Memory representation within the parahippocampal region

Network activity evoked by neocortical stimulation in area 36 of the guinea pig perirhinal cortex

The perirhinal cortex is a key structure involved in memory consolidation and retrieval. In spite of the extensive anatomical studies that describe the intrinsic and extrinsic associative connections of the perirhinal cortex, the activity generated within such a network has been poorly investigated. We describe here the pattern of synaptic interactions that subtend the responses evoked in area 36 of the perirhinal cortex by neocortical and local stimulation. The experiments were carried out in the in vitro isolated guinea pig brain. The synaptic perirhinal circuit was reconstructed by integrating results obtained during intracellular recordings from layer II-III neurons with simultaneous current source density analysis of laminar profiles performed with 16-channel silicon probes. Both neocortical and local stimulation of area 36 determined a brief monosynaptic excitatory potential in layer II-III neurons, followed by a biphasic synaptic inhibitory potential possibly mediated by a feed-forward inhibitory circuit at sites close to the stimulation electrode and a late excitatory postsynaptic potential (EPSP) that propagated at distance within area 36 along the rhinal sulcus. During a paired-pulse stimulation test, the inhibitory postsynaptic potential (IPSP) and the late EPSP were abolished in the second conditioned response, suggesting that they are generated by poli-synaptic circuits. Current source density analysis of the field responses demonstrated that 1) the monosynaptic activity was generated in layers II-III and 2) the sink associated to the disynaptic responses was localized within the superficial layer of area 36. We conclude that the neocortical input induces a brief monosynaptic excitation in area 36 of the perirhinal cortex, that is curtailed by a prominent inhibition and generates a recurrent excitatory associative response that travels at distance within area 36 itself. The results suggest that the perirhinal cortex network has the potentials to integrate multimodal incoming neocortical information on its way to the hippocampus.

Instability in the place field location of hippocampal place cells after lesions centered on the perirhinal cortex

The perirhinal cortex appears to play a key role in memory, and the neighboring hippocampus is critically involved in spatial processing. The possibility exists, therefore, that perirhinal-hippocampal interactions are important for spatial memory processes. The purpose of the present study was to investigate the contribution of the perirhinal cortex to the location-specific firing ("place field") of hippocampal complex-spike ("place") cells. The firing characteristics of dorsal CA1 place cells were examined in rats with bilateral ibotenic acid lesions centered on the perirhinal cortex (n = 4) or control surgeries (n = 5) as they foraged in a rectangular environment. The activity of individual place cells was also monitored after a delay period of either 2 min, or 1 or 24 hr, during which time the animal was removed from the environment. Although the perirhinal cortex lesion did not affect the place field size or place cell firing characteristics during a recording session, it was determined that the location of the place field shifted position across the delay period in 36% (10 of 28) of the cells recorded from lesioned animals. In contrast, none of the place cells (0 of 29) recorded from control animals were unstable by this measure. These data indicate that although the initial formation of place fields in the hippocampus is not dependent on perirhinal cortex, the maintenance of this stability over time is disrupted by perirhinal lesions. This instability may represent an erroneous "re-mapping" of the environment and suggests a role for the perirhinal cortex in spatial memory processing.

Recognition memory correlates of hippocampal theta cells

Investigations of hippocampal theta cell activity have typically involved behavioral tasks with modest cognitive demands. Recordings in rats locomoting through space or engaged in simple stimulus discrimination or conditioning have revealed some place specificity and S(+)/S(-) selectivity in addition to the hippocampal EEG theta-related behavioral/motor correlates. However, little data exist regarding theta cell activity during performance of more cognitively demanding, hippocampal-dependent recognition memory tasks. Here, we examined the cognitive firing correlates of theta cells in rats that were performing an olfactory recognition memory task with distinct sample and test phases. Discriminant analysis revealed odor and match/nonmatch memory correlates in theta cell activity comparable in relative magnitude to that of the principal cells. Odor-specific theta cell responses in the sample phase were restricted primarily to CA1 and linked to task performance. In the test recognition phase, match/nonmatch theta cells were found primarily in the CA3 and CA1 fields, most of which exhibited greater activity on correct nonmatch trials in which recognition occurred than on error match trials in which recognition failed. Odor selectivity of the match/nonmatch signaling was greatest in the dentate gyrus (DG) and CA3 and least in CA1. This inverted pattern of stimulus specificity in the sample versus test phase was similar to that observed in principal cells but with a greater contrast between the CA1 and DG/CA3 fields. Together, these findings suggest that theta cells actively participate in hippocampal recognition memory processing and play a specific role in shaping the cognitive firing properties of the hippocampal principal cells.

