Episodic memory and the self in a case of isolated retrograde amnesia

Isolated retrograde amnesia is defined as impaired recollection of experiences pre-dating brain injury with relatively preserved anterograde learning and memory. We present findings from a patient (M.L.) with isolated retrograde amnesia following severe traumatic brain injury (TBI) that address hypotheses of the interrelationships of focal neuropathology, episodic memory and the self. M.L. is densely amnesic for experiences predating his injury, but shows normal anterograde memory performance on a variety of standard tests of recall and recognition. The cognitive processes underlying this performance were examined with the remember/know technique, which permits separation of episodic from non-episodic contributions to memory tests by quantifying subjects' reports of re-experiencing aspects of the encoding episode. The results demonstrated that M.L. does not episodically re-experience post-injury events to the same extent as control subjects, although he can use familiarity or other non-episodic processes to distinguish events he has experienced from those he has not experienced. M.L.'s MRI showed damage to the right ventral frontal cortex and underlying white matter, including the uncinate fasciculus, a frontotemporal band of fibres previously hypothesized to mediate retrieval of specific events from one's personal past. Recent functional neuroimaging evidence of an association between right frontal lobe functioning and episodic retrieval demands suggest that M.L.'s memory deficits are related to this focal injury. This hypothesis was supported by right frontal polar hypoactivation in M.L. in response to episodic retrieval demands when he was examined with a cognitive activation H2(15)O PET paradigm that reliably activated this frontal region in both healthy controls and patients with TBI carefully matched to M.L. (but without isolated retrograde amnesia). He also showed increased left inferomedial temporal activation relative to control subjects, suggesting that his spared anterograde memory is mediated through increased reliance on medial temporal lobe structures. Re-experiencing events as part of one's past is based on autonoetic awareness, i.e. awareness of oneself as a continuous entity across time. This form of awareness also supports the formulation of future goals and the implementation of a behavioural guidance system to achieve them. The findings from this study converge to suggest that M.L. has impaired autonoetic awareness attributable to right ventral frontal lobe injury, including right frontal-temporal disconnection. Reorganized brain systems mediate certain preserved cognitive operations in M.L., but without the normal complement of information concerning the self with respect to both past and future events.

Hippocampal PET activations of memory encoding and retrieval: the HIPER model

A meta-analysis of experimentally induced changes in blood flow ("activations") in positron emission tomography (PET) studies of memory has revealed an orderly functional anatomic pattern: Activations in the hippocampal region associated with episodic memory encoding are located primarily in the rostral portions of the region, whereas activations associated with episodic memory retrieval are located primarily in the caudal portions. These findings are based on an analysis of a sample of 54 "hippocampal encoding and retrieval" activations that were culled from an overall database consisting of 52 published PET studies of memory. We refer to this general pattern of rostrocaudal gradient of encoding and retrieval PET activations as the HIPER (Hippocampal Encoding/Retrieval) model. The model suggests a division of memory-related labor between the rostral and caudal portions of the hippocampal formation. Because functional anatomic pattern of encoding and retrieval activation that defines the HIPER model was unprecedented and unexpected, it is difficult to relate the model to what is already known or thought about functional neuroanatomy of episodic memory in the hippocampal regions. The model is interesting primarily because its exploration may yield fresh insights into the neural basis of human memory.

Episodic and declarative memory: role of the hippocampus

The fact that medial temporal lobe structures, including the hippocampus, are critical for declarative memory is firmly established by now. The understanding of the role that these structures play in declarative memory, however, despite great efforts spent in the quest, has eluded investigators so far. Given the existing scenario, novel ideas that hold the promise of clarifying matters should be eagerly sought. One such idea was recently proposed by Vargha-Khadem and her colleagues (Science 1997; 277:376-380) on the basis of their study of three young people suffering from anterograde amnesia caused by early-onset hippocampal pathology. The idea is that the hippocampus is necessary for remembering ongoing life's experiences (episodic memory), but not necessary for the acquisition of factual knowledge (semantic memory). We discuss the reasons why this novel proposal makes good sense and why it and its ramifications should be vigorously pursued. We review and compare declarative and episodic theories of amnesia, and argue that the findings reported by Vargha-Khadem and her colleagues fit well into an episodic theory that retains components already publicized, and adds new ones suggested by the Vargha-Khadem et al. study. Existing components of this theory include the idea that acquisition of factual knowledge can occur independently of episodic memory, and the idea that in anterograde amnesia it is quite possible for episodic memory to be more severely impaired than semantic memory. We suggest a realignment of organization of memory such that declarative memory is defined in terms of features and properties that are common to both episodic and semantic memory. The organization of memory thus modified gives greater precision to the Vargha-Khadem et al. neuroanatomical model in which declarative memory depends on perihippocampal cortical regions but not on the hippocampus, whereas episodic memory, which is separate from declarative memory, depends on the hippocampus.

