Forgetting in H.M.: a second look

Neuropathology of a remarkable case of memory impairment informs human memory

Kent Cochrane (K.C.) has been investigated by researchers for nearly three decades after intracranial trauma from a motorcycle accident at age 30 resulted in a striking profile of amnesia. K.C. suffered severe anterograde amnesia in both verbal and non-verbal domains which was accompanied by selective retrograde amnesia for personal events experienced prior to the time of his injury (episodic memory), with relative preservation of memory for personal and world facts (semantic memory), and of implicit memory. This pattern of spared and impaired memory extended to spatial memory for large-scale environments and beyond memory to future imagining and decision-making. Post-mortem brain findings at age 62 included moderate diffuse atrophy, left orbitofrontal contusion, left posterior cerebral artery infarct, and left anterior frontal watershed infarct. Notably, there was severe neuronal loss and gliosis of the hippocampi bilaterally. The left hippocampus was severely affected anteriorly and posteriorly, but CA2, CA4, and the dentate gyrus (DG) were focally spared. There was associated degeneration of the left fornix. The right hippocampus showed near complete destruction anteriorly, with relative preservation posteriorly, mainly of CA4 and DG. Bilateral parahippocampal gyri and left anterior thalamus also showed neuron loss and gliosis. There was no evidence of co-existing neurodegenerative phenomena on beta-amyloid, phosphorylated tau, or TDP-43 immunostaining. The extent of damage to medial temporal lobe structures is in keeping with K.C.'s profound anterograde and retrograde amnesia, with the exception of the unexpected finding of preserved CA2/CA4 and DG. K.C.'s case demonstrates that relatively clean functional dissociations are still possible following widespread brain damage, with structurally compromised brain regions unlikely to be critical to cognitive functions found to be intact. In this way, the findings presented here add to K.C.'s significant contributions to our understanding of clinical-anatomical relationships in memory.

The effect of repetition on pronoun resolution in patients with memory impairment

Referring to things in the world - that woman, her idea, she - is a central component of language. Understanding reference requires the listener to keep track of the unfolding discourse history while integrating multiple sources of information to interpret the speech stream as it unfolds in time. Pronouns are a common way to establish reference. But due to their impoverished form, to understand them listeners must relate features of the pronoun (e.g., gender, animacy) with existing representations of potential discourse referents. Successful referential processing seems to place demands on memory. In a previous study, patients with hippocampal amnesia and healthy participants listened to short stories as their eye movements were monitored. When interpreting ambiguous pronouns, healthy participants demonstrated order-of-mention effects, whereby ambiguous pronouns are interpreted as referring to the first-mentioned referent in the story. By contrast, memory-impaired patients exhibited significant disruptions in their ability to use information about which character had been mentioned first to interpret pronouns. Repetition of the most salient information is a common clinical recommendation for improving pronoun resolution and communication in individuals with memory disorders (e.g., Alzheimer's disease) but this recommendation lacks an evidentiary basis. The present study seeks to determine whether the pronoun resolution performance of hippocampal patients can be improved, by repetition of the target referent, increasing its salience. Results indicate that patients with hippocampal damage demonstrate improved processing of pronouns following repetition of the target referent, but benefit from this repetition to a significantly smaller degree compared to healthy participants. These results provide further evidence for the role of the hippocampal-dependent memory system in language processing and point to the need for empirically tested communication interventions.

Network-wide abnormalities explain memory variability in hippocampal amnesia

Patients with hippocampal amnesia play a central role in memory neuroscience but the neural underpinnings of amnesia are hotly debated. We hypothesized that focal hippocampal damage is associated with changes across the extended hippocampal system and that these, rather than hippocampal atrophy per se, would explain variability in memory between patients. We assessed this hypothesis in a uniquely large cohort of patients (n = 38) after autoimmune limbic encephalitis, a syndrome associated with focal structural hippocampal pathology. These patients showed impaired recall, recognition and maintenance of new information, and remote autobiographical amnesia. Besides hippocampal atrophy, we observed correlatively reduced thalamic and entorhinal cortical volume, resting-state inter-hippocampal connectivity and activity in posteromedial cortex. Associations of hippocampal volume with recall, recognition, and remote memory were fully mediated by wider network abnormalities, and were only direct in forgetting. Network abnormalities may explain the variability across studies of amnesia and speak to debates in memory neuroscience.