Recognition memory: what are the roles of the perirhinal cortex and hippocampus?

The hallmark of medial temporal lobe amnesia is a loss of episodic memory such that patients fail to remember new events that are set in an autobiographical context (an episode). A further symptom is a loss of recognition memory. The relationship between these two features has recently become contentious. Here, we focus on the central issue in this dispute--the relative contributions of the hippocampus and the perirhinal cortex to recognition memory. A resolution is vital not only for uncovering the neural substrates of these key aspects of memory, but also for understanding the processes disrupted in medial temporal lobe amnesia and the validity of animal models of this syndrome.

Mental calculation in a prodigy is sustained by right prefrontal and medial temporal areas

Calculating prodigies are individuals who are exceptional at quickly and accurately solving complex mental calculations. With positron emission tomography (PET), we investigated the neural bases of the cognitive abilities of an expert calculator and a group of non-experts, contrasting complex mental calculation to memory retrieval of arithmetic facts. We demonstrated that calculation expertise was not due to increased activity of processes that exist in non-experts; rather, the expert and the non-experts used different brain areas for calculation. We found that the expert could switch between short-term effort-requiring storage strategies and highly efficient episodic memory encoding and retrieval, a process that was sustained by right prefrontal and medial temporal areas.

Avoidance task training potentiates phasic pontine-wave density in the rat: A mechanism for sleep-dependent plasticity

Behavioral studies of learning and memory in both humans and animals support a role for sleep in the consolidation and integration of memories. The present study explored possible physiological mechanisms of sleep-dependent behavioral plasticity by examining the relationship between learning and state-dependent phasic signs of rapid eye movement (REM) sleep. Cortical electroencephalogram, electromyogram, eye movement, hippocampal theta-wave, and pontine-wave (P-wave) measures were recorded simultaneously in freely moving rats after a session of conditioned avoidance learning or a control session. After learning trials, rats spent 25.5% more time in REM sleep and 180.6% more time in a transitional state between slow-wave sleep and REM sleep (tS-R) compared with that in control trials. Both REM sleep and tS-R behavioral states are characterized by the presence of P-waves. P-wave density was significantly greater in the first four episodes of REM sleep after the learning session compared with the control session. Furthermore, the P-wave density change between the first and third REM sleep episodes was proportional to the improvement of task performance between the initial training session and the post-sleep retest session. These findings show that the increase in P-wave density during the post-training REM sleep episodes is correlated with the effective consolidation and retention of avoidance task learning.

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.

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.

Computational modeling of entorhinal cortex

Computational modeling provides a means for linking the physiological and anatomical characteristics of entorhinal cortex at a cellular level to the functional role of this region in behavior. We have developed detailed simulations of entorhinal cortical neurons and networks, with an emphasis on the role of acetylcholine in entorhinal cortical function. Computational modeling suggests that when acetylcholine levels are high, this sets appropriate dynamics for the storage of stimuli during performance of delayed matching tasks. In particular, acetylcholine activates a calcium-sensitive nonspecific cation current which provides an intrinsic cellular mechanism which could maintain neuronal activity across a delay period. Simulations demonstrate how this phenomena could underlie entorhinal cortex delay activity as described in previous unit recordings. Acetylcholine also induces theta rhythm oscillations which may be appropriate for timing of afferent input to be encoded in hippocampus and for extraction of individual stored sequences from multiple stored sequences. Lower levels of acetylcholine may allow sharp wave dynamics which can reactivate associations encoded in hippocampus and drive the formation of additional traces in hippocampus and entorhinal cortex during consolidation.

The neurophysiology of memory

How do the structures of the medial temporal lobe contribute to memory? To address this question, we examine the neurophysiological correlates of both recognition and associative memory in the medial temporal lobe of humans, monkeys, and rats. These cross-species comparisons show that the patterns of mnemonic activity observed throughout the medial temporal lobe are largely conserved across species. Moreover, these findings show that neurons in each of the medial temporal lobe areas can perform both similar as well as distinctive mnemonic functions. In some cases, similar patterns of mnemonic activity are observed across all structures of the medial temporal lobe. In the majority of cases, however, the hippocampal formation and surrounding cortex signal mnemonic information in distinct, but complementary ways.

