Declarative memory: insights from cognitive neurobiology

Long-term transfer of learning from books and video during toddlerhood

Television viewing and picture book reading are prevalent activities during toddlerhood, and research has shown that toddlers can imitate from both books and videos after short delays. This is the first study to directly compare toddlers' long-term retention rates for target actions learned from a video or book. Toddlers (N=158) at 18- and 24-months of age saw an experimenter demonstrating how to make a novel three-step toy rattle via a prerecorded video or a picture book. The toddlers' imitation of the target actions was tested after a specific delay (e.g., 2, 4 weeks), and their performance was compared with that of age-matched controls who did not see a demonstration. The 18-month-olds retained the target actions for 2 weeks, exhibiting forgetting at 4 weeks, whereas the 24-month-olds retained the information for up to 4 weeks, exhibiting forgetting at 8 weeks. Retention rates for books and videos did not differ at either age. These findings demonstrate very impressive retention from a brief two-dimensional media demonstration, and they contribute to our overall understanding of long-term memory processes during infancy.

Functional dissociation of hippocampal mechanism during implicit learning based on the domain of associations

Traditionally, the medial temporal lobe (MTL) was linked to explicit or declarative memory in associative learning. However, recent studies have reported MTL involvement even when volunteers are not consciously aware of the learned contingencies. Therefore, the mechanism of the MTL-related learning process cannot be described sufficiently by the explicit/implicit distinction, and the underlying process in the MTL for associative learning needs a more functional characterization. A possible feature that would allow a functional specification also for implicit learning is the nature of the material that is learned. Given that implicit memory tasks often comprise a combination of perceptual and motor learning, we hypothesized that implicit learning of the perceptual but not the motor component entails MTL activation in these studies. To directly test this hypothesis, we designed a purely perceptual and a purely motor variant of the serial reaction time task. In two groups of human volunteers, behavioral results clearly showed that both variants were learned without awareness. Neuronal recordings using fMRI revealed that bilateral hippocampal activation was observed only for implicit learning of the perceptual sequence, not for the motor sequence. This dissociation clearly shows that the functional role of the hippocampus for learning is determined by the domain of the learned association and that the function of the medial temporal lobe system is the processing of contingencies between perceptual features regardless of the explicit or implicit nature of the ensuing memory.

Information-integration category learning and the human uncertainty response

The human response to uncertainty has been well studied in tasks requiring attention and declarative memory systems. However, uncertainty monitoring and control have not been studied in multi-dimensional, information-integration categorization tasks that rely on non-declarative procedural memory. Three experiments are described that investigated the human uncertainty response in such tasks. Experiment 1 showed that following standard categorization training, uncertainty responding was similar in information-integration tasks and rule-based tasks requiring declarative memory. In Experiment 2, however, uncertainty responding in untrained information-integration tasks impaired the ability of many participants to master those tasks. Finally, Experiment 3 showed that the deficit observed in Experiment 2 was not because of the uncertainty response option per se, but rather because the uncertainty response provided participants a mechanism via which to eliminate stimuli that were inconsistent with a simple declarative response strategy. These results are considered in the light of recent models of category learning and metacognition.

Visual recognition memory across contexts

In two experiments, we investigated the development of representational flexibility in visual recognition memory during infancy using the Visual Paired Comparison (VPC) task. In Experiment 1, 6- and 9-month-old infants exhibited recognition when familiarization and test occurred in the same room, but showed no evidence of recognition when familiarization and test occurred in different rooms. In contrast, 12- and 18-month-old infants exhibited recognition irrespective of testing room. Thus, flexibility across a change of room was observed at a younger age than flexibility across a change of background that has previously been seen with the VPC procedure (Robinson & Pascalis, 2004). To determine if limitations in representational flexibility across a change of background could be overcome by experiences during encoding, in Experiment 2, 6-, 9-, 12- and 18-month-old infants were familiarized with a picture on multiple backgrounds. At all ages, infants showed recognition across a change in background at test. These findings indicate that dissociating an item from its context during encoding may be an important factor in understanding the representational flexibility of visual recognition memory in infancy. Developmental changes in representational flexibility are likely driven by changes in the functional maturity of the hippocampal formation, and experience.

