesic individuals, with hippocampal lesions resulting in deficits in scene discrimination and perirhinal cortex damage affecting object discrimination. To complement these findings, the present functional magnetic resonance imaging study found that in healthy participants oddity judgment for novel trial-unique scenes, compared with face or size oddity, was associated with increased posterior hippocampus and parahippocampal cortex activity. In contrast, perirhinal and anterior hippocampus activity was observed during unfamiliar trial-unique face oddity judgment, when contrasted with scene or size oddity tasks. Activity in all of these regions decreased as the stimuli were repeated across trials, reflecting the participants' increasing familiarity with the stimuli. This change was significant in all areas, with the exception of the perirhinal cortex, right anterior hippocampus, and to a lesser extent the left anterior hippocampus during face oddity judgment. One possibility is that the activity in these regions may not reflect entirely episodic memory encoding but, in part, processes beyond the mnemonic domain. Thus, the perirhinal cortex, and possibly anterior hippocampus, may play a more generic role in the discrimination and processing of objects.

Effect of unitization on associative recognition in amnesia

We examined how associative recognition performance in amnesic patients is mediated by use of a unitized (i.e., holistic) encoding strategy, and the degree to which the unitization effect is related to sparing of familiarity-based recognition. Participants studied word pairs as either separate lexical units in sentences (i.e., nonunitized) or as compounds (unitized). Under standard recognition instructions, normal controls and patients with left-temporal lobe damage (previously determined to have impairments in both recollection and familiarity) showed no difference for unitized and nonunitized pairs, whereas hypoxics (previously determined to have impaired recollection but relatively preserved familiarity) showed an advantage of unitized over nonunitized pairs. This effect was reproduced in normal healthy participants under instructions to restrict responses to judgments of familiarity. The results indicate that unitization may mediate the degree of associative recognition impairment exhibited by some amnesic patients, and that the effect is related to preserved familiarity capacity. The relevance of the results to the debate over the importance of the hippocampus in memory for associations is discussed.

Asymmetrical participation of the left and right hippocampus for representing environmental geometry in homing pigeons

Control, right and left HF lesioned homing pigeons (Columba livia) were trained to locate a goal in one corner of a rectangular enclosure with a distinctive feature cue. Probe tests revealed that all groups were able to encode in parallel geometric (enclosure shape) and feature information, and in the absence of one of them, they could us the other to locate the goal. However, left HF lesioned pigeons learned the task at a faster rate, and when the geometric and feature information were set in conflict, they relied more on the feature cue compared to control and right HF lesioned pigeons. It was also found that pigeons, independent of group, trained to a goal adjacent to the feature cue learned the task in fewer sessions and relied more on feature information compared to pigeons trained to a goal opposite the feature cue. The latter group relied more on geometric information. The results support the hypothesis that the left HF plays a more important role in the representation of a goal location with respect to environmental shape/geometry. We further propose that the observed functional asymmetry can be explained by the lateralized properties of the pigeon tectofugal visual system.

The Effects of Aging on the Recognition of Different Types of Associations

The present study examined how aging influences item and associative recognition memory, and compared memory for two types of associations: associations between the same kinds of information and associations between different kinds of information. A group of young adults and a group of older adults performed a forced-choice face recognition task and two multitrial forced-choice associative recognition tasks, assessing memory for face-face and face-spatial location associations. The results showed disproportionate age-related decline of associative recognition compared to intact item recognition. Moreover, aging affected both types of associative tasks in the same way. The findings support an associative deficit hypothesis (Naveh-Benjamin, Journal of Experimental Psychology: Learning, Memory and Cognition, 26, 1170-1187, 2000), which attributes a substantial part of the age effect on episodic memory tasks to difficulty with binding individual components into a cohesive memory trace. This associative deficit seems to affect same-information associations, as well as different-information associations.

Elaborative verbal encoding and altered anterior parahippocampal activation in adolescents and young adults at genetic risk for schizophrenia using FMRI

BACKGROUND

First-degree relatives of persons with schizophrenia are at elevated risk for the illness, demonstrate deficits in verbal memory, and exhibit structural abnormalities in the medial temporal lobe (MTL). We used functional magnetic resonance imaging (fMRI) to assess brain activity in the MTL during novel and repeated word-pair encoding.

METHODS

Participants were 21 non-psychotic, first-degree relatives of persons with schizophrenia and 26 matched healthy controls (ages 13-28). fMRI signal change was measured using a Siemens 1.5T MR scanner, and data were analyzed using SPM-2. Verbal memory was assessed using the Miller Selfridge (MS) Context Memory test prior to scanning.

RESULTS

The groups were comparable on demographics, intelligence and post-scan word recognition. Relatives at genetic risk (GR) had significantly more psychopathology than controls and worse performance on the MS test (p < .05). GR participants exhibited greater repetition suppression of activation in the left and right anterior parahippocampus (PHA, in the region of the entorhinal cortex region), after controlling for possible confounders. Controls and GR participants with above-median MS performance showed significantly greater repetition suppression of activation in left inferior frontal gyrus than those scoring below the median.

CONCLUSIONS

This is the first study to demonstrate an alteration of brain activity in the PHA in persons at GR for schizophrenia.

Fornix transection impairs exploration but not locomotion in ambulatory macaque monkeys

Prompted by the theoretical prediction that damage to the hippocampal system should abolish exploratory behavior, the present study examined exploratory movements in control monkeys (CON) and monkeys with transection of the fornix (FNX), a major input/output pathway of the hippocampus. CON and FNX monkeys were introduced to a novel octagonal chamber for six daily sessions, each lasting 20 min. Both groups visited, punctuated by stops, the majority of the floor space of the environment in each of the sessions. The exploratory movements of CON and FNX groups were not significantly different on most of the measures taken over six consecutive days. These measures included cumulative distance traveled, number and duration of stops, traveling patterns, and proportion of time spent in each of 12 designated zones of floor space. The high degree of similarity in behavior between CON and FNX groups suggests that an intact hippocampal system is not necessary for the display of normal exploratory movement per se. On the other hand, the CON and FNX groups did behave differently on two measures. First, the CON group exhibited a decrement in distance traversed over consecutive epochs within the first test session, whereas FNX animals did not. Second, on those days in which the chamber was made visually asymmetrical, the CON animals tended to show a predilection for spending proportionally more time within one particular quadrant of the chamber. These observations are consistent with the idea that interrupting normal hippocampal system function by means of fornix transection is detrimental to learning about the spatial layout of environments. We therefore suggest that while monkeys with fornix transection still display intact locomotor and exploratory behavior patterns, their new learning of visuospatial context is impaired.