Prolonged synaptic integration in perirhinal cortical neurons

Layer II/III of rat perirhinal cortex (PR) contains numerous late-spiking (LS) pyramidal neurons. When injected with a depolarizing current step, these LS cells typically delay spiking for one or more seconds from the onset of the current step and then sustain firing for the duration of the step. This pattern of delayed and sustained firing suggested a specific computational role for LS cells in temporal learning. This hypothesis predicts and requires that some layer II/III neurons should also exhibit delayed and sustained spiking in response to a train of excitatory synaptic inputs. Here we tested this prediction using visually guided, whole cell recordings from rat PR brain slices. Most LS cells (19 of 26) exhibited delayed spiking to synaptic stimulation (>1 s latency from the train onset), and the majority of these cells (13 of 19) also showed sustained firing that persisted for the duration of the synaptic train (5-10 s duration). Delayed and sustained firing in response to long synaptic trains has not been previously reported in vertebrate neurons. The data are consistent with our model that a circuit containing late spiking neurons can be used for encoding long time intervals during associative learning.

Differential information processing by hippocampal and subicular neurons

It has been known for some years that hippocampal neurons are critically involved in processing of information necessary for encoding memories. What is less understood is the role of the subiculum in this process. We describe here differential response characteristics of subicular and hippocampal neurons in rats during execution of a delayed-nonmatch-to-sample short-term memory task. Subicular neurons, unlike hippocampal neurons, fire primarily in the delay interval of the task and appear to provide a temporal linkage between events encoded in hippocampus during the sample and nonmatch phases. Indeed, a large proportion of subicular neurons fire robustly for the entire duration of the delay only. Further analyses using electrical activation methods indicate that subicular neurons that receive short latency inputs from the anterior thalamus and do not project to cingulate cortex are the most responsive to stimuli with behavioral significance.

A locus and mechanism of action for associative morphine tolerance

Repeated administration of an opioid in the presence of specific environmental cues can induce tolerance specific to that setting (associative tolerance). Prolonged or repeated administration of an opioid without consistent contextual pairing yields non-associative tolerance. Here we demonstrate that cholecystokinin acting at the cholecystokinin-B receptor is required for associative but not non-associative morphine tolerance. Morphine given in the morphine-associated context increased Fos-like immunoreactivity in the lateral amygdala and hippocampal area CA1. Microinjection of the cholecystokinin B antagonist L-365,260 into the amygdala blocked associative tolerance. These results indicate that cholecystokinin acting in the amygdala is necessary for associative tolerance to morphine's analgesic effect.

Dynamic filtering of recognition memory codes in the hippocampus

Principal cells of the dentate gyrus (DG), CA3, and CA1 subfields of the hippocampus were recorded in rat during performance of an odor-guided delayed nonmatch-to-sample task with distinct sample and test phases. The hippocampus was found to possess multiple encoding modes. In the sample phase, odor-selective activity was restricted primarily to CA1 and, to a lesser extent, CA3. Odor representations in half of these cells were predictive of subsequent performance (i. e., correct vs error) in the test phase. Cells in each hippocampal subfield maintained elevated or suppressed activity in the delay interval relative to pre-odor baseline, but were indiscriminate with regard to sample odor identity. In the test phase, the regional distribution of odor-selective activity was inverse to that for the sample: maximal in DG and minimal in CA1. The inverted distribution of odor selectivity was also observed for cells that discriminated match/nonmatch trial types. Most match/nonmatch cells exhibited greater activity on correct nonmatch than error match trials, indicating the presence of a hippocampal recognition memory signal on trials where recognition occurred and its absence on trials where recognition failed. These findings reveal the hippocampus as a highly dynamic encoding device, restricting perceptual stimulus information to different subfields (or none, in the delay phase) depending on memory task contingencies. Moreover, the reduction in cue-specificity of match/nonmatch comparison signals as they pass through the hippocampal trisynaptic circuit may contribute to a generalized recognition signal for use in guiding behavior.