A new chapter in the field of memory: adult hippocampal neurogenesis

Understanding the cellular mechanisms underlying learning and memory is a major challenge in neurobiology. Structural and functional changes occurring in the hippocampus such as synaptic remodeling and long-term potentiation are key signatures of long-term memory processes. The discovery of a de novo hippocampal production of neurons in the adult brain has been a breakthrough in the field of plasticity and memory, introducing a new actor that could sustain memory processes. Here we will review our current knowledge on the role of these adult new neurons in memory. In particular we will provide evidence showing that they are required for learning and memory and that an alteration in their production rate or maturation leads to memory impairments. Through a thorough survey of the literature, we will also acknowledge that there are many controversies regarding the specific role played by newborn neurons. The emerging picture is that they are involved in the establishment of spatiotemporal relationships among multiple environmental cues for the flexible use of the acquired information. Indeed, newborn neurons have been found to be required for separating events based on their spatial and temporal characteristics, a process that preserves the uniqueness of a memory representation. Thus, adult-born neurons are required for allocentric space representation, for long-term memory retention and for flexible inferential memory expression. Finally, we will conclude by highlighting directions for future research, emphasizing that the exact participation of newborn neurons in memory processes will not be approached without considering the hippocampal network in general.

The epigenetics of estrogen: epigenetic regulation of hormone-induced memory enhancement

Epigenetic processes have been implicated in everything from cell proliferation to maternal behavior. Epigenetic alterations, including histone alterations and DNA methylation, have also been shown to play critical roles in the formation of some types of memory, and in the modulatory effects that factors, such as stress, drugs of abuse and environmental stimulation, have on the brain and memory function. Recently, we demonstrated that the ability of the sex-steroid hormone 17β-estradiol (E(2)) to enhance memory formation is dependent on histone acetylation and DNA methylation, a finding that has important implications for understanding how hormones influence cognition in adulthood and aging. In this article, we provide an overview of the literature demonstrating that epigenetic processes and E(2) influence memory, describe our findings indicating that epigenetic alterations regulate E(2)-induced memory enhancement, and discuss directions for future work on the epigenetics of estrogen.

A modeling study suggesting how a reduction in the context-dependent input on CA1 pyramidal neurons could generate schizophrenic behavior

The neural mechanisms underlying schizophrenic behavior are unknown and very difficult to investigate experimentally, although a few experimental and modeling studies suggested possible causes for some of the typical psychotic symptoms related to this disease. The brain region most involved in these processes seems to be the hippocampus, because of its critical role in establishing memories for objects or events in the context in which they occur. In particular, a hypofunction of the N-methyl-D-aspartate (NMDA) component of the synaptic input on the distal dendrites of CA1 pyramidal neurons has been suggested to play an important role for the emergence of schizophrenic behavior. Modeling studies have investigated this issue at the network and cellular level. Here, starting from the experimentally supported assumption that hippocampal neurons are very specific, sparse, and invariant in their firing, we explore an experimentally testable prediction at the single neuron level. The model shows how and to what extent a pathological hypofunction of a context-dependent distal input on a CA1 neuron can generate hallucinations by altering the normal recall of objects on which the neuron has been previously tuned. The results suggest that a change in the context during the recall phase may cause an occasional but very significant change in the set of active dendrites used for feature recognition, leading to a distorted perception of objects.

The impact of age-related ovarian hormone loss on cognitive and neural function

On average, women now live one-third of their lives after menopause. Because menopause has been associated with an elevated risk of dementia, an increasing body of research has studied the effects of reproductive senescence on cognitive function. Compelling evidence from humans, nonhuman primates, and rodents suggests that ovarian sex-steroid hormones can have rapid and profound effects on memory, attention, and executive function, and on regions of the brain that mediate these processes, such as the hippocampus and prefrontal cortex. This chapter will provide an overview of studies in humans, nonhuman primates, and rodents that examine the effects of ovarian hormone loss and hormone replacement on cognitive functions mediated by the hippocampus and prefrontal cortex. For humans and each animal model, we outline the effects of aging on reproductive function, describe how ovarian hormones (primarily estrogens) modulate hippocampal and prefrontal physiology, and discuss the effects of both reproductive aging and hormone treatment on cognitive function. Although this review will show that much has been learned about the effects of reproductive senescence on cognition, many critical questions remain for future investigation.

Transfer of learning between 2D and 3D sources during infancy: Informing theory and practice

The ability to transfer learning across contexts is an adaptive skill that develops rapidly during early childhood. Learning from television is a specific instance of transfer of learning between a 2-Dimensional (2D) representation and a 3-Dimensional (3D) object. Understanding the conditions under which young children might accomplish this particular kind of transfer is important because by 2 years of age 90% of US children are viewing television on a daily basis. Recent research shows that children can imitate actions presented on television using the corresponding real-world objects, but this same research also shows that children learn less from television than they do from live demonstrations until they are at least 3 years old; termed the video deficit effect. At present, there is no coherent theory to account for the video deficit effect; how learning is disrupted by this change in context is poorly understood. The aims of the present review are (1) to review the conditions under which children transfer learning between 2D images and 3D objects during early childhood, and (2) to integrate developmental theories of memory processing into the transfer of learning from media literature using Hayne's (2004) developmental representational flexibility account. The review will conclude that studies on the transfer of learning between 2D and 3D sources have important theoretical implications for general developmental theories of cognitive development, and in particular the development of a flexible representational system, as well as policy implications for early education regarding the potential use and limitations of media as effective teaching tools during early childhood.