Space, time and learning in the hippocampus: how fine spatial and temporal scales are expanded into population codes for behavioral control

The hippocampus participates in multiple functions, including spatial navigation, adaptive timing and declarative (notably, episodic) memory. How does it carry out these particular functions? The present article proposes that hippocampal spatial and temporal processing are carried out by parallel circuits within entorhinal cortex, dentate gyrus and CA3 that are variations of the same circuit design. In particular, interactions between these brain regions transform fine spatial and temporal scales into population codes that are capable of representing the much larger spatial and temporal scales that are needed to control adaptive behaviors. Previous models of adaptively timed learning propose how a spectrum of cells tuned to brief but different delays are combined and modulated by learning to create a population code for controlling goal-oriented behaviors that span hundreds of milliseconds or even seconds. Here it is proposed how projections from entorhinal grid cells can undergo a similar learning process to create hippocampal place cells that can cover a space of many meters that are needed to control navigational behaviors. The suggested homology between spatial and temporal processing may clarify how spatial and temporal information may be integrated into an episodic memory. The model proposes how a path integration process activates a spatial map of grid cells. Path integration has a limited spatial capacity, and must be reset periodically, leading to the observed grid cell periodicity. Integration-to-map transformations have been proposed to exist in other brain systems. These include cortical mechanisms for numerical representation in the parietal cortex. As in the grid-to-place cell spatial expansion, the analog representation of number is extended by additional mechanisms to represent much larger numbers. The model also suggests how visual landmarks may influence grid cell activities via feedback projections from hippocampal place cells to the entorhinal cortex.

Memory, perception, and the ventral visual-perirhinal-hippocampal stream: Thinking outside of the boxes

The prevailing paradigm in cognitive neuroscience assumes that the brain can be best understood as consisting of modules specialised for different psychological functions. Within the field of memory, we assume modules for different kinds of memory. The most influential version of this view posits a module called the "medial temporal lobe memory system" which operates in the service of "declarative memory." This system can be contrasted with a separate "perceptual representation system" in the ventral visual stream, which is critical for perceptual learning and memory, an example of nondeclarative function. Here we question this modular memory systems view and suggest that a better way to understand the ventral visual-perirhinal-hippocampal stream is as a hierarchically organised representational continuum. We suggest that in general, rather than trying to map psychological functions onto brain modules, we could benefit by instead attempting to understand the functions of brain regions in terms of the representations they contain, and the computations they perform.

The human medial temporal lobe processes online representations of complex objects

There has been considerable debate as to whether structures in the medial temporal lobe (MTL) support both memory and perception, in particular whether the perirhinal cortex may be involved in the perceptual discrimination of complex objects with a large number of overlapping features. Similar experiments testing the discrimination of blended images have obtained contradictory findings, and it remains possible that reported deficits in object perception are due to subtle learning in controls, but not patients. To address this issue, a series of trial-unique object "oddity" tasks, in which subjects selected the odd stimulus from a visual array, were administered to amnesic patients with either selective bilateral damage to the hippocampus or more extensive damage to MTL regions, including the perirhinal cortex. Whereas patients with damage limited to the hippocampus performed similarly to controls on all conditions, patients with perirhinal damage were significantly impaired when the task required discrimination between objects with a large number of features in common. By contrast, when the same stimuli could be discriminated using simple visual features, patients with perirhinal damage performed normally. These results are consistent with a theoretical view which holds that rostral inferotemporal cortical regions, including perirhinal cortex, represent the complex conjunctions of stimulus features necessary for both perception and memory of objects.

Event-related potential signatures of relational memory

Various lines of evidence suggest that memory for the relations among arbitrarily paired items acquired prior to testing can influence early processing of a probe stimulus. The event-related potential experiment reported here was designed to explore how early in time memory for a previously established face-scene relationship begins to influence processing of faces, under sequential presentation conditions in which a preview of the scene can promote expectancies about the to-be-presented face. Prior to the current work, the earliest component documented to be sensitive to memory for the relations among arbitrarily paired items was the late positive complex (LPC), but here relational memory effects were evident as early as 270-350 msec after face onset. The latency of these relational memory effects suggests that they may be the precursor to similar effects observed in eye movement behavior. As expected, LPC amplitude was also affected by memory for face-scene relationships, and N400 amplitude reflected some combination of memory for items and memory for the relations among items.

Remembering: functional organization of the declarative memory system

How do brain systems support our subjective experience of recollection and our senses of familiarity and novelty? A new functional imaging study concludes that each of these functions is accomplished by a distinct component of the medial temporal lobe, shedding new light on the functional organization of this memory system.

Telencephalon and geometric space in goldfish

Neuroanatomical evidence indicates that the lateral pallium (LP) of ray-finned fishes could be homologous to the hippocampus of mammals and birds. Recent studies have found that hippocampus of mammals and birds is critical for learning geometric properties of space. In this work, we studied the effects of lesions to the lateral pallium of goldfish on the encoding of geometric spatial information. Goldfish with telencephalic lesions were trained to search for a goal in a rectangular-shaped arena containing one different wall that served as the only distinctive environmental feature. Although fish with lateral pallium lesions learned the task even faster than sham and medial pallium (MP)-lesioned animals, subsequent probe trials showed that they were insensitive to geometric information. Sham and medial pallium-lesioned animals could use both geometric and feature information to locate the goal. By contrast, fish with lateral palium lesions relied exclusively on the feature information provided by the wall of a different colour. These results indicate that lesions to the lateral pallium of goldfish, like hippocampal lesions in mammals and birds, selectively impair the encoding of geometric spatial information of environmental space. Thus, the forebrain structures of teleost fish that are neuroanatomically equivalent to the mammalian and avian hippocampus also share a central role in supporting spatial cognition. Present results suggest that the presence of a hippocampal-dependent memory system implicated in the processing of geometric spatial information is an ancient feature of the vertebrate forebrain that has been conserved during the divergent evolution of different vertebrate groups.

Perirhinal cortex activity during visual object discrimination: An event-related fMRI study

Previous fMRI studies have demonstrated preferential involvement of the perirhinal cortex and hippocampus in tasks of object and spatial memory, respectively. Here we investigated whether similar activity would also be present when object and spatial discrimination was assessed in the absence of explicit declarative memory demands. On each trial in the scanner, participants were presented simultaneously with two arrays of objects and were asked to indicate whether both arrays were identical, differed with respect to the identity of one object or differed with respect to the spatial arrangement of the objects. It was found that the detection of an object identity change was associated with significant right perirhinal cortex activity. We suggest that this perirhinal activity indicates a role of this structure in processes beyond declarative memory, for example, short-term visual working memory or higher order perception. Significantly greater hippocampal activity was not, however, observed during the spatial arrangement condition, perhaps due to the relatively low spatial processing demands of this task.