Bridging the gap: integrating cellular and functional magnetic resonance imaging studies of the hippocampus

The importance of the medial temporal lobe in memory has been studied extensively at the neuronal, neural ensemble, and systems level. In this report, we discuss recent systems level neuroimaging results in relation to neurophysiological studies of the hippocampus and related structures within the medial temporal lobe. By combining our knowledge across the cellular and systems levels we sought to gain theoretical insight and a better understanding of the function of the hippocampus and related medial temporal lobe structures. The integration of information from studies carried out at the cellular and neural ensemble level with studies at the systems level is difficult because of the vast differences in spatial and temporal resolution of the different research methodologies, differences in neuroanatomy across species, and differences in the types of behavioral and cognitive paradigms used in rat, nonhuman primate, and human studies. Despite these methodological and species-related differences, the neurophysiological studies offer insight into many of the questions raised by recent neuroimaging studies. For instance, there is physiological evidence that suggests that the hippocampal memory system is functionally heterogeneous, which may explain some of the discrepancies in the location and extent of activation reported by different imaging studies of the medial temporal lobe. In addition, we describe recent computational models of the hippocampus which may be useful for bridging the gap between neurophysiological and neuroimaging data.

Level of sustained entorhinal activity at study correlates with subsequent cued-recall performance: a functional magnetic resonance imaging study with high acquisition rate

Functional magnetic resonance imaging (fMRI) with high acquisition rate was performed during the intentional memorizing of words to specify which medial temporal lobe structure is important in determining what words are subsequently remembered in a cued-recall test and to characterize the time course of activation in that structure. Functional images of six healthy young subjects were analyzed by two subject- and voxel-wise statistics: First, to identify brain areas transiently engaged in encoding of words, brain activity during memorizing visually presented words and watching a fixation cross was compared by a Kolmogorov-Smirnov statistic (KS-test). Second, to identify brain areas whose activity correlates with memory encoding success, a Kendall's correlation was calculated between signal intensity at study and performance in a subsequent cued-recall test. Averaged signal intensities were plotted as a function of time to depict the time course of brain activity detected by both statistical tests. The level of slowly modulated, sustained activity in Brodmann area 28 (entorhinal cortex) did not respond transiently as study words appeared, but did correlate positively with subsequent test performance. More left than right activity in Brodmann area 45 (dorso-lateral prefrontal cortex) and bilateral activity in Brodmann area 44 (premotor cortex) exhibited transient hemodynamic responses that did not show any relation to subsequent memory performance. Thus, the study identified a novel pattern of slowly modulated brain activity in human entorhinal cortex that may represent a declarative memory encoding state whose level predicts whether experiences will be remembered or forgotten.

Crossmodal associative memory representations in rodent orbitofrontal cortex

Firing patterns of neurons in the orbitofrontal cortex (OF) were analyzed in rats trained to perform a task that encouraged incidental associations between distinct odors and the places where their occurrence was detected. Many of the neurons fired differentially when the animals were at a particular location or sampled particular odors. Furthermore, a substantial fraction of the cells exhibited odor-specific firing patterns prior to odor presentation, when the animal arrived at a location associated with that odor. These findings suggest that neurons in the OF encode cross-modal associations between odors and locations within long-term memory.

Localization of pontine PGO wave generation sites and their anatomical projections in the rat

A number of experimental and theoretical reports have suggested that the ponto-geniculo-occipital (PGO) wave-generating cells are involved in the generation of rapid eye movement (REM) sleep and REM sleep dependent cognitive functions. No studies to date have examined anatomical projections from PGO-generating cells to those brain structures involved in REM sleep generation and cognitive functions. In the present study, pontine PGO wave-generating sites were mapped by microinjecting carbachol in 74 sites of the rat brainstem. Those microinjections elicited PGO waves only when made in the dorsal part of the nucleus subcoeruleus of the pons. In six rats, the anterograde tracer biotinylated dextran amine (BDA) was microinjected into the physiologically identified cholinoceptive pontine PGO-generating site to identify brain structures receiving efferent projections from those PGO-generating sites. In all cases, small volume injections of BDA in the cholinoceptive pontine PGO-generating sites resulted in anterograde labeling of fibers and terminals in many regions of the brain. The most important output structures of those PGO-generating cells were the occipital cortex, entorhinal cortex, piriform cortex, amygdala, hippocampus, and many other thalamic, hypothalamic, and brainstem nuclei that participate in the generation of REM sleep. These findings provide anatomical evidence for the hypothesis that the PGO-generating cells in the pons could be involved in the generation of REM sleep. Since PGO-generating cells project to the entorhinal cortex, piriform cortex, amygdala, and hippocampus, these PGO-generating cells could also be involved in the modulation of cognitive functions.

