Hypotheses on mnemonic information processing by the brain

Behavioral, neurological, and psychiatric frailty of autobiographical memory

Autobiographical-episodic memory is considered to be the most complex of the five long-term memory systems. It is autonoetic, which means, self-reflective, relies on emotional colorization, and needs the features of place and time; it allows mental time traveling. Compared to the other four long-term memory systems-procedural memory, priming, perceptual, and semantic memory-it develops the latest in phylogeny and ontogeny, and is the most vulnerable of the five systems, being easily impaired by brain damage and psychiatric disorders. Furthermore, it is characterized by its fragility and proneness to distortion due to environmental influences and subsequent information. On the brain level, a distinction has to be made between memory encoding and consolidating, memory storage, and memory retrieval. For encoding, structures of the limbic system, with the hippocampus in its center, are crucial, for storage of widespread cortical networks, and for retrieval again a distributed recollection network, in which the prefrontal cortex plays a crucial role, is engaged. Brain damage and psychiatric diseases can lead to what is called "focal retrograde amnesia." In this context, the clinical picture of dissociative or functional or psychogenic amnesia is central, as it may result in autobiographical-emotional amnesia of the total past with the consequence of an impairment of the self as well. The social environment therefore can have a major impact on the brain and on autobiographical-episodic memory processing. This article is categorized under: Psychology > Memory.

Dynamics of memory engrams

In this update article, we focus on "memory engrams", which are traces of long-term memory in the brain, and emphasizes that they are not static but dynamic. We first introduce the major findings in neuroscience and psychology reporting that memory engrams are sometimes diffuse and unstable, indicating that they are dynamically modified processes of consolidation and reconsolidation. Second, we introduce and discuss the concepts of cell assembly and engram cell, the former has been investigated by psychological experiments and behavioral electrophysiology and the latter is defined by recent combination of activity-dependent cell labelling with optogenetics to show causal relationships between cell population activity and behavioral changes. Third, we discuss the similarities and differences between the cell assembly and engram cell concepts to reveal the dynamics of memory engrams. We also discuss the advantages and problems of live-cell imaging, which has recently been developed to visualize multineuronal activities. The last section suggests the experimental strategy and background assumptions for future research of memory engrams. The former encourages recording of cell assemblies from different brain regions during memory consolidation-reconsolidation processes, while the latter emphasizes the multipotentiality of neurons and regions that contribute to dynamics of memory engrams in the working brain.

Functional (dissociative) retrograde amnesia

Retrograde amnesia is described as condition which can occur after direct brain damage, but which occurs more frequently as a result of a psychiatric illness. In order to understand the amnesic condition, content-based divisions of memory are defined. The measurement of retrograde memory is discussed and the dichotomy between "organic" and "psychogenic" retrograde amnesia is questioned. Briefly, brain damage-related etiologies of retrograde amnesia are mentioned. The major portion of the review is devoted to dissociative amnesia (also named psychogenic or functional amnesia) and to the discussion of an overlap between psychogenic and "brain organic" forms of amnesia. The "inability of access hypothesis" is proposed to account for most of both the organic and psychogenic (dissociative) patients with primarily retrograde amnesia. Questions such as why recovery from retrograde amnesia can occur in retrograde (dissociative) amnesia, and why long-term new learning of episodic-autobiographic episodes is possible, are addressed. It is concluded that research on retrograde amnesia research is still in its infancy, as the neural correlates of memory storage are still unknown. It is argued that the recollection of episodic-autobiographic episodes most likely involves frontotemporal regions of the right hemisphere, a region which appears to be hypometabolic in patients with dissociative amnesia.

Memory and self-neuroscientific landscapes

Relations between memory and the self are framed from a number of perspectives-developmental aspects, forms of memory, interrelations between memory and the brain, and interactions between the environment and memory. The self is seen as dividable into more rudimentary and more advanced aspects. Special emphasis is laid on memory systems and within them on episodic autobiographical memory which is seen as a pure human form of memory that is dependent on a proper ontogenetic development and shaped by the social environment, including culture. Self and episodic autobiographical memory are seen as interlocked in their development and later manifestation. Aside from content-based aspects of memory, time-based aspects are seen along two lines-the division between short-term and long-term memory and anterograde-future-oriented-and retrograde-past-oriented memory. The state dependency of episodic autobiographical is stressed and implications of it-for example, with respect to the occurrence of false memories and forensic aspects-are outlined. For the brain level, structural networks for encoding, consolidation, storage, and retrieval are discussed both by referring to patient data and to data obtained in normal participants with functional brain imaging methods. It is elaborated why descriptions from patients with functional or dissociative amnesia are particularly apt to demonstrate the facets in which memory, self, and personal temporality are interwoven.

