Two functional components of the hippocampal memory system

Alcohol-induced neurodegeneration, suppression of transforming growth factor-beta, and cognitive impairment in rats: prevention by group II metabotropic glutamate receptor activation

BACKGROUND

Glutamatergic neurotransmission has been implicated in mechanisms of alcohol-induced neurodegeneration and cognitive impairment, but the underlying mechanism remains unknown. Here, we examined whether the group II metabotropic glutamate receptor agonist LY379268 prevents neuronal death and learning deficits in a rat model of binge-like exposure to alcohol.

METHODS

Following 4-day binge alcohol exposure concurrent with LY379268 or vehicle treatment, Fluoro-Jade B and transforming growth factor-beta (TGF-beta) staining were carried out, and reversal learning in the Morris water maze was assessed.

RESULTS

Fluoro-Jade B staining indicating neurodegeneration was most extensive in the ventral hippocampus and the entorhinal cortex (EC). LY379268 was potently neuroprotective in the EC but not in the dentate gyrus of the hippocampus. In parallel, binge alcohol exposure suppressed TGF-beta expression in both the EC and dentate gyrus, whereas LY379268 increased TGF-beta in the EC only. Finally, neuroprotective effects of LY379268 were accompanied by prevention of deficits in spatial reversal learning.

CONCLUSIONS

Our data support a neuroprotective role for group II metabotropic glutamate receptor agonists and TGF-beta in alcohol-induced neurodegeneration.

A single-trace dual-process model of episodic memory: a novel computational account of familiarity and recollection

Dual-process theories of episodic memory state that retrieval is contingent on two independent processes: familiarity (providing a sense of oldness) and recollection (recovering events and their context). A variety of studies have reported distinct neural signatures for familiarity and recollection, supporting dual-process theory. One outstanding question is whether these signatures reflect the activation of distinct memory traces or the operation of different retrieval mechanisms on a single memory trace. We present a computational model that uses a single neuronal network to store memory traces, but two distinct and independent retrieval processes access the memory. The model is capable of performing familiarity and recollection-based discrimination between old and new patterns, demonstrating that dual-process models need not to rely on multiple independent memory traces, but can use a single trace. Importantly, our putative familiarity and recollection processes exhibit distinct characteristics analogous to those found in empirical data; they diverge in capacity and sensitivity to sparse and correlated patterns, exhibit distinct ROC curves, and account for performance on both item and associative recognition tests. The demonstration that a single-trace, dual-process model can account for a range of empirical findings highlights the importance of distinguishing between neuronal processes and the neuronal representations on which they operate.

Medial temporal lobe activity during complex discrimination of faces, objects, and scenes: Effects of viewpoint

The medial temporal lobe (MTL), a set of heavily interconnected structures including the hippocampus and underlying entorhinal, perirhinal and parahippocampal cortex, is traditionally believed to be part of a unitary system dedicated to declarative memory. Recent studies, however, demonstrated perceptual impairments in amnesic individuals with MTL damage, with hippocampal lesions causing scene discrimination deficits, and perirhinal lesions causing object and face discrimination deficits. The degree of impairment on these tasks was influenced by the need to process complex conjunctions of features: discriminations requiring the integration of multiple visual features caused deficits, whereas discriminations that could be solved on the basis of a single feature did not. Here, we address these issues with functional neuroimaging in healthy participants as they performed a version of the oddity discrimination task used previously in patients. Three different types of stimuli (faces, scenes, novel objects) were presented from either identical or different viewpoints. Consistent with studies in patients, we observed increased perirhinal activity when participants distinguished between faces and objects presented from different, compared to identical, viewpoints. The posterior hippocampus, by contrast, showed an effect of viewpoint for both faces and scenes. These findings provide convergent evidence that the MTL is involved in processes beyond long-term declarative memory and suggest a critical role for these structures in integrating complex features of faces, objects, and scenes into view-invariant, abstract representations.

The storehouse/correspondence partition in memory research: Promises and perils

The novel correspondence metaphor outlined by Koriat & Goldsmith offers important advantages for studying critical issues of memory-accuracy. It also fits well with the current emphasis on the reconstructive nature of memory and on the role of cognitive, metacognitive, and motivational factors in memory performance. These positive features notwithstanding, the storehouse/correspondence framework faces potential perils having to do with its implied linkage to the laboratory/real-life controversy and its proposal of studying correspondence issues in isolation from memory phenomena captured by the storehouse paradigm.