Using olfaction to study memory

In a series of studies we have been exploring the role of hippocampal function in memory using the model system of olfactory-hippocampal pathways and odor learning in rats. Our experiments show that hippocampus itself is not essential to memory for single odors, but is critical for forming the representations of relations among odor memories, and for the expression of odor memory representations in novel situations. These studies that exploit the exceptional qualities of olfactory learning are helping to clarify the nature of higher order memory processes in all mammals, and extending to declarative memory in humans.

Septocingulate and septohippocampal cholinergic pathways: involvement in working/episodic memory

The contribution of the septohippocampal cholinergic pathway to performance of a working/episodic memory task was compared to that of the septocingulate cholinergic path. The septocingulate and septohippocampal cholinergic pathways were selectively destroyed in male Sprague-Dawley rats using site-specific injections of the anti-neuronal immunotoxin 192-IgG saporin into either the hippocampus or the cingulate cortex. 192-IgG-saporin selectively destroys cholinergic neurons and terminals that express the p75 neurotrophin receptor. Following extensive pre-operative training, working memory was assessed using a delayed nonmatch to sample eight arm radial maze task, with delays of 1, 4 and 8 h. The group with lesions of the septohippocampal cholinergic pathway displayed performance deficits on this task which were not related to length of delay. In contrast, the group with lesions of the septocingulate cholinergic pathway did display delay-dependent deficits which were observed at the 4- and 8-h delays, but not at the 1-h delay. These data suggest that the septocingulate cholinergic pathway is critically involved in working/episodic memory but that the septohippocampal cholinergic pathway is either not contributing to working/episodic memory per se or it is involved only at shorter delays.

Entorhinal cortex and fimbria-fornix role in rat's passive avoidance response memorization

The stereotaxic administration of tetrodotoxin (TTX) was employed to induce the fully reversible inactivation of the fimbria-fornix complex (FF) and of the entorhinal cortex (EC), in order to ascertain the role of these structures in the memorization of a passive avoidance response (PAR). On permanently cannulated rats TTX (5 ng in 0.5 microliter saline) or saline (0.5 microliter) was injected uni- or bilaterally, respectively, in the FF and in the EC, 60 min before PAR acquisition, immediately after PAR acquisition and 60 min before PAR retrieval, always performed 48 h after the acquisition trial. It was shown that EC unilateral or bilateral pre-acquisition inactivation was followed by amnesia, while TTX inactivation in post-acquisition and pre-retrieval had no effects. Identical results were obtained by TTX administration in FF. The experimental evidence indicates that both EC and FF play a role during acquisition of PAR engram. The results are discussed in comparison with previous ones concerning dorsal and ventral hippocampus TTX inactivation effects on rat's PAR, and in relation to hippocampal and medial septal area connectivity.

Recognition memory: neuronal substrates of the judgement of prior occurrence

Recognition memory relies on two processes: (i) identification and (ii) judgement concerning prior occurrence. A system centred on perirhinal cortex appears to be responsible for judgement of prior occurrence based on discrimination of the familiarity of stimuli or their recency of occurrence; in contrast, a hippocampal system probably supplies information concerning the episodic, contextual aspects of recognition memory. This review chiefly concerns the perirhinal system and, in particular, neurones that signal the prior occurrence of stimuli by a decrease in response. Details concerning such decremental responses are given and it is argued that such responses in perirhinal cortex are adequate for and central to discrimination of stimulus familiarity and recency in a wide range of situations. Information is given of similar types of neuronal responses in anatomically related brain regions and what may be deduced about the operation of the recognition memory system. The possibility is discussed that the neuronal responses that signal information concerning the recent occurrence of stimuli may contribute to repetition priming as well as recognition memory. Other described changes in the activity of individual neurones such as response enhancements, or sustained (delay) activity may allow solution of specialised forms of recognition memory tasks where relatively short-term working memory is adequate. Implications of the multi-faceted nature of recognition memory for the interpretation of results are emphasised. Unsolved problems and avenues for future experimentation, including determining the nature of possible underlying synaptic plastic changes, are discussed.