Unilateral medial temporal lobe memory impairment: type deficit, function deficit, or both?

Previous research has characterized memory deficits resulting from unilateral hippocampal system damage as 'material specific', suggesting that left damage results in verbal memory impairment with preservation of visuospatial function and the converse with right damage. Implicit within this hypothesis are the assumptions that the systems are independent and memory is lateralized for each type of material. To test the verbal component of this hypothesis, unilateral hippocampal lesion and commissurotomy patients were compared with controls on a multiple-list free-recall task. The material specific hypothesis predicts severe impairment only with left lesions; right lesions and commissurotomy patients should be only minimally impaired. However, secondary memory was compromised at immediate recall for all patient groups, with both unilateral groups showing comparable and severe verbal episodic memory deficits. Final testing across all lists also revealed severe impairment in commissurotomy patients. Finding both unilateral groups to be similarly impaired for verbal material is taken as evidence against a material specific deficit during this verbal episodic memory task. Although previous data suggest that left patients are considerably more impaired during some verbal tasks, this may not be specific to the material, but rather the combination of material and task demands. Implications for the material specific hypothesis are discussed.

Functional brain imaging of episodic and semantic memory with positron emission tomography

Human memory is composed of several independent but interacting systems. These include a system for remembering general knowledge, semantic memory, and a system for recollection of personal events, episodic memory. The results of positron emission tomography (PET) studies of regional cerebral blood flow indicate that networks of distributed brain regions subserve episodic and semantic memory. Some networks seem to be generally engaged in memory processes whereas the involvement of others is specific to factors such as the type of information to be remembered or the level of retrieval success. The PET findings help to understand memory dysfunction (a) by showing that multiple brain regions are involved in different memory processes and (b) by sharpening the interpretation of the functional role of different brain regions.

Age-related differences in effective neural connectivity during encoding and recall

Age-related differences in brain activity may reflect local neural changes in the regions involved or they may reflect a more global transformation of brain function. To investigate this issue, we applied structural equation modeling to the results of a positron emission tomography (PET) study in which young and old adults encoded and recalled word pairs. In the young group there was a shift from positive interactions involving the left prefrontal cortex during encoding to positive interactions involving the right prefrontal cortex during recall, whereas in the old group frontal interactions were mixed during encoding and bilaterally positive during recall. The present results suggest that age-related changes in neural activation are partly due to age-related changes in effective connectivity in the neural network underlying the task.

Brain regions differentially involved in remembering what and when: a PET study

Recollecting a past episode involves remembering not only what happened but also when it happened. We used positron emission tomography (PET) to directly contrast the neural correlates of item and temporalorder memory. Subjects studied a list of words and were then scanned while retrieving information about what words were in the list or when they occurred within the list. Item retrieval was related to increased neural activity in medial temporal and basal forebrain regions, whereas temporal-order retrieval was associated with activations in dorsal prefrontal, cuneus/precuneus, and right posterior parietal regions. The dissociation between temporal and frontal lobe regions confirms and extends previous lesion data. The results show that temporal-order retrieval involves a network of frontal and posterior brain regions.

Event-related brain potential correlates of two states of conscious awareness in memory

We report an event-related potential (ERP) experiment of human recognition memory that explored the relation between conscious awareness and electrophysiological activity of the brain. We recorded ERPs from healthy adults while they made "remember" and "know" recognition judgments about previously seen words. These two kinds of judgments reflect "autonoetic" and "noetic" awareness, respectively. The ERP effects differed between the two kinds of awareness while they were similar for "true" and "false" recognition. Noetic awareness was associated with a temporoparietal positivity in the N400 range (325-600 ms) and a late (600-1,000 ms) frontocentral negativity, whereas autonoetic awareness was associated with a widespread, late, bifrontal and left parietotemporal (600-1000 ms) positivity. In the very late (1,300-1, 900 ms) time window, a right frontal positivity was observed for both remember and know judgments of both true and false targets. These results provide physiological evidence for two types of conscious awareness in episodic memory retrieval.