The right hippocampus participates in short-term memory maintenance of object–location associations

Doubts have been cast on the strict dissociation between short- and long-term memory systems. Specifically, several neuroimaging studies have shown that the medial temporal lobe, a region almost invariably associated with long-term memory, is involved in active short-term memory maintenance. Furthermore, a recent study in hippocampally lesioned patients has shown that the hippocampus is critically involved in associating objects and their locations, even when the delay period lasts only 8 s. However, the critical feature that causes the medial temporal lobe, and in particular the hippocampus, to participate in active maintenance is still unknown. This study was designed in order to explore hippocampal involvement in active maintenance of spatial and non-spatial associations. Eighteen participants performed a delayed-match-to-sample task in which they had to maintain either object-location associations, color-number association, single colors, or single locations. Whole-brain activity was measured using event-related functional magnetic resonance imaging and analyzed using a random effects model. Right lateralized hippocampal activity was evident when participants had to maintain object-location associations, but not when they had to maintain object-color associations or single items. The present results suggest a hippocampal involvement in active maintenance when feature combinations that include spatial information have to be maintained online.

Cooperation and competition between the dorsal hippocampus and lateral amygdala in spatial discrimination learning

The conditioned cue preference (CCP) was used to study how rats discriminate between adjacent arms on a radial maze. Chai and White (Behav Neurosci 2004, 118:770-784) showed that an intact dorsal hippocampus is required to learn this discrimination and that an amygdala-based conditioned approach response that produces an equal tendency to enter both arms is simultaneously acquired. In the present experiments, rats were preexposed to the maze with no food and trained by alternately confining them at the ends of two adjacent arms, one that contained food and one that did not. When given a choice between these arms with no food present, the rats spent more time on their food-paired arms, suggesting they had learned to discriminate their locations. Temporary inactivation of the dorsal hippocampus with muscimol during confinement on the food-paired arm had no effect on the discrimination, but inactivation while on the no-food arm impaired it. This pattern of effects was reversed in rats with amygdala lesions (inactivation on the food-paired arm impaired, but inactivation on the no-food arm had no effect on the discrimination), showing that hippocampus-based and amygdala-based learning interact to influence the behavior of normal rats in this situation. The dorsal hippocampus learns about locations that contain food and about locations that do not contain food. The amygdala-based tendency to enter the food-paired arm cooperates with hippocampus-based foraging for food on the food-paired, but the amygdala-based tendency to enter the no-food arm competes with hippocampus-based learning about the absence of food on that arm.

Single-item memory, associative memory, and the human hippocampus

We tested recognition memory for items and associations in memory-impaired patients with bilateral lesions thought to be limited to the hippocampal region. In Experiment 1 (Combined memory test), participants studied words and then took a memory test in which studied words, new words, studied word pairs, and recombined word pairs were presented in a mixed order. In Experiment 2 (Separated memory test), participants studied single words and then took a memory test involving studied word and new words. In a separate test, they studied word pairs and then took a memory test involving studied word pairs and recombined word pairs. In both experiments, patients were impaired at memory for single items as well as memory for associations, suggesting that the hippocampus is important for both of these memory functions. In Experiment 1, patients appeared to be more impaired at associative memory than item memory. In Experiment 2, patients were similarly impaired at associative memory and item memory. These different findings are considered, including the fact that in Experiment 1 the results depended on the fact that controls produced unexpectedly low false-alarm rates to recombined pairs. We discuss single-item and associative memory from the perspective that the hippocampus and adjacent cortex work cooperatively to signal recognition and that simple dichotomies do not adequately describe the division of labor within the medial temporal lobe.

A volumetric MRI study of the hippocampus and the parahippocampal region after unilateral medial temporal lobe resection

Segmentation guidelines on high-resolution MRI designed to assess remaining volumes of the hippocampus and the parahippocampal cortices after medial temporal lobe (MTL) surgery could provide a useful tool to investigate the involvement of these anatomical regions in surgical outcomes and in human memory. For this purpose, we implemented an MRI volumetric analysis, already applied to healthy population or epileptic patient before surgery, to quantify the volume of the hippocampus, the temporopolar cortex and the regions of the parahippocampal gyrus (perirhinal, entorhinal and parahippocampal cortices) spared after unilateral MTL resection carried out to treat medically uncontrolled temporal lobe epilepsy (TLE). Based on the locations of remaining anatomical landmarks, we quantified the volume of these regions in 24 patients after MTL resection and in 16 control participants. Our results show that (1) mean volumes of these regions contralateral to the epileptic focus were similar to those of normal subjects, (2) volumetric measures obtained from the resected side were much smaller than those from the non-resected side or from normal values and (3) the extent of MTL resection was comparable in right or left MTL surgery. Individual analysis of patients showed that the parahippocampal cortex, as opposed to the other regions, was not systematically removed across patients. As a post-operative MRI-based method, it therefore proves valuable to assess group data as well as to explore differences between individual patients.

Neural dynamics of autistic behaviors: cognitive, emotional, and timing substrates

What brain mechanisms underlie autism, and how do they give rise to autistic behavioral symptoms? This article describes a neural model, called the Imbalanced Spectrally Timed Adaptive Resonance Theory (iSTART) model, that proposes how cognitive, emotional, timing, and motor processes that involve brain regions such as the prefrontal and temporal cortex, amygdala, hippocampus, and cerebellum may interact to create and perpetuate autistic symptoms. These model processes were originally developed to explain data concerning how the brain controls normal behaviors. The iSTART model shows how autistic behavioral symptoms may arise from prescribed breakdowns in these brain processes, notably a combination of underaroused emotional depression in the amygdala and related affective brain regions, learning of hyperspecific recognition categories in the temporal and prefrontal cortices, and breakdowns of adaptively timed attentional and motor circuits in the hippocampal system and cerebellum. The model clarifies how malfunctions in a subset of these mechanisms can, through a systemwide vicious circle of environmentally mediated feedback, cause and maintain problems with them all.

Memory for famous faces and the temporal pole: functional imaging findings in temporal lobe epilepsy

The ability to recognize, name, and provide information about famous persons is deficient in patients with temporal lobe epilepsy (TLE), although the neural basis for these deficits is not well understood. We examined the relationship of resting metabolism of the temporal poles, as determined by [18F] fluorodeoxyglucose positron emission tomography, to performance on a task of famous face recognition, naming, and generation of semantic information in 12 patients with TLE. Correlations between metabolic measures of the temporal poles and performance on the Famous Faces Task revealed strong relationships between all aspects of the Famous Faces Task and the left temporal pole, whereas Famous Faces Task correlations with the right temporal pole were not significant. These findings indicate that the left temporal pole is associated with lexical and semantic retrieval of knowledge of famous persons in patients with TLE. Further study appears warranted to elucidate the networks involved in semantic knowledge for famous faces.