A contextual binding theory of episodic memory: systems consolidation reconsidered

Episodic memory reflects the ability to recollect the temporal and spatial context of past experiences. Episodic memories depend on the hippocampus but have been proposed to undergo rapid forgetting unless consolidated during offline periods such as sleep to neocortical areas for long-term storage. Here, we propose an alternative to this standard systems consolidation theory (SSCT) - a contextual binding account - in which the hippocampus binds item-related and context-related information. We compare these accounts in light of behavioural, lesion, neuroimaging and sleep studies of episodic memory and contend that forgetting is largely due to contextual interference, episodic memory remains dependent on the hippocampus across time, contextual drift produces post-encoding activity and sleep benefits memory by reducing contextual interference.

Awareness of what is learned as a characteristic of hippocampus-dependent memory

We explored the relationship between memory performance and conscious knowledge (or awareness) of what has been learned in memory-impaired patients with hippocampal lesions or larger medial temporal lesions. Participants viewed familiar scenes or familiar scenes where a change had been introduced. Patients identified many fewer of the changes than controls. Across all of the scenes, controls preferentially directed their gaze toward the regions that had been changed whenever they had what we term robust knowledge about the change: They could identify that a change occurred, report what had changed, and indicate where the change occurred. Preferential looking did not occur when they were unaware of the change or had only partial knowledge about it. The patients, overall, did not direct their gaze toward the regions that had been changed, but on the few occasions when they had robust knowledge about the change they (like controls) did exhibit this effect. Patients did not exhibit this effect when they were unaware of the change or had partial knowledge. The findings support the idea that awareness of what has been learned is a key feature of hippocampus-dependent memory.

Cortical Networks Involved in Memory for Temporal Order

We examined the neurobiological basis of temporal resetting, an aspect of temporal order memory, using a version of the delayed-match-to-multiple-sample task. While in an fMRI scanner, participants evaluated whether an item was novel or whether it had appeared before or after a reset event that signified the start of a new block of trials. Participants responded "old" to items that were repeated within the current block and "new" to both novel items and items that had last appeared before the reset event (pseudonew items). Medial-temporal, prefrontal, and occipital regions responded to absolute novelty of the stimulus-they differentiated between novel items and previously seen items, but not between old and pseudonew items. Activation for pseudonew items in the frontopolar and parietal regions, in contrast, was intermediate between old and new items. The posterior cingulate cortex extending to precuneus was the only region that showed complete temporal resetting, and its activation reflected whether an item was new or old according to the task instructions regardless of its familiarity. There was also a significant Condition (old/pseudonew) × Familiarity (second/third presentations) interaction effect on behavioral and neural measures. For pseudonew items, greater familiarity decreased response accuracy, increased RTs, increased ACC activation, and increased functional connectivity between ACC and the left frontal pole. The reverse was observed for old items. On the basis of these results, we propose a theoretical framework in which temporal resetting relies on an episodic retrieval network that is modulated by cognitive control and conflict resolution.

The mammillary bodies and memory: more than a hippocampal relay

Although the mammillary bodies were one of the first neural structures to be implicated in memory, it has long been assumed that their main function was to act primarily as a hippocampal relay, passing information on to the anterior thalamic nuclei and from there to the cingulate cortex. This view not only afforded the mammillary bodies no independent role in memory, it also neglected the potential significance of other, nonhippocampal, inputs to the mammillary bodies. Recent advances have transformed the picture, revealing that projections from the tegmental nuclei of Gudden, and not the hippocampal formation, are critical for sustaining mammillary body function. By uncovering a role for the mammillary bodies that is independent of its subicular inputs, this work signals the need to consider a wider network of structures that form the neural bases of episodic memory.