On giving a more active and selective role to consciousness

An active role for conscious processes in the production of behaviour is proposed, involving top level controls in a hierarchy of behavioural control. It is suggested that by inhibiting or sensitizing lower levels in the hierarchy conscious processes can play a role in the organization of ongoing behaviour. Conscious control can be more or less evident, according to prevailing circumstances.

The control of consciousness via a neuropsychological feedback loop

Gray's neuropsychological model of consciousness uses a hierarchical feedback loop framework that has been extensively discussed by many others in psychology. This commentary therefore urges Gray to integrate with, or at least acknowledge previous models. It also points out flaws in his feedback model and suggests directions for further theoretical work.

Memory, metamemory, and conditional statistics

Koriat & Goldsmith's distinction between encoding processes and metamnemonic decision processes is theoretically and practically important, as is their methodology for separating the two. However, their accuracy measure is a conditional statistic, subject to the unfathomable selection effects that have hindered analogous measures in the past. We also find their arguments concerning basic and applied research mostly beside the point.

The hippocampal system: Dissociating its functional components and recombining them in the service of declarative memory

Continuing commentary raised several issues concerning our proposal that the hippocampus, parahippocampal region, and cortical association areas mediate different aspects of memory function. Recent relevant findings strengthen our argument that neocortical areas and the parahippocampal region maintain persistent encodings of specific single items and that the hippocampus mediates representations of the relations among these items. The reciprocally and closely interconnected structures that compose the hippocampal memory system work interactively to support flexible memory expression that is relevant to the natural behavior of animals and to conscious recollection in humans.

Don't leave the “un” off “consciousness”

Gray extrapolates from circuit models of psychopathology to propose neural substrates for the contents of consciousness. I raise three concerns: (1) knowledge of synaptic arrangements may be inadequate to fully support his model; (2) latent inhibition deficits in schizophrenia, a focus of this and related models, are complex and deserve replication; and (3) this conjecture omits discussion of the neuropsychological basis for the contents of the unconscious.

Consciousness is for other people

Gray has expanded his account of schizophrenia to explain consciousness as well. His theory explains neither phenomenon adequately because he treats individual minds (and brains) in isolation. The primary function of consciousness is to permit high level interactions with other conscious beings. The key symptoms of schizophrenia reflect a failure of this mechanism.

The hunting of the hippocampal function

Eichenbaum et al.'s (1994a) theory suffers from a lack of ecological validation. It is not at all clear why the hypothesized faculties would have evolved and what their adaptive value would be. I argue that hippocampal function can only be understood if the animal is seen in its natural context.

Comparators, functions, and experiences

The comparator model is insufficient for three reasons. First, consciousness is involved in the process of comparison as well as in the output. Second, we still do not have enough neurophysiological information to match the events of consciousness, although such knowledge is growing. Third, the anatomical localisation proposed can be damaged bilaterally but consciousness will persist.

Possible roles for a predictor plus comparator mechanism in human episodic recognition memory and imitative learning

This commentary is divided into two parts. The first considers a possible role for Gray's predictor plus comparator mechanism in human episodic recognition memory. It draws on the computational specifications of recognition outlined in Humphreys et al. (1994) to demonstrate how the logically necessary components of recognition tasks might be mapped onto the mechanism. The second part demonstrates how the mechanism outlined by Gray might be implicated in a form of imitative learning suitable for the acquisition of complex tasks.

Thinking about repairing thinking

The work of Sinden et al. suggests that it may be possible to produce improvement in the “highest” areas of brain function by transplanting brain tissue. What appears to be the limiting factor is not the complexity of the mental process under consideration but the discreteness of the lesion which causes the impairment and the appropriateness and accuracy of placement of the grafted tissue.

Consciousness does not seem to be linked to a single neural mechanism

On the basis of neuropsychological evidence, it is clear that attention should be given a role in any model (or conjecture) of consciousness. What is known about the many instances of dissociation between explicit and implicit knowledge after brain damage suggests that conscious experience might not be linked to a restricted area of the brain. Even if it were true that there is a single brain area devoted to consciousness, the subicular area would seem to be an unlikely possibility.