Rule shift under long-term PCP challenge in rats

Performance of Long-Evans rats repeatedly treated with a high dose of phencyclidine (PCP) was compared with that of controls on two tasks involving rule shifts. Fifteen hours after receiving 12 daily doses of PCP (10mg/kg) or saline, rats were tested in the first task where a fixed or variable goal had to be recognized among four maze-arm locations. After receiving the last of 33 injections, rats were subjected to a second task: they had to discriminate the relevant perceptual dimension in a pair of stimuli and choose the appropriate stimulus in each problem. Results from both tasks revealed no difference between controls and rats injected with PCP either during acquisition using a constant rule or during testing with rule shifts. Consequently, PCP appears to lack consistency in disturbing frontal cognitive functions in animal models of schizophrenia.

Conditional Associative Learning of Spatial and Object Information in Children with Attention Deficit/Hyperactivity Disorder

The present study assessed frontostriatal mediated memory functions in children with ADHD (N=12) and healthy control participants (N=12) using two tests of conditional associative learning (i.e., object and spatial) that shared similar stimulus-response association structures but that differed in terms of the demands placed upon strategic processes. Children with ADHD displayed normal performance on the object learning task but were impaired on the spatial learning task that placed greater demands on internally derived strategic processes. Secondary analyses further indicated that this strategic processing impairment cannot be attributed specifically to perseverative or working memory errors but rather appears to be related to a more general inability to maintain a high degree of consistency in responding across trials. Although the results of this study must be interpreted in light of the small sample sizes, they suggest that ADHD does not produce a basic deficit in acquiring stimulus-response associations previously shown to be associated with basal ganglia dysfunction. Rather, these findings suggest that the impaired conditional associative learning performance of children with ADHD is attributable to deficits in strategic processes previously been found to be dependent upon the integrity of the prefrontal cortex.

Spatial relational memory in 9-month-old macaque monkeys

This experiment assesses spatial and nonspatial relational memory in freely moving 9-mo-old and adult (11-13-yr-old) macaque monkeys (Macaca mulatta). We tested the use of proximal landmarks, two different objects placed at the center of an open-field arena, as conditional cues allowing monkeys to predict the location of food rewards hidden in one of two sets of three distinct locations. Monkeys were tested in two different conditions: (1) when local visual cues marked the two sets of potentially baited locations, so that monkeys could use both local and spatial information to discriminate these locations from never-baited locations; and (2) when no local visual cues marked the two sets of potentially baited locations, so that monkeys had to rely on a spatial relational representation of the environment to discriminate these locations. No 9-mo-old or adult monkey associated the presence of the proximal landmarks, at the center of the arena, with the presence of food in one set of three distinct locations. All monkeys, however, discriminated the potentially baited locations in the presence of local visual cues, thus providing evidence of visual discrimination learning. More importantly, all 9-mo-old monkeys tested discriminated the potentially baited locations in absence of the local visual cues, thus exhibiting evidence of spatial relational learning. These findings indicate that spatial memory processes characterized by a relational representation of the environment are present as early as 9 mo of age in macaque monkeys.

Oscillatory neuronal dynamics during language comprehension

Language comprehension involves two basic operations: the retrieval of lexical information (such as phonologic, syntactic, and semantic information) from long-term memory, and the unification of this information into a coherent representation of the overall utterance. Neuroimaging studies using hemodynamic measures such as PET and fMRI have provided detailed information on which areas of the brain are involved in these language-related memory and unification operations. However, much less is known about the dynamics of the brain's language network. This chapter presents a literature review of the oscillatory neuronal dynamics of EEG and MEG data that can be observed during language comprehension tasks. From a detailed review of this (rapidly growing) literature the following picture emerges: memory retrieval operations are mostly accompanied by increased neuronal synchronization in the theta frequency range (4-7 Hz). Unification operations, in contrast, induce high-frequency neuronal synchronization in the beta (12-30 Hz) and gamma (above 30 Hz) frequency bands. A desynchronization in the (upper) alpha frequency band is found for those studies that use secondary tasks, and seems to correspond with attentional processes, and with the behavioral consequences of the language comprehension process. We conclude that it is possible to capture the dynamics of the brain's language network by a careful analysis of the event-related changes in power and coherence of EEG and MEG data in a wide range of frequencies, in combination with subtle experimental manipulations in a range of language comprehension tasks. It appears then that neuronal synchrony is a mechanism by which the brain integrates the different types of information about language (such as phonological, orthographic, semantic, and syntactic information) represented in different brain areas.

No effect of hippocampal lesions on perirhinal cortex-dependent feature-ambiguous visual discriminations

Previous studies have shown that perirhinal cortex lesions in monkeys impair visual discriminations with a high degree of "feature ambiguity," a property of visual discriminations that can emerge when features are a part of both rewarded and unrewarded stimuli. The effects of damage to the hippocampus on these perirhinal-dependent feature-ambiguous tasks are, however, unknown. Prominent theories of medial temporal lobe function predict similar effects of perirhinal cortex and hippocampal lesions on cognitive tasks. In contrast, our hypothesis is that perirhinal cortex, and not the hippocampus, is important for nonspatial complex feature-ambiguous discriminations. We sought to distinguish between these competing theories in a straightforward way, by testing rhesus monkeys with hippocampal lesions on the same feature-ambiguous tasks shown previously to depend on perirhinal cortex. It was found that hippocampal lesions had no effects on any of these tasks. The findings support the perceptual-mnemonic/feature conjunction model of perirhinal cortex function, and provide further evidence for heterogeneity of function within the putative medial temporal lobe memory system.

Neuropharmacology of cognition and memory: A unifying theory of neuromodulator imbalance in psychiatry and amnesia