A computational theory of episodic memory formation in the hippocampus

A quantitative computational theory of the operation of the hippocampus as an episodic memory system is described. The CA3 system operates as a single attractor or autoassociation network to enable rapid, one-trial associations between any spatial location (place in rodents or spatial view in primates) and an object or reward and to provide for completion of the whole memory during recall from any part. The theory is extended to associations between time and object or reward to implement temporal order memory, also important in episodic memory. The dentate gyrus performs pattern separation by competitive learning to produce sparse representations, producing for example neurons with place-like fields from entorhinal cortex grid cells. The dentate granule cells produce by the very small number of mossy fibre connections to CA3 a randomizing pattern separation effect important during learning but not recall that separates out the patterns represented by CA3 firing to be very different from each other, which is optimal for an unstructured episodic memory system in which each memory must be kept distinct from other memories. The direct perforant path input to CA3 is quantitatively appropriate to provide the cue for recall in CA3, but not for learning. The CA1 recodes information from CA3 to set up associatively learned backprojections to neocortex to allow subsequent retrieval of information to neocortex, providing a quantitative account of the large number of hippocampo-neocortical and neocortical-neocortical backprojections. Tests of the theory including hippocampal subregion analyses and hippocampal NMDA receptor knockouts are described and support the theory.

The hippocampus contributes to memory expression during transitive inference in mice

There is substantial evidence that the hippocampus plays a role in transitive inference, the capacity to link overlapping memories and subsequently make novel judgments between elements of those memories that are only indirectly related. However, it is unclear whether the hippocampus is involved primarily during the original acquisition of the overlapping memories, or additionally during the flexible expression of those memories during transitive judgments. Here, we demonstrated that selective hippocampal damage produced after acquisition of the overlapping memories resulted in a severe impairment in subsequent transitive inference judgments, indicating that the hippocampus does play an important role beyond the initial learning phase. Furthermore, this study extends to mice a role for the hippocampus in transitive inference, as previously observed in other species.

A new approach to understanding the molecular mechanisms through which estrogens affect cognition

Traditional approaches to the study of hormones and cognition have been primarily observational or correlational in nature. Because this work does not permit causal relationships to be identified, very little is known about the specific molecules and cellular events through which hormones affect cognitive function. In this review, we propose a new approach to study hormones and memory, where the systematic blocking of cellular events can reveal which such events are necessary for hormones to influence memory consolidation. The discussion will focus on the modulation of the hippocampus and hippocampal memory by estrogens, given the extensive literature on this subject, and will illustrate how the application of this approach is beginning to reveal important new information about the molecular mechanisms through which estrogens modulate memory consolidation. The clinical relevance of this work will also be discussed.

Neuropeptides in learning and memory processes with focus on galanin

Neuropeptides represent by far the most common signalling molecules in the central nervous system. They are involved in a wide range of physiological functions and can act as neurotransmitters, neuromodulators or hormones in the central nervous system and in the periphery. Accumulating evidence during the past 40 years has implicated a number of neuropeptides in various cognitive functions including learning and memory. A major focus has been on the possibility that neuropeptides, by coexisting with classical neurotransmitters, can modulate classical transmitter function of importance for cognition. It has become increasingly clear that most transmitter systems in the brain can release a cocktail of signalling molecules including classical transmitters and several neuropeptides. However, the neuropeptides seem to come into action mainly under conditions of severe stress or aversive events, which have linked their action also to regulation of affective components of behaviour. This paper summarises some of the results of three neuropeptides, which can impact on hippocampal cognition by intrinsic (dynorphins, nociceptin) or extrinsic (galanin) modulation. The results obtained with these neuropeptides in rodent studies indicate that they are important for various aspects of hippocampal learning and memory as well as hippocampal plasticity. Recent studies in humans have also shown that dysregulation of these neuropeptides may be of importance for both neurodegenerative and neuropsychiatric disorders associated with cognitive impairments. It is concluded that compounds acting on neuropeptide receptor subtypes will represent novel targets for a number of disorders, which involve cognitive deficiencies.

Relational memory during infancy: evidence from eye tracking

Here we report evidence from a new eye-tracking measure of relational memory that suggests that 9-month-old infants can encode memories in terms of the relations among items, a function putatively subserved by the hippocampus. Infants learned about the association between faces that were superimposed on unique scenic backgrounds. During test trials, infants were shown three faces presented on a familiar scene. All three faces were equally familiar; however, one had been presented with the test background earlier. Visual behavior was recorded continuously using a TOBII eye tracker. Infants looked preferentially at the face that matched the test background very early in the trial; however, the time course of this preferential looking effect varied as a function of delay. These results suggest that by 9 months of age infants can form memories that represent the relations among items and maintain them over short delays.