Segmentalized consciousness in schizophrenia

Segmentalized consciousness in schizophrenia reflects a loss of the normal Gestalt organization and contextualization of perception. Grays model explains such segmentalization in terms of septohippocampal dysfunction, which is consistent with known neuropsychological impairment in schizophrenia. However, other considerations suggest that everyday perception and its failure in schizophrenia also involve prefrontal executive mechanisms, which are only minimally elaborated by Gray.

Toward a theory of human memory: Data structures and access processes

Starting from Marr's ideas about levels of explanation, a theory of the data structures and access processes in human memory is demonstrated on 10 tasks. Functional characteristics of human memory are captured implementation-independently. Our theory generates a multidimensional task classification subsuming existing classifications such as the distinction between tasks that are implicit versus explicit, data driven versus conceptually driven, and simple associative (two-way bindings) versus higher order (threeway bindings), providing a broad basis for new experiments. The formal language clarifies the binding problem in episodic memory, the role of input pathways in both episodic and semantic (lexical) memory, the importance of the input set in episodic memory, and the ubiquitous calculation of an intersection in theories of episodic and lexical access.

Unitary consciousness requires distributed comparators and global mappings

Gray, like other recent authors, seeks a scientific approach to consciousness, but fails to provide a biologically convincing description, partly because he implicitly bases his model on a computationalist foundation that embeds the contents of thought in irreducible symbolic representations. When patterns of neural activity instantiating conscious thought are shorn of homuncular observers, it appears most likely that these patterns and the circuitry that compares them with memories and plans should be found distributed over large regions of neocortex.

On seeking the mythical fountain of consciousness

Because consciousness has an organizational, or functional, center, Gray supposes that there must be a corresponding physical center in the brain. He proposes further that since this center generates consciousness, ablating it would eliminate consciousness, while leaving behavior intact. But the center of consciousness is simply the product of the functional linkages among sensory input, memory, inner speech, and so on, and behavior.

The alternative to the storehouse metaphor

Koriat and Goldsmith clearly show the need for an alternative to the storehouse metaphor; however, the alternative metaphor they choose – the correspondence metaphor – is problematic. A more suitable one is the capacity metaphor.

Prospects for a cognitive neuroscience of consciousness

In this commentary, I point out some weaknesses in Gray's target article and, in the light of that discussion, I attempt to delineate the kinds of problem a cognitive neuroscience of consciousness faces on its way to a scientific understanding of subjective experience.

Memory metaphors and the real-life/laboratory controversy: Correspondence versus storehouse conceptions of memory

The study of memory is witnessing a spirited clash between proponents of traditional laboratory research and those advocating a more naturalistic approach to the study of “real-life” or “everyday” memory. The debate has generally centered on the “what” (content), “where” (context), and “how” (methods) of memory research. In this target article, we argue that the controversy discloses a further, more fundamental breach between two underlying memory metaphors, each having distinct implications for memory theory and assessment: Whereas traditional memory research has been dominated by thestorehousemetaphor, leading to a focus on thenumberof items remaining in store and accessible to memory, the recent wave of everyday memory research has shifted toward acorrespondencemetaphor, focusing on theaccuracyof memory in representing past events. The correspondence metaphor calls for a research approach that differs from the traditional one in important respects: in emphasizing the intentional –representational function of memory, in addressing the wholistic and graded aspects of memory correspondence, in taking an output-bound assessment perspective, and in allowing more room for the operation of subject-controlled metamemory processes and motivational factors. This analysis can help tie together soine of the what, where, and how aspects of the “real-life/laboratory” controversy. More important, however, by explicating the unique metatheoretical foundation of the accuracy-oriented approach to memory we aim to promote a more effective exploitation of the correspondence metaphor inbothnaturalistic and laboratory research contexts.

Therapeutic uses for neural grafts: Progress slowed but not abandoned

In spite of Stein and Glasier's justifiable conclusion that initial optimism concerning the immediate clinical applicability of neural transplantation was premature, there exists much experimental evidence to support the potential for incorporating this procedure into a therapeutic arsenal in the future. To realize this potential will require continued evolution of our knowledge at multiple levels of the clinical and basic neurosciences.

Septohippocampal comparator: Consciousness generator or attention feedback loop?

As Gray insists, his comparator model proposes a brute correlation only – of consciousness with septohippocampal output. I suggest that the comparator straddles a feedback loop that boosts the activation ofnovelrepresentations, thus helping them feature in present or recollected experience. Such a role in organizing conscious contents would transcend correlation and help explain how consciousness emerges from brain function.

What do memories correspond to?