Towards solving the riddle of forgetting in functional amnesia: recent advances and current opinions

Remembering the past is a core feature of human beings, enabling them to maintain a sense of wholeness and identity and preparing them for the demands of the future. Forgetting operates in a dynamic neural connection with remembering, allowing the elimination of unnecessary or irrelevant information overload and decreasing interference. Stress and traumatic experiences could affect this connection, resulting in memory disturbances, such as functional amnesia. An overview of clinical, epidemiological, neuropsychological, and neurobiological aspects of functional amnesia is presented, by preponderantly resorting to own data from patients with functional amnesia. Patients were investigated medically, neuropsychologically, and neuroradiologically. A detailed report of a new case is included to illustrate the challenges posed by making an accurate differential diagnosis of functional amnesia, a condition that may encroach on the boundaries between psychiatry and neurology. Several mechanisms may play a role in "forgetting" in functional amnesia, such as retrieval impairments, consolidating defects, motivated forgetting, deficits in binding and reassembling details of the past, deficits in establishing a first person autonoetic connection with personal events, and loss of information. In a substantial number of patients, we observed a synchronization abnormality between a frontal lobe system, important for autonoetic consciousness, and a temporo-amygdalar system, important for evaluation and emotions, which provides empirical support for an underlying mechanism of dissociation (a failure of integration between cognition and emotion). This observation suggests a mnestic blockade in functional amnesia that is triggered by psychological or environmental stress and is underpinned by a stress hormone mediated synchronization abnormality during retrieval between processing of affect-laden events and fact-processing.

Hippocampal activity, olfaction, and sniffing: an olfactory input to the dentate gyrus

Experiments in freely moving rats showed that olfactory stimuli elicit a burst of rhythmical 15-30 Hz waves in or near the hilus of the dentate gyrus but not in adjacent regions of CA1. This fast wave burst is not elicited by visual, auditory, or somatosensory inputs and is not related to motor activity. Electrical stimulation of the olfactory bulb evokes a complex potential in the hilus of the dentate gyrus but not in adjacent regions of CA1. Experiments making use of wave-triggered averaging demonstrated that there is a degree of phase-locking between (a) hippocampal RSA and sniffing or other respiratory patterns, (b) hippocampal RSA and the initiation of jumping, and (c) respiration and the initiation of jumping. An early hypothesis that the hippocampus and dentate gyrus are part of an olfacto-motor mechanism may merit re-examination.

Location of lesions in Korsakoff's syndrome: neuropsychological and neuropathological data on two patients

Psychometric and neuropathological findings on two Korsakoff amnesics are described. Both patients showed anterograde and retrograde amnesia, poor performance on the Peterson short-term memory task, on the Wisconsin card sort test and on certain visuo-spatial tasks. Patient J.W. performed consistently worse on tests of anterograde, but not retrograde amnesia, whereas patient B.C. showed more perseverative difficulties and, unlike J.W., his measured intelligence seemed to have declined from its premorbid level. Both patients showed marked neuronal loss from the medial mammillary bodies and a narrow band of gliosis in the medial thalamus, adjacent to the wall of the third ventricle, a region known as the paratenial nucleus. Only B.C. showed visible signs of cortical atrophy. Morphometric measures did, however, reveal reduced nucleolar volumes in layers III and V of the frontal cortex, with B.C. also showing more marked neuronal loss from these layers. B.C. also showed neuronal loss from the CA1 region of the hippocampus and reduced nucleolar volumes in the septum. Significantly, both patients had normal neuronal numbers and nucleolar volumes in the nucleus basalis of Meynert. J.W. only showed greater dysfunction than B.C. in one region: the locus caeruleus. This finding was related to his more severe amnesia.