The effect of mGlu8 deficiency in animal models of psychiatric diseases

The metabotropic glutamate receptor subtype 8 (mGlu(8)) is presynaptically located and regulates the release of the transmitter. Dysfunctions of this mechanism are involved in the pathophysiology of different psychiatric disorders. mGlu(8) deficient mice have been previously investigated in a range of studies, but the results are contradictory and there are still many open questions. Therefore, we tested mGlu(8)-deficient animals in different behavioral tasks that are commonly used in neuropsychiatric research. Our results show a robust contextual fear deficit in mGlu(8)-deficient mice. Furthermore, novel object recognition, chlordiazepoxide-facilitated extinction of operant conditioning and the acoustic startle response were attenuated by mGlu(8) deficiency. We found no changes in sensory processing, locomotor activity, prepulse inhibition, phencyclidine-induced changes in locomotion or prepulse inhibition, operant conditioning, conditioned fear to a discrete cue or in animal models of innate fear and post-traumatic stress disorder. We conclude that mGlu(8) might be a potential target for disorders with pathophysiological changes in brain areas where mGlu(8) modulates glutamate and gamma-amino butyric acid (GABA) transmission. Our data especially point to anxiety disorders involving exaggerated contextual fear, such as generalized anxiety disorders, and to conditions with disturbed declarative memory.

Modulation of medial temporal lobe activity in epilepsy patients with hippocampal sclerosis during verbal working memory

It has been traditionally assumed that medial temporal lobe (MTL) is not required for working memory (WM). However, animal lesion and electrophysiological studies and human neuropsychological and neuroimaging studies have provided increasing evidences of a critical involvement of MTL in WM. Based on previous findings, the central aim of this study was to investigate the contribution of the MTL to verbal WM encoding. Here, we used magnetoencephalography (MEG) to compare the patterns of MTL activation of 9 epilepsy patients suffering from left hippocampal sclerosis with those of 10 healthy matched controls while they performed a verbal WM task. MEG recordings allow detailed tracking of the time course of MTL activation. We observed impaired WM performance associated with changes in the dynamics of MTL activity in epilepsy patients. Specifically, whereas patients showed decreased activity in damaged MTL, activity in the contralateral MTL was enhanced, an effect that became significant in the 600- to 700-ms interval after stimulus presentation. These findings strongly support the crucial contribution of MTL to verbal WM encoding and provide compelling evidence for the proposal that MTL contributes to both episodic memory and WM. Whether this pattern is signaling reorganization or a normal use of a damaged structure is discussed.

The 5-HT7 receptor is involved in allocentric spatial memory information processing

The hippocampus has been implicated in aspects of spatial memory. Its ability to generate new neurons has been suggested to play a role in memory formation. Hippocampal serotonin (5-HT) neurotransmission has also been proposed as a contributor to memory processing. Studies have shown that the 5-HT(7) receptor is present in the hippocampus in relatively high abundance. Thus the aim of the present study was to investigate the possible role of the 5-HT(7) receptor in spatial memory using 5-HT(7) receptor-deficient mice (5-HT(7)(-/-)). A hippocampus-associated spatial memory deficit in 5-HT(7)(-/-) mice was demonstrated using a novel location/novel object test. A similar reduction in novel location exploration was observed in C57BL/6J mice treated with the selective 5-HT(7) receptor antagonist SB-269970. These findings prompted an extended analysis using the Barnes maze demonstrating that 5-HT(7)(-/-) mice were less efficient in accommodating to changes in spatial arrangement than 5-HT(7)(+/+) mice. 5-HT(7)(-/-) mice had specific impairments in memory compilation required for resolving spatial tasks, which resulted in impaired allocentric spatial memory whereas egocentric spatial memory remained intact after the mice were forced to switch back from striatum-dependent egocentric to hippocampus-dependent allocentric memory. To further investigate the physiological bases underlining these behaviors we compared hippocampal neurogenesis in 5-HT(7)(+/+) and 5-HT(7)(-/-) mice employing BrdU immunohistochemistry. The rate of cell proliferation in the dentate gyrus was identical in the two genotypes. From the current data we conclude that the 5-HT(7)(-/-) mice performed by remembering a simple sequence of actions that resulted in successfully locating a hidden target in a static environment.