Toward a theory of episodic memory: the frontal lobes and autonoetic consciousness

Adult humans are capable of remembering prior events by mentally traveling back in time to re-experience those events. In this review, the authors discuss this and other related capabilities, considering evidence from such diverse sources as brain imaging, neuropsychological experiments, clinical observations, and developmental psychology. The evidence supports a preliminary theory of episodic remembering, which holds that the prefrontal cortex plays a critical, supervisory role in empowering healthy adults with autonoetic consciousness-the capacity to mentally represent and become aware of subjective experiences in the past, present, and future. When a rememberer mentally travels back in subjective time to re-experience his or her personal past, the result is an act of retrieval from episodic memory.

Cognitive subtractions may not add up: the interaction between semantic processing and response mode

Determining the areas of brain activity associated with cognitive processing has typically relied on the use of a subtraction paradigm, which is based on the premise that the neural processes underlying behavior are additive. If the additivity assumption is valid then brain regions associated with a semantic processing task should be the same regardless of how participants make a response. To investigate this proposition, participants underwent six PET scans, in which they made semantic or letter word judgments, responding "yes" or "no" in three different modes: mouse-clicking, spoken response, or silent thought. Analyses showed an increase in regional cerebral blood flow associated with semantic processing in the left inferior frontal cortex, anterior cingulate, and right cerebellum for all three response conditions. However, there was a significant interaction: the greatest increase was observed in the mouse-click condition and the weakest change seen with silent thought. Moreover, other areas of the brain were uniquely activated for each response mode. The results indicate that different areas of the brain were recruited for semantic processing depending on how participants had to organize their responses. Implications for the additivity assumption and methods of analysis to be used in conjunction with the subtraction technique are discussed.

Memory beyond the hippocampus

Improved neuroanatomical knowledge, technical and methodological innovations (such as PET), and more refined conceptualizations of memory have inspired a reappraisal of theoretical beliefs regarding the role of the hippocampus in memory. In the past few years, it has become apparent that the influence of the medial temporal lobe regions extends beyond memory and that memory processes (such as encoding, consolidation and retrieval) involve not only the hippocampus and the medial temporal and diencephalic regions, but also widely distributed neocortical and perhaps even cerebellar regions.

Functional Neuroanatomy of Recall and Recognition: A PET Study of Episodic Memory

The purpose of this study was to directly compare the brain regions involved in episodic-memory recall and recognition. Changes in regional cerebral blood flow were measured by positron emission tomography while young healthy test persons were either recognizing or recalling previously studied word pairs. Reading of previously nonstudied pairs served as a reference task for subtractive comparisons. Compared to reading, both recall and recognition were associated with higher blood flow (activation) at identical sites in the right prefrontal cortex (areas 47, 45, and 10) and the anterior cingulate. Compared to recognition, recall was associated with higher activation in the anterior cingulate, globus pallidus, thalamus, and cerebellum, suggesting that these components of the cerebello-frontal pathway play a role in recall processes that they do not in recognition. Compared to recall, recognition was associated with higher activation in the right inferior parietal cortex (areas 39, 40, and 19), suggesting a larger perceptual component in recognition than in recall. Contrary to the expectations based on lesion data, the activations of the frontal regions were indistinguishable in recall and recognition. This finding is consistent with the notion that frontal activations in explicit memory tasks are related to the general episodic retrieval mode or retrieval attempt, rather than to specific mechanisms of ecphory (recovery of stored information).

Age-related differences in neural activity during memory encoding and retrieval: a positron emission tomography study

Positron emission tomography (PET) was used to compare regional cerebral blood flow (rCBF) in young (mean 26 years) and old (mean 70 years) subjects while they were encoding, recognizing, and recalling word pairs. A multivariate partial-least-squares (PLS) analysis of the data was used to identify age-related neural changes associated with (1) encoding versus retrieval and (2) recognition versus recall. Young subjects showed higher activation than old subjects (1) in left prefrontal and occipito-temporal regions during encoding and (2) in right prefrontal and parietal regions during retrieval. Old subjects showed relatively higher activation than young subjects in several regions, including insular regions during encoding, cuneus/precuneus regions during recognition, and left prefrontal regions during recall. Frontal activity in young subjects was left-lateralized during encoding and right-lateralized during recall [hemispheric encoding/retrieval asymmetry (HERA)], whereas old adults showed little frontal activity during encoding and a more bilateral pattern of frontal activation during retrieval. In young subjects, activation in recall was higher than that in recognition in cerebellar and cingulate regions, whereas recognition showed higher activity in right temporal and parietal regions. In old subjects, the differences in blood flow between recall and recognition were smaller in these regions, yet more pronounced in other regions. Taken together, the results indicate that advanced age is associated with neural changes in the brain systems underlying encoding, recognition, and recall. These changes take two forms: (1) age-related decreases in local regional activity, which may signal less efficient processing by the old, and (2) age-related increases in activity, which may signal functional compensation.