The case of HM, a man with intractable epilepsy who became amnesic following bilateral medial temporal lobe surgery nearly half a century ago has instigated ongoing research and theoretical speculation on the nature of memory and the role of the hippocampus. Neuropsychological testing showed that although HM had extensive anterograde memory loss he could still acquire motor and cognitive skills implicitly, but could not remember the context of this learning. This has lead to declarative and procedural descriptions of the memory process. Cholinergic and monoaminergic neurotransmitter systems have also been implicated in the memory process and anticholinergic drugs traditionally have been associated with impairment of declarative memory. The cholinergic hypothesis of Alzheimer's disease is a classic example of an application of these neuropharmacological findings. In schizophrenia, preattentive deficits have been amply demonstrated by unconscious priming studies. Memory processes are also impaired in these patients. Dopamine, glutamate and even cholinergic dysfunction has been implicated in the clinical picture of schizophrenia. The present paper will attempt to bring together both the anatomical and pharmacological data from these disparate fields of research under a cohesive theory of cognition and memory. A hypothesis is presented for an inverse relationship between monoaminergic and cholinergic systems in the modulation of implicit (unconscious) and explicit (conscious) cognitive processes. It is postulated that muscarinic cholinergic receptors and monoaminergic systems facilitate unconscious and conscious processes, respectively, and they disfacilitate conscious and unconscious processes, respectively (the purported inverse relationship). In fact, the muscarinic and monoaminergic modulations of a neural network are proposed to be finely balanced such that, if, the activity of one receptor system is modified then this by necessity has effects on the other system. It takes into account receptor subtypes and their effects mediated through excitatory and inhibitory G-protein complexes. For example, m1/D2 and D1/m4 paired receptor subtypes, colocalized on separate neurons would have opposing functional effects. A theory is then presented that the critical underlying pathophysiology of schizophrenia involves a hypofunctional muscarinic cholinergic system, which induces abnormal facilitation of monoaminergic subsystems such as dopamine (e.g., a decrease in m1R function would potentiate D2R function). This extends the idea of an inverted U function for optimal monoaminergic concentrations. Not only would this impair unconscious preattentive processes, but according to the hypothesis, explicit cognition as well including memory deficits and would underlie the mechanism of psychosis. Contrary to current thinking a different view is also presented for the role of the hippocampus in the memory process. It is postulated that long-term explicit memory traces in the neocortex are laid down by phasic coactivation of forebrain projecting monoaminergic systems above some basal firing rate, such as the rostral serotonergic raphe, which projects diffusely to the cortex and according to a modified Hebbian principle. This is the proposed principal function of the hippocampal theta rhythm. The phasic activation of the cholinergic basal forebrain is mediated by projections from a separate cortical structure, possibly the lateral prefrontal cortex. Phasic muscarinic receptor activation is proposed to strengthen implicit memory traces (at a synaptic level) in the neocortex. Thus, the latter are spared by medial temporal surgery explaining the dissociation of explicit from implicit memory.

Conjunction errors, recollection-based rejections, and forgetting in a continuous recognition task

Six experiments investigated conjunction memory errors (e.g., falsely remembering blackbird after studying parent words blackmail and jailbird) in a continuous recognition procedure with a parent-conjunction lag manipulation. In 4 experiments (1A, 1B, 2A, and 2B) "recollect" judgments, which indexed recall of parent words, showed that participants can use recollection to prevent conjunction errors. "Recollect" judgments, as well as overt recall of parent words (in Experiments 2A and 2B), dropped sharply from a lag of 0 to 1 word, then stabilized from a lag of 1 to 20 words. Thus, the "recollect" responses and overt recall demonstrate a step function of forgetting over short intervals. These data generalized to cued recall in Experiments 3A and 3B with the first morpheme (e.g., black) as the cue, though recall conjunction errors occurred infrequently relative to recognition conjunction errors. Overall, the results support the idea that automatic and controlled processes contribute to memory performance.

Hippocampal contributions to recollection in retrograde and anterograde amnesia

Lesions restricted to the hippocampal formation and/or extended hippocampal system (hippocampal formation, fornix, mammillary bodies, and anterior thalamic nuclei) can disrupt conscious recollection in anterograde amnesia, while leaving familiarity-based memory relatively intact. Familiarity may be supported by extra-hippocampal medial temporal lobe (MTL) structures. Within-task dissociations in recognition memory best exemplify this distinction in anterograde amnesia. The authors report for the first time comparable dissociations within recognition memory in retrograde amnesia. An amnesic patient (A.D.) with bilateral fornix and septal nuclei lesions failed to recognize details pertaining to personal past events only when recollection was required, during recognition of episodic details. His intact recognition of generic and semantic details pertaining to the same events was ascribed to intact familiarity processes. Recollective processes in the controls were reflected by asymmetrical Receiver's Operating Characteristic curves, whereas the patient's Receiver's Operating Characteristic was symmetrical, suggesting that his inferior recognition performance on episodic details was reliant on familiarity processes. Anterograde and retrograde memories were equally affected, with no temporal gradient for retrograde memories. By comparison, another amnesic person (K.C.) with extensive MTL damage (involving extra-hippocampal MTL structures in addition to hippocampal and fornix lesions) had very poor recognition and no recollection of either episodic or generic/semantic details. These data suggest that the extended hippocampal system is required to support recollection for both anterograde and retrograde memories, regardless of their age.

Temporal processing of information: The role of the medial prefrontal cortex and hippocampus: Theoretical comment on gilmartin and mcechron (2005)

M. R. Gilmartin and M. D. McEchron (2005) reported that single cells recorded in the prelimbic cortex of rats during the acquisition of trace fear conditioning display multiple patterns of neuronal firing during the trace. These finding are discussed in the context of the role of the prelimbic cortex in processing temporal information during trace conditioning and delayed matching- or nonmatching-to-sample paradigms based on both electrophysiology and lesion evidence. In addition, evidence is provided for a role of the hippocampus in supporting temporal processing of information and its potential interaction with the prelimbic cortex.

Methods for reducing interference in the Complementary Learning Systems model: oscillating inhibition and autonomous memory rehearsal

The stability-plasticity problem (i.e. how the brain incorporates new information into its model of the world, while at the same time preserving existing knowledge) has been at the forefront of computational memory research for several decades. In this paper, we critically evaluate how well the Complementary Learning Systems theory of hippocampo-cortical interactions addresses the stability-plasticity problem. We identify two major challenges for the model: Finding a learning algorithm for cortex and hippocampus that enacts selective strengthening of weak memories, and selective punishment of competing memories; and preventing catastrophic forgetting in the case of non-stationary environments (i.e. when items are temporarily removed from the training set). We then discuss potential solutions to these problems: First, we describe a recently developed learning algorithm that leverages neural oscillations to find weak parts of memories (so they can be strengthened) and strong competitors (so they can be punished), and we show how this algorithm outperforms other learning algorithms (CPCA Hebbian learning and Leabra at memorizing overlapping patterns. Second, we describe how autonomous re-activation of memories (separately in cortex and hippocampus) during REM sleep, coupled with the oscillating learning algorithm, can reduce the rate of forgetting of input patterns that are no longer present in the environment. We then present a simple demonstration of how this process can prevent catastrophic interference in an AB-AC learning paradigm.

Hippocampal mechanisms for the context-dependent retrieval of episodes

Behaviors ranging from delivering newspapers to waiting tables depend on remembering previous episodes to avoid incorrect repetition. Physiologically, this requires mechanisms for long-term storage and selective retrieval of episodes based on the time of occurrence, despite variable intervals and similarity of events in a familiar environment. Here, this process has been modeled based on the physiological properties of the hippocampal formation, including mechanisms for sustained activity in entorhinal cortex and theta rhythm oscillations in hippocampal subregions. The model simulates the context-sensitive firing properties of hippocampal neurons including trial-specific firing during spatial alternation and trial by trial changes in theta phase precession on a linear track. This activity is used to guide behavior, and lesions of the hippocampal network impair memory-guided behavior. The model links data at the cellular level to behavior at the systems level, describing a physiologically plausible mechanism for the brain to recall a given episode which occurred at a specific place and time.