Word sequence production in cognitive decline: forward ever, backward never

Backward recall of automatic word sequences involves declarative and working memory abilities known to be impaired in the early stages of cognitive decline. Yet its utility in the diagnosis of mild cognitive impairment and mild dementia has not been studied in detail. We analysed word sequence production in 234 participants drawn from three categories: subjective cognitive impairment, mild cognitive impairment, and mild dementia in Alzheimer's disease. The names of the months were used as a diagnostic target for investigating forward versus backward sequence production. Forward production remained normal across categories. In contrast, backward speed was significantly decreased in mild cognitive impairment. In dementia both speed and accuracy were impaired. Backward production had significant diagnostic classificatory power. We conclude that word sequence production yields data relevant to the diagnosis of dementia with a minimum of time and expense.

Infant imitation from television using novel touch screen technology

Infants learn less from a televised demonstration than from a live demonstration, the video deficit effect. The present study employs a novel approach, using touch screen technology to examine 15-month olds' transfer of learning. Infants were randomly assigned either to within-dimension (2D/2D or 3D/3D) or cross-dimension (3D/2D or 2D/3D) conditions. For the within-dimension conditions, an experimenter demonstrated an action by pushing avirtual button on a 2D screen or a real button on a 3D object. Infants were then given the opportunity to imitate using the same screen or object. For the 3D/2D condition, an experimenter demonstrated the action on the 3D object, and infants were given the opportunity to reproduce the action on a 2D touch screen (and vice versa for the 2D/3D condition). Infants produced significantly fewer target actions in the cross-dimension conditions than in the within-dimension conditions. These findings have important implications for infants' understanding and learning from 2D images and for their using 2D media as the basis of actions in the real world.

Experience-dependent eye movements reflect hippocampus-dependent (aware) memory

We investigated the relationship between experience-dependent eye movements, hippocampus-dependent memory, and aware memory. We measured eye movements in young adults, older adults, and memory-impaired patients with damage to the medial temporal lobe as they viewed 120 novel scenes and 120 previously viewed scenes. Participants indicated if each scene was old or new and also gave a confidence rating for the memory judgment. Young adults and older adults explored old scenes less than they explored new scenes, but the patients did not. For the young and older adults, this effect was observed only when participants were aware of the scene's familiar or novel status. In a second experiment, young adults viewed scenes that were either new, had been viewed previously, or had been viewed previously but had been changed (i.e., an object within the scene was either added or removed). The only instructions were to pay attention to the scenes and view each scene as it appeared, and there was no expectation that memory would be tested. Directly after the first altered scene was presented, participants were asked to classify the scene as new, old, or old but changed. Participants who were aware of the manipulation preferentially viewed the changed region, but participants who were unaware did not. These findings suggest that experience-dependent eye movements reflect hippocampus-dependent (and aware) memory, even when participants have no expectation that memory is being tested; and they are consistent with the view that awareness of what is learned is a fundamental characteristic of hippocampus-dependent memory.

Estrogens and age-related memory decline in rodents: what have we learned and where do we go from here?

The question of whether ovarian hormone therapy can prevent or reduce age-related memory decline in menopausal women has been the subject of much recent debate. Although numerous studies have demonstrated a beneficial effect of estrogen and/or progestin therapy for certain types of memory in menopausal women, recent clinical trials suggest that such therapy actually increases the risk of cognitive decline and dementia. Because rodent models have been frequently used to examine the effects of age and/or ovarian hormone deficiency on mnemonic function, rodent models of age-related hormone and memory decline may be useful in helping to resolve this issue. This review will focus on evidence suggesting that estradiol modulates memory, particularly hippocampal-dependent memory, in young and aging female rats and mice. Various factors affecting the mnemonic response to estradiol in aging females will be highlighted to illustrate the complications inherent to studies of estrogen therapy in aging females. Avenues for future development of estradiol-based therapies will also be discussed, and it is argued that an approach to drug development based on identifying the molecular mechanisms underlying estrogenic modulation of memory may lead to promising future treatments for reducing age-related mnemonic decline.

Multiple memory systems are unnecessary to account for infant memory development: an ecological model

How the memory of adults evolves from the memory abilities of infants is a central problem in cognitive development. The popular solution holds that the multiple memory systems of adults mature at different rates during infancy. The early-maturing system (implicit or nondeclarative memory) functions automatically from birth, whereas the late-maturing system (explicit or declarative memory) functions intentionally, with awareness, from late in the first year. Data are presented from research on deferred imitation, sensory preconditioning, potentiation, and context for which this solution cannot account and present an alternative model that eschews the need for multiple memory systems. The ecological model of infant memory development (N. E. Spear, 1984) holds that members of all species are perfectly adapted to their niche at each point in ontogeny and exhibit effective, evolutionarily selected solutions to whatever challenges each new niche poses. Because adults and infants occupy different niches, what they perceive, learn, and remember about the same event differs, but their raw capacity to learn and remember does not.