The neural correlates of intentional learning of verbal materials: a PET study in humans

The purpose of this study was to identify the brain regions invoked when subjects attempt to learn verbal materials for a subsequent memory test. Twelve healthy subjects undertook two different tasks: reading and encoding of word pairs, while they were being scanned using [15O]H2O positron emission tomography (PET). As expected, the encoding pairs were remembered much better (recall 39% vs. 8%; P < 0.001) than reading pairs in a subsequent memory test. The encoding scans, as compared to reading scans, showed activation of the left prefrontal cortex, the anterior cingulate cortex and the left medial temporal cortex. The left prefrontal activations were in two discrete regions: (i) a left anterior and inferior left prefrontal (Brodmann's areas 45, 46) which we attribute to semantic processing; and (ii) a left posterior mid-frontal region (BA 6, 44) which may reflect rote rehearsal. We interpret the data to suggest that when subjects use cognitive strategies of semantic processing and rote-rehearsal to learn words, they invoke discrete regions of the left prefrontal cortex. And this activation of the left prefrontal cortex along with the medial temporal region leads to a neurophysiological memory trace which can be used to guide subsequent memory retrieval.

General and specific brain regions involved in encoding and retrieval of events: what, where, and when

Remembering an event involves not only what happened, but also where and when it occurred. We measured regional cerebral blood flow by positron emission tomography during initial encoding and subsequent retrieval of item, location, and time information. Multivariate image analysis showed that left frontal brain regions were always activated during encoding, and right superior frontal regions were always activated at retrieval. Pairwise image subtraction analyses revealed information-specific activations at (i) encoding, item information in left hippocampal, location information in right parietal, and time information in left fusiform regions; and (ii) retrieval, item in right inferior frontal and temporal, location in left frontal, and time in anterior cingulate cortices. These results point to the existence of general encoding and retrieval networks of episodic memory whose operations are augmented by unique brain areas recruited for processing specific aspects of remembered events.

Network analysis of positron emission tomography regional cerebral blood flow data: ensemble inhibition during episodic memory retrieval

Two important objectives in the neuroscience of memory are (1) identification of neural pathways involved in memory processes; and (2) characterization of the pattern of interactions between these pathways. Functional neuroimaging can contribute to both of these goals. Using image subtraction analysis of regional cerebral blood flow data measured with positron emission tomography, we identified brain regions that changed activity during episodic memory retrieval (visual work recognition). Relative to a baseline reading task, decreased activity was observed in bilateral prefrontal, bilateral anterior and posterior temporal, and posterior cingulate cortices. Brain regions showing increased activity were the right prefrontal (different from deactivated regions), left anterior cingulate, and left occipital cortices, and vermis of cerebellum. We then performed a network analysis with structural equation modeling to test the hypothesis that regional decreases came about through active inhibition by regions showing increased activity during retrieval. This analysis demonstrated that the influence of activated regions on deactivated regions was more negative during retrieval than during reading, confirming the inhibition hypothesis. Such confirmation could not have been made from the subtraction analysis alone because decreases can come about, at the very least, through reduction of functional influences as well as by active inhibition. The concepts of ensemble excitation and inhibition, as defined through network analysis, are introduced. We argue that is is critical to examine the combined pattern of excitatory and inhibitory influences to fully appreciate the neural basis of episodic memory.

Activation of medial temporal structures during episodic memory retrieval

Medial temporal lobe structures have been implicated in human episodic memory. Patients with medial temporal lesions show memory deficits, and functional neuroimaging studies have revealed activation in this region during episodic encoding and retrieval when data are averaged over a sample of subjects. The relevance of such observations for memory performance has remained unclear, however. Here we have used positron emission tomography (PET) to examine cerebral blood flow related to verbal episodic retrieval. We observed strong positive correlations between retrieval and blood flow in left medial temporal structures in individual normal human subjects. In addition, multivariate analysis showed that regions in the left medial temporal lobe were dominant components of a pattern of brain regions that distinguished a high-retrieval condition from conditions of lower retrieval. These results suggest that medial temporal activity is related to retrieval success rather than retrieval attempt, possibly by reflecting reactivation of stored patterns.