On aims and methods in the neuroimaging of derived relations

Ingenious and seemingly powerful technologies have been developed recently that enable the visualization in some detail of events in the brain concomitant upon the ongoing behavioral performance of a human participant. Measurement of such brain events offers at the very least a new set of dependent variables in relation to which the independent variables familiarly manipulated in the operant laboratory may be explored. Two related paradigms in which a start has been made in such research concern the derivation of novel or emergent relations from a baseline set of trained relations, and include the phenomenon of transitive inference (TI), observed in studies of stimulus equivalence (SE) and serial learning (SL) or seriation. This paper reviews some published and forthcoming neuroimaging studies of these and related phenomena, and considers how this line of research both demands and represents a welcome synthesis between types of question and levels of explanation in behavioral science that often have been seen as antithetical.

Functional role of entorhinal cortex in working memory processing

Our learning and memory system has the challenge to work in a world where items to learn are dispersed in space and time. From the information extracted by the perceptual systems, the learning system must select and combine information. Both these operations may require a temporary storage where significance and correlations could be assessed. This work builds on the common hypothesis that hippocampus and subicular, entorhinal and parahippocampal/postrhinal areas are essential for the above-mentioned functions. We bring up two examples of models; the first one is modeling of in vivo and in vitro data from entorhinal cortex layer II of delayed match-to-sample working memory experiments, the second one studying mechanisms in theta rhythmicity in EC. In both cases, we discuss how cationic currents might be involved and relate their kinetics and pharmacology to behavioral and cellular experimental results.

Novelty responses to relational and non-relational information in the hippocampus and the parahippocampal region: a comparison based on event-related fMRI

We conducted two functional magnetic resonance imaging (fMRI) experiments that examined novelty responses in the human medial temporal lobe (MTL) to determine whether the hippocampus makes contributions to memory processing that differ from those of structures in the adjacent parahippocampal region. In light of proposals that such differential contributions may pertain to relational processing demands, we assessed event-related fMRI responses in the MTL for novel single objects and for novel spatial and non-spatial object relationships; subjects were asked to detect these different types of novelties among previously studied items, and they successfully performed this task during scanning. A double dissociation that emerged from the response pattern of regions in the hippocampus and perirhinal cortex provided the strongest support for functional specialization in the MTL. A region in the right middle hippocampus responded to the novelty of spatial and non-spatial relationships but not to the novelty of individual objects. By contrast, a region in right perirhinal cortex, situated in the anterior collateral sulcus, responded to the novelty of individual objects but not to that of either type of relationship. Other MTL regions that responded to novelty in the present study showed no reliable difference in their response to the various novelty types; these regions included anterior parts of the hippocampus and posterior aspects of parahippocampal cortex. Together, our findings indicate that relational processing demands are a critical determinant of functional specialization in the human MTL. They also suggest, however, that a neuroanatomical framework that only distinguishes between the hippocampus and the parahippocampal region is not sufficiently refined to account for all functional differences and similarities observed with respect to relational processes in the human MTL.

Lag-sensitive repetition suppression effects in the anterior parahippocampal gyrus

Single-unit recording studies of monkeys have shown that neurons in perirhinal and entorhinal cortex exhibit activity reductions following stimulus repetition, and some have suggested that these "repetition suppression" effects may represent neural signals that support recognition memory. Critically, repetition suppression effects are most pronounced at short intervals between stimulus repetitions. Here, we used event-related functional magnetic resonance imaging (fMRI) to identify repetition suppression effects in the human medial temporal lobe and determine whether these effects are sensitive to the length of the interval between repetitions. Twenty-one participants were scanned while performing a continuous recognition memory task in which the interval between item repetitions was parametrically varied from 2 to 32 intervening items. We found evidence of repetition suppression in the anterior parahippocampal gyrus, but only when the repetition interval was relatively short. Moreover, bilateral hippocampal regions showed lag-sensitive repetition effects. Our results demonstrate that activity in the human medial temporal cortex, like that of monkeys, exhibits repetition suppression effects that are sensitive to the length of the interval between repetitions.

Hippocampal lesions that abolish spatial maze performance spare object recognition memory at delays of up to 48 hours

The hippocampus is widely considered to be a critical component of a medial temporal lobe memory system, necessary for normal performance on tests of declarative memory. Object recognition memory is thought to be a classic test of declarative memory function. However, previous tests of the effects of hippocampal lesions on object recognition memory have not always supported this view. One possible reason for this inconsistency is that previously reported effects of hippocampal lesions on object recognition memory tasks may have stemmed not from a deficit in object recognition memory per se, but as a result of spatial and contextual confounds in the task. Thus, in the present study, we used a spontaneous object recognition test in a modified apparatus designed to minimize spatial and contextual factors. A group of rats with complete excitotoxic lesions of the hippocampus and a group of control rats were tested on this modified spontaneous object recognition task with retention delays of up to 48 h. These rats were also tested on a spatial nonmatching-to-place task. Spatial memory performance was abolished following hippocampal lesions, whereas performance on the recognition memory task was intact at all delays tested.

Alzheimer's disease behaviors from past self-identities: an exploration of the memory and cognitive features

Alzheimer's disease (AD) patients have been reported by caregivers to display "behaviors from past self-identities " (BPSI); however, there is little known about these distinct behaviors. This study, the first to explore BPSI, hypothesized that BPSI were associated with self-memory and cognitive impairments. Its purpose was to determine if AD subjects with and without BPSI differed on measures of autobiographical memory, selective attention, and fluency. The cross-sectional design compared 35 moderate-stage AD subjects from an AD research center. Subjects demonstrating BPSI (37 percent) recalled significantly fewer recent autobiographical memories than AD subjects without BPSI. The results establish BPSI as a common behavior among moderate-stage AD patients and suggest that paucity of recent self-memories contributes to BPSI.

Functional neuroanatomy of remote episodic, semantic and spatial memory: a unified account based on multiple trace theory

We review lesion and neuroimaging evidence on the role of the hippocampus, and other structures, in retention and retrieval of recent and remote memories. We examine episodic, semantic and spatial memory, and show that important distinctions exist among different types of these memories and the structures that mediate them. We argue that retention and retrieval of detailed, vivid autobiographical memories depend on the hippocampal system no matter how long ago they were acquired. Semantic memories, on the other hand, benefit from hippocampal contribution for some time before they can be retrieved independently of the hippocampus. Even semantic memories, however, can have episodic elements associated with them that continue to depend on the hippocampus. Likewise, we distinguish between experientially detailed spatial memories (akin to episodic memory) and more schematic memories (akin to semantic memory) that are sufficient for navigation but not for re-experiencing the environment in which they were acquired. Like their episodic and semantic counterparts, the former type of spatial memory is dependent on the hippocampus no matter how long ago it was acquired, whereas the latter can survive independently of the hippocampus and is represented in extra-hippocampal structures. In short, the evidence reviewed suggests strongly that the function of the hippocampus (and possibly that of related limbic structures) is to help encode, retain, and retrieve experiences, no matter how long ago the events comprising the experience occurred, and no matter whether the memories are episodic or spatial. We conclude that the evidence favours a multiple trace theory (MTT) of memory over two other models: (1) traditional consolidation models which posit that the hippocampus is a time-limited memory structure for all forms of memory; and (2) versions of cognitive map theory which posit that the hippocampus is needed for representing all forms of allocentric space in memory.