Quality-of-life considerations in substitute decision-making for severely disabled neonates: the problem of developing awareness

Substitute decision-makers for severely disabled neonates who can be kept alive but who will require constant medical interventions and will die at the latest in their teens are faced with a difficult decision when trying to decide whether to keep the infant alive. By and large, the primary focus of their decision-making centers on what is in the best interests of the newborn. The best-interests criterion, in turn, is importantly conditioned by quality-of-life considerations. However, the concept of quality of life is logically and ethically different for patients with a developing as opposed to a developed awareness. Unfortunately, this difference is ignored by current quality-of-life considerations, there are no quality-of-life measures that take this difference into account, and decision-making proceeds entirely without acknowledging this fact. This note outlines why this is a problem and why there is a need for a new set of tools that incorporates this distinction if the substitute decision-makers are to apply the best-interest criterion in a meaningful way.

Memory systems do not divide on consciousness: Reinterpreting memory in terms of activation and binding

There is a popular hypothesis that performance on implicit and explicit memory tasks reflects 2 distinct memory systems. Explicit memory is said to store those experiences that can be consciously recollected, and implicit memory is said to store experiences and affect subsequent behavior but to be unavailable to conscious awareness. Although this division based on awareness is a useful taxonomy for memory tasks, the authors review the evidence that the unconscious character of implicit memory does not necessitate that it be treated as a separate system of human memory. They also argue that some implicit and explicit memory tasks share the same memory representations and that the important distinction is whether the task (implicit or explicit) requires the formation of a new association. The authors review and critique dissociations from the behavioral, amnesia, and neuroimaging literatures that have been advanced in support of separate explicit and implicit memory systems by highlighting contradictory evidence and by illustrating how the data can be accounted for using a simple computational memory model that assumes the same memory representation for those disparate tasks.

Neuropeptide Y signalling on hippocampal stem cells in health and disease

Neuropeptides are emerging as key components in the hippocampal neurogenic niche in health and disease, regulating many aspects of neurogenesis and the synaptic integration of newly generated neurons. This review focuses on the role of neuropeptide Y in the control of stem/precursor cells in the postnatal and adult hippocampus. It is likely that neuropeptide Y releasing interneurons are key sensors of neural activity, modulating neurogenesis appropriately. This is likely to be a fruitful area of research for extending our understanding of the control of stem cells in the normal and diseased brain.

Magnetic resonance spectroscopy in anxiety disorders

OBJECTIVE

Magnetic resonance spectroscopy (MRS) is a non-invasive in vivo method used to quantify metabolites that are relevant to a wide range of brain processes. This paper briefly describes neuroimaging using MRS and provides a systematic review of its application to anxiety disorders.

METHOD

A literature review was performed in the PubMed, Lilacs and Scielo databases using the keywords spectroscopy and anxiety disorder. References of selected articles were also hand-searched for additional citations.

RESULTS

Recent studies have shown that there are significant metabolic differences between patients with anxiety disorders and healthy controls in various regions of the brain. Changes were mainly found in N-acetylaspartate, which is associated with neuronal viability, but some of them were also seen in creatine, a substance that is thought to be relatively constant among individuals with different pathological conditions.

CONCLUSIONS

MRS is a sophisticated neuroimaging technique that has provided useful insights into the biochemical and neurobiological basis of many anxiety disorders. Nevertheless, its utilization in some anxiety disorders is still modest, particularly social phobia and generalised anxiety. Although it is an extremely useful advance in neuroimaging, further research in other brain areas and patient populations is highly advisable.

Gradual changes in hippocampal activity support remembering the order of events

The hippocampus is thought to contribute to episodic memory in part by binding stimuli to their spatiotemporal context. The present study examined how hippocampal neuronal populations encode spatial and temporal context as rats performed a task in which they were required to remember the order of trial-unique sequences of odors. The results suggest that a gradual change in the pattern of hippocampal activity served as a temporal context for odor-sampling events and was important for successful subsequent memory of the order of those odors.

Nicotine and hippocampus-dependent learning: implications for addiction

Addiction is a complex disorder because many factors contribute to the development and maintenance of addiction. One factor is learning. For example, drug-context associations that develop during drug use could facilitate drug craving upon re-exposure to contexts previously associated with drugs. Additionally, deficits in cognitive processes associated with withdrawal could precipitate relapse in attempts to ameliorate those deficits. Because addiction and learning involve common neural areas and cell signaling cascades, addiction-related changes in processes underlying plasticity may contribute to addiction. This article examines similarities between addiction and learning at the behavioral, neural, and cellular levels, with emphasis on the neural substrates underlying the effects of acute nicotine, chronic nicotine, and withdrawal from chronic nicotine on hippocampus-dependent contextual learning.