Neither the storehouse nor the correspondence metaphor is an appropriate conceptual framework for memory research. Instead a meaning-based account of human memory is required. The correspondence metaphor is an advance over previous suggestions but entails an oversimple view of “accuracy.” Freud's account of memory may provide a more fruitful approach to memory and meaning.

Perspective, reflection, transparent explanation, and other minds

Perspective and reflection (whether involving conceptual or nonconceptual content) have each been considered in some way basic to phenomenal consciousness. Each has possible evolutionary value, though neither seems sufficient for consciousness. Consider an account of consciousness in terms of the combination of perspective and reflection, its relationship to the problem of other minds, and its capacity to inherit evolutionary explanation from its components.

Psychopathology and the discontinuity of conscious experience

It is accepted that “primary awareness” may emerge from the integration of two classes of information. It is unclear, however, why this cannot take place within the comparator rather than in conjunction with feedback to the perceptual systems. The model has plausibility in relation to the continuity of conscious experience in the normal waking state and may be extended to encompass certain aspects of the “sense of self” which are frequently disrupted in psychotic patients.

Sequential processing of “items” and “relations”

Eichenbaum et al. (1994a) hypothesized that perceptually distinct items and the relations among them are processed sequentially by the parahippocampal region and the hippocampal formation, respectively. Predictions based solely on their model's sequential-processing feature might prove easier to disconfirm than those based on its representational features. Two such predictions are discussed: (1) double dissociations should be impossible following hippocampal vs. parahippocampal lesions, and (2) hippocampal lesions should not exacerbate impairments that follow complete parahippocampal lesions.

The structure, operation, and functionality of intracerebral grafts

The concept of structure, operation, and functionality, as they may be understood by clinicians or researchers using neural transplantation techniques, are briefly defined. Following Stein & Glasier, we emphasize that the question of whether an intracerebral graft is really functional should be addressed not only in terms of what such a graft does in a given brain structure, but also in terms of what it does at the level of the organism.

Consciousness, memory, and the hippocampal system: What kind of connections can we make?

Gray's account is remarkable in its depth and scope but too little attention is paid to poor correspondences with the literature on hippocampal/subicular damage, the theta rhythm, and novelty detection. An alternative account, focusing on hippocampal involvement in organizing memories in a way that makes them accessible to conscious recollection but not in access to consciousness per se, avoids each of these limitations.

Communication and consciousness: A neural network conjecture

The communicative aspects of the contents of consciousness are analyzed in the framework of a neural network model of animal communication. We discuss some issues raised by Gray, such as the control of the contents of consciousness, the adaptive value of consciousness, conscious and unconscious behaviors, and the nature of a model's consciousness.

The NGF superfamily of neurotrophins: Potential treatment for Alzheimer's and Parkinson's disease

Stein & Glasier suggest embryonic neural tissue grafts as a potential treatment strategy for Alzheimer's and Parkinson's disease. As an alternative, we suggest that the family of nerve growth factor-related neurotrophins and their trk (tyrosine kinase) receptors underlie cholinergic basal forebrain (CBF) and dopaminergic substantia nigra neuron degeneration in these diseases, respectively. Therefore, treatment approaches for these disorders could utilize neurotrophins.

Overworking the hippocampus

The postulated hippocampal comparator, like any other subsystem, must rely on “syntactic” patterns in its “input,” and hence could not have the extraordinary powers Gray supposes. It may play a more modest role, but it is not the place “where it all comes together” for consciousness.

Consciousness and its (dis)contents

The first claim in the target article was that there is as yet no transparent, causal account of the relations between consciousness and brain-and-behaviour. That claim remains firm. The second claim was that the contents of consciousness consist, psychologically, of the outputs of a comparator system; the third consisted of a description of the brain mechanisms proposed to instantiate the comparator. In order to defend these claims against criticism, it has been necessary to clarify the distinction between consciousness-as-such and the contents of consciousness, to widen the description of the neural machinery instantiating the comparator system, and to clarify the relationship between the contents of consciousness in the here-and-now and episodic memory.