Neural correlates of memory for items and for associations: an event-related functional magnetic resonance imaging study

Although results from cognitive psychology, neuropsychology, and behavioral neuroscience clearly suggest that item and associative information in memory rely on partly different brain regions, little is known concerning the differences and similarities that exist between these two types of information as a function of memory stage (i.e., encoding and retrieval). We used event-related functional magnetic resonance imaging to assess neural correlates of item and associative encoding and retrieval of simple images in 18 healthy subjects. During encoding, subjects memorized items and pairs. During retrieval, subjects made item recognition judgments (old vs. new) and associative recognition judgments (intact vs. rearranged). Relative to baseline, item and associative trials activated bilateral medial temporal and prefrontal regions during both encoding and retrieval. Direct contrasts were then performed between item and associative trials for each memory stage. During en- coding, greater prefrontal, hippocampal, and parietal activation was observed for associations, but no significant activation was observed for items at the selected threshold. During recognition, greater activation was observed for associative trials in the left dorsolateral prefrontal cortex and superior parietal lobules bilaterally, whereas item recognition trials showed greater activation of bilateral frontal regions, bilateral anterior medial temporal areas, and the right temporo-parietal junction. Post hoc analyses suggested that the anterior medial temporal activation observed during item recognition was driven mainly by new items, confirming a role for this structure in novelty detection. These results suggest that although some structures such as the medial temporal and prefrontal cortex play a general role in memory, the pattern of activation in these regions can be modulated by the type of information (items or associations) interacting with memory stages.

A Hierarchy of Associations in Hippocampo-Cortical Systems: Cognitive Maps and Navigation Strategies

In this letter we describe a hippocampo-cortical model of spatial processing and navigation based on a cascade of increasingly complex associative processes that are also relevant for other hippocampal functions such as episodic memory. Associative learning of different types and the related pattern encoding-recognition take place at three successive levels: (1) an object location level, which computes the landmarks from merged multimodal sensory inputs in the parahippocampal cortices; (2) a subject location level, which computes place fields by combination of local views and movement-related information in the entorhinal cortex; and (3) a spatiotemporal level, which computes place transitions from contiguous place fields in the CA3-CA1 region, which form building blocks for learning temporospatial sequences.

At the cell population level, superficial entorhinal place cells encode spatial, context-independent maps as landscapes of activity; populations of transition cells in the CA3-CA1 region encode context-dependent maps as sequences of transitions, which form graphs in prefrontal-parietal cortices. The model was tested on a robot moving in a real environment; these tests produced results that could help to interpret biological data.

Two different goal-oriented navigation strategies were displayed depend-ing on the type of map used by the system.

Thanks to its multilevel, multimodal integration and behavioral imple-mentation, the model suggests functional interpretations for largely un-accounted structural differences between hippocampo-cortical systems. Further, spatiotemporal information, a common denominator shared by several brain structures, could serve as a cognitive processing frame and a functional link, for example, during spatial navigation and episodic memory, as suggested by the applications of the model to other domains, temporal sequence learning and imitation in particular.

The acquisition of face and person identity information following anterior temporal lobectomy

Thirty unilateral anterior temporal lobectomy (ATL) subjects (15 right and 15 left) and 15 controls were presented a multitrial learning task in which unfamiliar faces were paired with biographical information (occupation, city location, and a person's name). Face recognition hits were similar between groups, but the right ATL group committed more false-positive errors to face foils. Both left and right ATL groups were impaired relative to controls in acquiring biographical information, but the deficit was more pronounced for the left ATL group. Recall levels also varied for the different types of biographical information; occupation was most commonly recalled followed by city name and person name. In addition, city and person name recall was more likely when occupation was also recalled. Overall, recall of biographical information was positively correlated with clinical measures of anterograde episodic memory. Findings are discussed in terms of the role of the temporal lobe and associative learning ability in the successful acquisition of new face semantic (biographical) representations.

Neural correlates of social odor recognition and the representation of individual distinctive social odors within entorhinal cortex and ventral subiculum

Recognition of individual conspecifics is important for social behavior and requires the formation of memories for individually distinctive social signals. Individual recognition is often mediated by olfactory cues in mammals, especially nocturnal rodents such as golden hamsters. In hamsters, this form of recognition requires main olfactory system input to the lateral entorhinal cortex (LEnt). Here, we tested whether neurons in LEnt and the nearby ventral subiculum (VS) would show cellular correlates of this natural form of recognition memory. Two hundred ninety single neurons were recorded from both superficial (SE) and deep layers of LEnt (DE) and VS while male hamsters investigated volatile odorants from female vaginal secretions. Many neurons encoded differences between female's odors with many discriminating between odors from different individual females but not between different odor samples from the same female. Other neurons discriminated between odor samples from one female and generalized across collections from other females. LEnt and VS neurons showed enhanced or suppressed cellular activity during investigation of previously presented odors and in response to novel odors. A majority of SE neurons decreased firing to odor repetition and increased activity to novel odors. In contrast, DE neurons often showed suppressed activity in response to novel odors. Thus, neurons in LEnt and VS of male hamsters encode information that is critical for the identification and recognition of individual females by odor cues. This study reveals cellular mechanisms in LEnt and VS that may mediate a natural form of recognition memory in hamsters. These neuronal responses were similar to those observed in rats and monkeys during performance in standard recognition memory tasks. Consequently, the present data extend our understanding of the cellular basis for recognition memory and suggest that individual recognition requires similar neural mechanisms as those employed in laboratory tests of recognition memory.

Medial temporal lobe activation during encoding and retrieval of novel face-name pairs

The human medial temporal lobe (MTL) is known to be involved in declarative memory, yet the exact contributions of the various MTL structures are not well understood. In particular, the data as to whether the hippocampal region is preferentially involved in the encoding and/or retrieval of associative memory have not allowed for a consensus concerning its specific role. To investigate the role of the hippocampal region and the nearby MTL cortical areas in encoding and retrieval of associative versus non-associative memories, we used functional magnetic resonance imaging (fMRI) to measure brain activity during learning and later recognition testing of novel face-name pairs. We show that there is greater activity for successful encoding of associative information than for non-associative information in the right hippocampal region, as well as in the left amygdala and right parahippocampal cortex. Activity for retrieval of associative information was greater than for non-associative information in the right hippocampal region also, as well as in the left perirhinal cortex, right entorhinal cortex, and right parahippocampal cortex. The implications of these data for a clear functional distinction between the hippocampal region and the MTL cortical structures are discussed.