Decreased N-acetyl-aspartate levels in anterior cingulate and hippocampus in subjects with post-traumatic stress disorder: a proton magnetic resonance spectroscopy study

The purpose of this study was to investigate the concentration of N-acetyl-aspartate (NAA) in the brain and its relationship with clinical characteristics in patients with post-traumatic stress disorder (PTSD). Proton magnetic resonance spectroscopy was performed in order to measure NAA concentrations in the anterior cingulate cortex (ACC) and bilateral hippocampus in 26 subjects with fire-related PTSD, who were survivors of a subway fire in South Korea, and 25 age- and sex-matched healthy comparison subjects. There were decreased NAA levels in the ACC (t = -3.88, d.f. = 49, P < 0.001) and bilateral hippocampus (right, t = -3.88, d.f. = 49, P < 0.001; left, t = -3.62, d.f. = 49, P < 0.001) in the PTSD group relative to the healthy comparison group. Also, NAA levels of the ACC (r = -0.43, n = 26, P = 0.027) and bilateral hippocampus (right, r = -0.48, n = 26, P = 0.013; left, r = -0.40, n = 26, P = 0.04) were negatively correlated with re-experience symptom scores in subjects with PTSD. In conclusion, our findings suggest that subjects with PTSD had decreased neuronal viabilities in the ACC and bilateral hippocampus, and that these deficits may play an important role in the pathophysiology of PTSD, especially regarding the re-experiencing of traumatic events.

Experience-dependent eye movements, awareness, and hippocampus-dependent memory

We asked what kind of memory is operating when eye movements change as the result of experience. Participants viewed scenes that were either novel, repeated, or manipulated (i.e., a change was introduced in one region of the scene). Eye movements differed depending on the past viewing history of each scene. Participants made fewer fixations and sampled fewer regions when scenes were repeated than when scenes were novel. When scenes were altered, participants made more fixations in the altered region, spent more time looking at the altered region, and made more transitions into and out of the altered region than in unchanged (matched) regions in the repeated scenes. Importantly, these effects occurred only when individuals were aware that a change had occurred. Participants who were unaware that the scene had been altered looked at the changed scenes in the same way that they looked at repeated scenes. Thus, there was no indication that eye movements could reveal an unaware (unconscious) form of memory. Instead, eye movements reflected conscious memory of whether the scene was repeated or manipulated. The findings were the same when awareness was assessed after viewing all the scenes (experiment 1) and when awareness was assessed after each scene was presented (experiment 2). In experiment 3, memory-impaired patients with damage limited to the hippocampus were impaired at deciding whether scenes were novel, repeated, or manipulated. Thus, the ability to consciously recollect recent encounters with scenes reflects a form of hippocampus-dependent memory. The findings show that experience-dependent eye movements in response to altered scenes reflect conscious, declarative memory, and they support the link between aware memory, declarative memory, and hippocampus-dependent memory.

Intra- and Inter-Item Associations Doubly Dissociate the Electrophysiological Correlates of Familiarity and Recollection

Single-process models of recognition memory posit that recognizing is based on a unidimensional value of global memory strength. By contrast, dual-process models propose the existence of two independent processes subserving the explicit recognition of previously encountered episodes, namely "familiarity" and "recollection." Familiarity represents a noncontextual form of recognition that may only support the retrieval of associative information when the to-be-associated information can be unitized, such as when two photographs depicting the same person are memorized (intra-item associations). Conversely, recollection enables retrieving associations between arbitrarily linked information, such as associations between photographs of different persons (inter-item associations). By measuring event-related brain potentials (ERPs), we obtained a double dissociation of familiarity and recollection that strongly favors dual-process accounts of recognition memory: the electrophysiological correlate of familiarity was significantly larger for intra- than for inter-item associations. Conversely, the electrophysiological correlate of recollection was significantly larger for inter- than for intra-item associations.

“The seven sins” of the Hebbian synapse: Can the hypothesis of synaptic plasticity explain long-term memory consolidation?

Memorizing new facts and events means that entering information produces specific physical changes within the brain. According to the commonly accepted view, traces of memory are stored through the structural modifications of synaptic connections, which result in changes of synaptic efficiency and, therefore, in formations of new patterns of neural activity (the hypothesis of synaptic plasticity). Most of the current knowledge on learning and initial stages of memory consolidation ("synaptic consolidation") is based on this hypothesis. However, the hypothesis of synaptic plasticity faces a number of conceptual and experimental difficulties when it deals with potentially permanent consolidation of declarative memory ("system consolidation"). These difficulties are rooted in the major intrinsic self-contradiction of the hypothesis: stable declarative memory is unlikely to be based on such a non-stable foundation as synaptic plasticity. Memory that can last throughout an entire lifespan should be "etched in stone." The only "stone-like" molecules within living cells are DNA molecules. Therefore, I advocate an alternative, genomic hypothesis of memory, which suggests that acquired information is persistently stored within individual neurons through modifications of DNA, and that these modifications serve as the carriers of elementary memory traces.