Some practical and theoretical issues concerning fetal brain tissue grafts as therapy for brain dysfunctions

Grafts of embryonic neural tissue into the brains of adult patients are currently being used to treat Parkinson's disease and are under serious consideration as therapy for a variety of other degenerative and traumatic disorders. This target article evaluates the use of transplants to promote recovery from brain injury and highlights the kinds of questions and problems that must be addressed before this form of therapy is routinely applied. It has been argued that neural transplantation can promote functional recovery through the replacement of damaged nerve cells, the reestablishment of specific nerve pathways lost as a result of injury, the release of specific neurotransmitters, or the production of factors that promote neuronal growth. The latter two mechanisms, which need not rely on anatomical connections to the host brain, are open to examination for nonsurgical, less intrusive therapeutic use. Certain subjective judgments used to select patients who will receive grafts and in assessment of the outcome of graft therapy make it difficult to evaluate the procedure. In addition, little long-term assessment of transplant efficacy and effect has been done in nonhuman primates. Carefully controlled human studies, with multiple testing paradigms, are also needed to establish the efficacy of transplant therapy.

The contents of consciousness: A neuropsychological conjecture

Drawing on previous models of anxiety, intermediate memory, the positive symptoms of schizophrenia, and goal-directed behaviour, a neuropsychological hypothesis is proposed for the generation of the contents of consciousness. It is suggested that these correspond to the outputs of a comparator that, on a moment-by-moment basis, compares the current state of the organism's perceptual world with a predicted state. An outline is given of the information-processing functions of the comparator system and of the neural systems which mediate them. The hypothesis appears to be able to account for a number of key features of the contents of consciousness. However, it is argued that neitherthis nor any existing comparable hypothesis is yet able to explain why the brain should generate conscious experience of any kind at all.

The correspondence metaphor of memory: Right, wrong, or useful?

Our response to the commentators covers four general issues: (1) How useful is our proposed conceptualization of the real-life/laboratory controversy in terms of the contrast between the correspondence and storehouse metaphors? (2) What is the relationship between these two metaphors? (3) What are the unique implications of the correspondence metaphor for memory assessment and theory? (4) What are the nature and role of memory metaphors in memory research? We stress that the correspondence metaphor can be usefully exploited independent of the real-life/laboratory controversy, but that a variety of other metaphors, including the storehouse, should also be utilized in order to more fully capture the myriad facets and functions of memory in everyday life.

The elusive quale

If sensations were behaviorally conceived, as they should be, as complex functional patterns of interaction between overt behavior and the environment, there would be no point in searching for them as instantaneous psychic elements (qualia) within the brain or as internal products of the brain.

Beyond the correspondence metaphor: When accuracy cannot be assessed

Koriat & Goldsmith propose that the correspondence metaphor captures the essence of everyday memory research. We suggest that correspondence is often not at issue because objective assessments of everyday events are frequently lacking. In these cases, other questions arise, such as how individuals evaluate the validity of memories and the significance they attach to those evaluations.

Contexts and functions of retrieval

Koriat & Goldsmith provide an excellent analysis of the flexibility of retrieval processes and how they are situationally dependent. I agree with their emphasis on functional considerations and argue that the traditional laboratory experiment motivates the subject to be accurate. However, I disagree with their strong claim that the quantity–accuracy distinction implies an essential discontinuity between traditional and naturalistic approaches to the study of memory.

Recovered consciousness: A proposal for making consciousness integral to neuropsychological theories of memory in humans and nonhumans

Why is consciousness associated with recovery of memories that are initially dependent on the hippocampal system? A hypothesis is proposed that the medial temporal lobe/hippocampal complex (MTL/H) receives as its input only information that is consciously apprehended. By a process termed “cohesion,” the MTL/H binds into a memory trace those neural elements that mediated the conscious experience so that effectively, “consciousness” is an integral part of the memory trace. It is the phenomenological records of events (Conway 1992), integrated consciousness-content packets, that are recovered when memory traces are retrieved.

Implications of output-bound measures for laboratory and field research in memory

Everyday memory tasks often require that researchers focus on output-bound measures of memory. As a result, nonmemorial processes (e.g., report option and grain size) may influence recall accuracy. These nonmemorial processes, usually eliminated by laboratory researchers, have the potential to explain some anomalous results and may even be useful to enhance everyday recollection.

Information synthesis in cortical areas as an important link in brain mechanisms of mind

To explore the mechanism of sensation correlations between EP (evoked potential) component amplitude and signal detection indices (d' and criterion) were studied. The time of sensation coincided with the peak latency of those EP components that showed a correlation with both indices. The components presumably reflected information synthesis in projection cortical neurons. A mechanism providing the synthesis process is proposed.