Verbal memory in brain damaged patients under different conditions of retrieval aids: a study of frontal, temporal, and diencephalic damaged subjects

The performance of 36 patients, divided into six groups, and 13 control subjects was investigated in paired-associate learning. The patients had right or left prefrontal, right or left anterior lateral temporopolar or medial temporal lobe damage, or lesions restricted to diencephalic areas. As tasks, two lists of paired words had to be learned, with the first list presenting only the word pairs, and the second one embedding the word pairs in sentences of a highly imaginable content. Recall consisted of immediate or delayed (48 hrs) free recall of the first list (condition I), of immediate or delayed recall of the second list (with visual imagery as a learning aid; condition II), and of cued recall (in which the sentence form was presented with a blank space where the word to be recalled had been previously; condition III). Control subjects clearly performed best under all conditions, manifesting a ceiling effect for the second and third ones under immediate recall. Among the brain-damaged groups the diencephalic subjects were poorest and gained only little from the aids given for learning and recall. Of the four patients with medial temporal lobe damage, those two with bilateral lesions were nearly as bad as the diencephalic lesioned subjects. The other patients were markedly inferior to the control subjects, but gained considerably under conditions II and III. These statements hold for immediate and delayed recall, though for the delayed recall conditions all groups showed a reduction in performance which amounted to roughly half of the values they had had under immediate recall. It is concluded that increasing the possibilities for depth of information processing assists brain damaged (as well as normal) subjects in verbal learning, but that the advantage of aiding them at the moment of encoding and retrieval is highest for patients with restricted lesions and/or with lesions not invading the two regions most regularly implicated in long-term information processing (the diencephalon and the medial temporal lobe area).

Single unit activity in cat prefrontal and parietal cortex during performance of a symmetrically reinforced go-no go task

Relationships between the performance in a symmetrically reinforced go-no go task and cellular firing patterns in prefrontal and parietal association areas of the neocortex were studied in six cats. During recordings, animals lay in a box, with their heads fixed to a stereotaxic frame, and performed an auditory go-no go task by pressing a retractable lever in front of them. Units obtained were classified into eight types according to the correlation of their activity changes with aspects of the task and/or with sensory stimuli. These types were (poly-) sensory, reward related, EMG-related, EOG-related, event-related, movement-initiating, expressing expectancy or novelty, and nonspecific or task-unrelated active units. Between the two recording areas a considerable degree of similarity was obtained in unit firing patterns. It was concluded that within the cerebral cortex, and especially within its association areas, a considerable functional overlap exists, that neurons may be involved in the processing of several and rather different phenomena, and that the processing of information at this level of the brain is generally done via widespread, interwoven neuronal nets so that only the average network activity, but not that of a particular, single neuron, represents a stimulus or an event.

The neuropathology of amnesia

Relations between brain damage and memory disturbance are outlined with emphasis on the so-called amnesic syndrome. Following a brief introduction into forms of memory and memory failures, the basic causes of brain damaage (with relevance to amnestic failures) are described. Thereafter, the two best-known forms of brain damage-amnesia relations are reviewed: the consequences of damage to medial temporal lobe structures and to diencephalic regions. For the cases with medial temporal lobe damage, evidence is reported in greater detail for H.M., who has been examined more than any other amnesic patient for more than 30 years now, as a considerable amount of literature has accumulated on his behavior in diverse situations. Other cases with more or less circumscribed damage to medial temporal lobe structures are reviewed so as to outline criteria for or against the hypothesis that there are regions within the medial temporal lobe whose damage might be critical for the amnesic syndrome. Two cases of diencephalic amnesia are summarized in particular (cases of Mair et al., 1979) as they have received extensive neuropsychological and neuropathological investigation. Other cases with, for example, Korsakoff's disease are reviewed, as well as cases with diencephalic, or combined mesencephalic-diencephalic damage without nutritional causes. A third group of patients with massive, but still selective amnesic disturbances are then described: cases of basal forebrain damage, followed by descriptions of Alzheimer's disease which has similarities in the underlying neuropathology. This leads over to cases with more generalized intellectual deteriorations (dementia), which may have developed on the basis of primarily cortical damage or damage principally to basal ganglia structures. After reviewing cases with mainly material-specific memory failures--usually as a consequence of restricted neocortical damage--a separate section follows on patients in whom retrograde amnesia is the prominent symptom. The contribution of animal models of human amnesia is critically reviewed and discrepancies are analyzed between human and animal memory disturbances. This section emphasizes the value of investigating inter-dependencies between brain structures by pointing out that relations between memory disturbances and brain damage may be more complicated than apparent from a simple structure-function assignment. This aspect is further followed up in the conclusions.