Novelty and familiarity activations in PET studies of memory encoding and retrieval

Nine young right-handed men viewed colored pictures of people, scenes, and landscapes. Then, 24 hr later while undergoing PET scanning, they viewed previously studied (OLD) pictures in one type of scan, and previously not seen (NEW) pictures in another. The OLD-NEW subtraction of PET images indicates familiarity, and the NEW-OLD indicates novelty. Familiarity activations, signalling aspects of retrieval, were observed in the left and right frontal areas, and posterior regions bilaterally. Novelty activations were in the right limbic regions, and bilaterally in temporal and parietal regions, including area 37. These latter activations were located similarly to novelty activations in previous PET studies using visual words and auditory sentences, suggesting the existence of brain regions specializing in transmodal novelty assessment. The effects of novelty are seen both behaviorally and in replicable patterns of cortical and subcortical activation. We propose a 'novelty/encoding hypothesis': (1) novelty assessment represents an early stage of long-term memory encoding; (2) elaborate, meaning-based encoding processes operate on the incoming information to the extent of its novelty, and therefore (3) the probability of long-term storage of information varies directly with the novelty of the information.

Functional brain maps of retrieval mode and recovery of episodic information

Positron emission tomography (PET) was used to identify brain regions associated with two component processes of episodic retrieval; those related to thinking back in subjective time (retrieval mode) and those related to actual recovery of stored information (ecphory). Healthy young subjects recognized words that had been encoded with respect to meaning or the speaker's voice. Regardless of how the information had been encoded, recognition was associated with increased activation in regions in right prefrontal cortex, left anterior cingulate, and cerebellum. These activations reflect retrieval mode. Recognition following meaning encoding was specifically associated with increased activation in left temporal cortex, and recognition following voice encoding involved regions in right orbital frontal and parahippocampal cortex. These activations reflect ecphory of differentially encoded information.

Frontal lobe damage produces episodic memory impairment

This article reports the outcome of a meta-analysis of the relation between the frontal lobes and memory as measured by tests of recognition, cued recall, and free recall. We reviewed experiments in which patients with documented, circumscribed frontal pathology were compared with normal control subjects on these three types of tests. Contrary to conventional wisdom, there is strong evidence that frontal damage disrupts performance on all three types of tests, with the greatest impairment in free recall, and the smallest in recognition.

Functional role of the prefrontal cortex in retrieval of memories: a PET study

Retrieval of information from episodic memory involves the processes invoked by the attempt to remember (retrieval attempt) as well as processes associated with the successful retrieval of stored information (ecphory). Previous PET studies of memory have shown an activation of the prefrontal cortex in memory retrieval tasks, and we hypothesised that this activation represents retrieval attempt, not ecphory. This hypothesis was directly directed using [15O]H2 PET imaging in 19 healthy subjects who performed three matched tasks which involved different levels of retrieval attempt and ecphory. The results showed that retrieval attempt was associated with activation of the prefrontal cortex, right greater than left, while ecphory involved the posterior cortical regions. These findings illuminate the functional role of the different neuroanatomical regions involved in episodic remembering.

Cognitive processes and cerebral cortical fundi

Human brain evolution has resulted in a large increase in cortical folding as a result of which 60% of the cerebral cortical mantle is buried within sulci. Cortical regions within the sulci, and especially in the fundal zones (fundi) at the bottom of sulci, differ from the rest of the cortex in a number of ways with respect to anatomical and histological morphology. Although physiological implications of the fundal morphology have been discussed from time to time, and although scattered evidence hints at a special functional role for fundi, until recently there have been few empirical facts to guide the inquiry into a possibly special physiological function of fundal zones. In this article we review findings yielded by positron emission tomography studies showing that the peaks of changes in neuronal activity are frequently observed in and near fundi. We discuss, but do not accept, the possibility that these findings reflect either the partial volume effect or the course of cerebral blood vessels. Instead, because of a coarse correlation observed between fundal fraction (the proportion of fundally related activity peaks) and the apparent cognitive complexity of the tasks probed, and in light of the anatomical evidence reviewed, we propose the hypothesis that cortical sulcal and fundal regions play a distinctive role in higher cognitive processing.