An analysis of independence and interactions of brain substrates that subserve multiple attributes, memory systems, and underlying processes

It is proposed that memory is organized into event-based, knowledge-based, and rule-based memory systems. Furthermore, each system is composed of the same set of multiple attributes and characterized by a set of process oriented operating characteristics that are mapped onto multiple neural regions and interconnected neural circuits. Based on this theoretical model of memory, it is possible to investigate the independence and interaction among brain regions between any two systems for any of the proposed attributes or processes. This applies also to the investigation of independence and interactions between any two attributes within a system and between processes associated with a system for any of the proposed attributes. In this article, research evidence is presented to suggest that there are both dissociations and interactions between the hippocampus and caudate nucleus in mediating spatial and response attributes within the event-based memory system, between the hippocampus and the parietal cortex in subserving the spatial attribute within the event-based and knowledge-based memory systems, and between the hippocampus and the prefrontal cortex in subserving the spatial attribute within the event-based and rule-based memory systems.

Activation of human hippocampal formation reflects success in both encoding and cued recall of paired associates

Contemporary theories of hippocampal function suggest that both encoding and retrieval of episodic memories may be accomplished by neural circuitry embedded within the same anatomical structures, but neuroimaging support for this hypothesis has been ambiguous. Recent studies suggest that the best available indicators of hippocampal encoding and retrieval operations are selective activations due to novelty, encoding success, and recall success in a paired associate learning paradigm. In the current study, both encoding and cued recall of paired associate words were conducted during a single session of fMRI scanning. Bilateral activation in the medial temporal lobe was detected for encoding word pairs vs. a fixation baseline and for encoding novel word pairs vs. repeated word pairs. These activations were stronger in subjects who successfully memorized more word pairs. In cued recall, greater responses were seen in higher performing subjects. In lower performing subjects, responses were greater to cue words whose paired associate was correctly recalled than to cue words whose correct associate had been forgotten (or not encoded). The difference between correct and incorrect trials was more pronounced on repeated presentations of the same cue words, but not apparent on their first presentation alone. Overlap of encoding and retrieval effects was maximal in the middle of the longitudinal extent of the right hippocampus, with one additional locus of overlap outside the MTL, in left occipitotemporal cortex. The conjunction of these effects suggests that it is correct to view both encoding and recall of associative memories as functions of an integrated hippocampal system.

New Semantic Learning and Generalization in a Patient With Amnesia

New learning of semantic information is impaired in amnesia. Several reports have demonstrated that "errorless" learning techniques have allowed patients with amnesia to acquire at least some form of new semantic information, although this information appears to be relatively inflexible. Using insights and principles from connectionist modeling of cortical and medial temporal lobe memory systems, the authors describe why errorless learning procedures act as a poor proxy for the medial temporal lobe, suggesting that they artificially eliminate the variability that defines semantic information. The authors trained a patient with severe amnesia on new semantic sentences both with and without variance and then tested him on both repeated and related novel sentences to assess generalization. He successfully learned new semantic information in both conditions but demonstrated better generalization of semantic concepts following training with variance.

Mediotemporal contributions to semantic processing: fMRI evidence from ambiguity processing during semantic context verification

The medial temporal lobe (MTL) is well known to be crucial for various types of memory; however, controversy remains as to which of its substructures contribute to semantic processing and, if so, to what extent. The current study addresses the issue of MTL contributions to semantic processing during lexico-semantic ambiguity processing by using functional magnetic resonance imaging (fMRI) in combination with a context verification task (CVT). The CVT required decisions on the semantic fit of congruent and incongruent target words to the overall meaning of preceding sentential contexts with and without semantic ambiguity. In two of the four experimental conditions (congruent homographic, incongruent homographic), target decisions were critically dependent on the successful processing of prior sentence-final lexico-semantic ambiguity. Semantic context verification per se evidenced bilateral activations of the hippocampus that were part of a functional network including inferior prefrontal and superior parietal cortices. Commonalities in activation differences pertaining to the specific cognitive component of lexico-semantic ambiguity processing were found in a left temporal lobe network that comprised activation foci in the temporal pole, the parahippocampal and fusiform gyri. The present results suggest that the hippocampus may well contribute to semantic processing, namely by a mnemonic function that serves to link the target meaning representation with the meaning of a prior sentence context. Contrary to previous reports from human lesion studies, the present findings further suggest, that the specific cognitive component of lexico-semantic ambiguity processing is neither dependent on the hippocampus nor exclusively subserved by the temporal pole, but also recruits an associative semantic memory function from the parahippocampal gyrus as well as a more general (bottom-up) semantic function from the fusiform gyrus.

Does hippocampus associate discontiguous events? Evidence from event-related fMRI

To examine the hypothesis that the hippocampus is necessary to overcome temporal or spatial "discontiguity" (Wallenstein et al., Trends Neurosci 1998; 21:317-323), subjects were imaged by functional magnetic resonance imaging (fMRI) when they were making judgments as to whether two words were semantically related. Two words were presented, either at the same time (the Simultaneous Presentation Condition) or one after the other with a short unfilled rest period (the Delayed Presentation Condition). The latter condition, relative to the former, was proposed to involve the process of "discontiguity association." Event-related fMRI results of eight subjects showed that, relative to the binding of simultaneously presented words, the binding of delay presented words was associated with left hippocampus activity. This result provided direct neuroimaging evidence for the role of the hippocampus in "discontiguity association."

Hippocampal nitric oxide synthase and arginase and age-associated behavioral deficits

The present study investigated age-related changes in nitric oxide synthase (NOS) and arginase in the subregions of the hippocampus and their correlations with animals' performance in the open field, T-maze, and water maze tasks. Aged rats (24 months old) showed reduced exploratory activity and poorer spatial learning relative to the young adults (4 months old). Significant increases in total NOS activity were found in the aged dentate gyrus and a dramatic decrease in endothelial NOS expression was observed in the aged CA2/3. Activity or protein expression of inducible NOS was not detected in any subregion of the hippocampus. There were no age-related changes in total arginase activity or arginase I and arginase II protein expression. Correlation analysis revealed that animals' motor ability was associated with CA1 NOS and arginase, as well as hippocampal function. The present findings provide further support for the involvement of NOS/NO and arginase in the normal aging process. A strong positive correlation between CA1 eNOS protein expression and swimming speed in the water maze task may reflect a relationship between the local cerebral blood flow and neuronal activity.