A computational theory of hippocampal function, and empirical tests of the theory

The main aim of the paper is to present an up-to-date computational theory of hippocampal function and the predictions it makes about the different subregions (dentate gyrus, CA3 and CA1), and to examine behavioral and electrophysiological data that address the functions of the hippocampus and particularly its subregions. Based on the computational proposal that the dentate gyrus produces sparse representations by competitive learning and via the mossy fiber pathway forces new representations on the CA3 during learning (encoding), it has been shown behaviorally that the dentate gyrus supports spatial pattern separation during learning. Based on the computational proposal that CA3-CA3 autoassociative networks are important for episodic memory, it has been shown behaviorally that the CA3 supports spatial rapid one-trial learning, learning of arbitrary associations where space is a component, pattern completion, spatial short-term memory, and sequence learning by associations formed between successive items. The concept that the CA1 recodes information from CA3 and sets up associatively learned backprojections to neocortex to allow subsequent retrieval of information to neocortex, is consistent with findings on consolidation. Behaviorally, the CA1 is implicated in processing temporal information as shown by investigations requiring temporal order pattern separation and associations across time; computationally this could involve temporal decay memory, and temporal sequence memory which might also require CA3. The perforant path input to DG is implicated in learning, to CA3 in retrieval from CA3, and to CA1 in retrieval after longer time intervals ("intermediate-term memory").

Declarative memory, awareness, and transitive inference

A characteristic usually attributed to declarative memory is that what is learned is accessible to awareness. Recently, the relationship between awareness and declarative (hippocampus-dependent) memory has been questioned on the basis of findings from transitive inference tasks. In transitive inference, participants are first trained on overlapping pairs of items (e.g., A+B-, B+C-, C+D-, and D+E-, where + and - indicate correct and incorrect choices). Later, participants who choose B over D when presented with the novel pair BD are said to demonstrate transitive inference. The ability to exhibit transitive inference is thought to depend on the fact that participants have represented the stimulus elements hierarchically (i.e., A>B>C>D>E). We found that performance on five-item and six-item transitive inference tasks was closely related to awareness of the hierarchical relationship among the elements of the training pairs. Participants who were aware of the hierarchy performed near 100% correct on all tests of transitivity, but participants who were unaware of the hierarchy performed poorly (e.g., on transitive pair BD in the five-item problem; on transitive pairs BD, BE, and CE in the six-item problem). When the five-item task was administered to memory-impaired patients with damage thought to be limited to the hippocampal region, the patients were impaired at learning the training pairs. All patients were unaware of the hierarchy and, like unaware controls, performed poorly on the BD pair. The findings indicate that awareness is critical for robust performance on tests of transitive inference and support the view that awareness of what is learned is a fundamental characteristic of declarative memory.

Molecular heterogeneity along the dorsal-ventral axis of the murine hippocampal CA1 field: a microarray analysis of gene expression

There has been increasing interest in functional heterogeneity along the septotemporal, dorsal-ventral (D-V) axis of the hippocampus. Although anatomical connectivity and lesion studies point to discrete roles for these sub-regions, the contribution of differential gene expression across this axis has not been systematically studied. Here we present findings from an Affymetrix microarray screen aimed at identifying genes in the CA1 region of the adult murine hippocampus that show significant differential expression along the D-V axis. Our results indicate that the vast majority of monitored genes (>90%) had tissue expression levels that differed by less than 20% between regions, while less than 0.1% of genes had expression levels that varied more than three-fold by sub-region. Only 23 probes showed a CA1 dorsoventral signal intensity ratio greater than three: 18 enriched dorsally and five enriched ventrally. Probes with the greatest difference in expression levels represent a range of genes with known functions in patterning and signaling, as well as genes without known function. Selective screening with digoxigenin-labeled in situ hybridization confirms the existence of CA1 sub-regionalized expression, with some genes exhibiting a graded expression pattern across the D-V axis, and others restricted to a discrete region. Our findings demonstrate that there are gene expression differences across the D-V axis of the adult murine hippocampus within traditionally recognized cytoarchitecturally defined boundaries. Combined with the previously recognized differences in connectivity and results from lesion studies, our data further confirm the existence of functional heterogeneity along the D-V axis.

Category learning and multiple memory systems

Categorization is a vitally important skill that people use every day. Early theories of category learning assumed a single learning system, but recent evidence suggests that human category learning may depend on many of the major memory systems that have been hypothesized by memory researchers. As different memory systems flourish under different conditions, an understanding of how categorization uses available memory systems will improve our understanding of a basic human skill, lead to better insights into the cognitive changes that result from a variety of neurological disorders, and suggest improvements in training procedures for complex categorization tasks.