H.M.'s contributions to neuroscience: a review and autopsy studies

H.M., Henry Molaison, was one of the world's most famous amnesic patients. His amnesia was caused by an experimental brain operation, bilateral medial temporal lobe resection, carried out in 1953 to relieve intractable epilepsy. He died on December 2, 2008, and that night we conducted a wide variety of in situ MRI scans in a 3 T scanner at the Massachusetts General Hospital (Mass General) Athinoula A. Martinos Center for Biomedical Imaging. For the in situ experiments, we acquired a full set of standard clinical scans, 1 mm isotropic anatomical scans, and multiple averages of 440 μm isotropic anatomical scans. The next morning, H.M.'s body was transported to the Mass General Morgue for autopsy. The photographs taken at that time provided the first documentation of H.M.'s lesions in his physical brain. After tissue fixation, we obtained ex vivo structural data at ultra-high resolution using 3 T and 7 T magnets. For the ex vivo acquisitions, the highest resolution images were 210 μm isotropic. Based on the MRI data, the anatomical areas removed during H.M.'s experimental operation were the medial temporopolar cortex, piriform cortex, virtually all of the entorhinal cortex, most of the perirhinal cortex and subiculum, the amygdala (except parts of the dorsal-most nuclei-central and medial), anterior half of the hippocampus, and the dentate gyrus (posterior head and body). The posterior parahippocampal gyrus and medial temporal stem were partially damaged. Spared medial temporal lobe tissue included the dorsal-most amygdala, the hippocampal-amygdalo-transition-area, ∼2 cm of the tail of the hippocampus, a small part of perirhinal cortex, a small portion of medial hippocampal tissue, and ∼2 cm of posterior parahippocampal gyrus. H.M.'s impact on the field of memory has been remarkable, and his contributions to neuroscience continue with a unique dataset that includes in vivo, in situ, and ex vivo high-resolution MRI.

Comparison of explicit and incidental learning strategies in memory-impaired patients

Declarative memory for rapidly learned, novel associations is thought to depend on structures in the medial temporal lobe (MTL), whereas associations learned more gradually can sometimes be supported by nondeclarative memory and by structures outside the MTL. A recent study suggested that even rapidly learned associations can be supported by structures outside the MTL when an incidental encoding procedure termed "fast mapping" (FM) is used. We tested six memory-impaired patients with bilateral damage to hippocampus and one patient with large bilateral lesions of the MTL. Participants saw photographs and names of animals, plants, and foods that were previously unfamiliar (e.g., mangosteen). Instead of asking participants to study name-object pairings for a later memory test (as with traditional memory instructions), participants answered questions that allowed them to infer which object corresponded to a particular name. In a second condition, participants learned name-object associations of unfamiliar items by using standard, explicit encoding instructions (e.g., remember the mangosteen). In FM and explicit encoding conditions, patients were impaired (and performed no better than a group that was given the same tests but had not previously studied the material). The same results were obtained in a second experiment that used the same procedures with modifications to allow for more robust learning and more reliable measures of performance. Thus, our results with the FM procedure and memory-impaired patients yielded the same deficits in learning and memory that have been obtained by using other more traditional paradigms.

Memory consolidation in aging and MCI after 1 week

OBJECTIVE

To assess consolidation in amnestic mild cognitive impairment (aMCI), controlling for differences in initial learning and using a protracted delay period for recall.

METHOD

15 individuals with aMCI were compared with 15 healthy older adult controls on a story learning task. Subjects were trained to criteria to equalize initial learning across subjects. Recall was tested at both the 30-min typically used delay and a 1-week delay used to target consolidation.

RESULTS

Using repeated measures ANOVAs adjusted for age, we found group × time point interactions across the entire task between the final trial and 30-min delay, and again between the 30-min and 1-week delay periods, with aMCI having greater declines in recall as compared with controls. Significant group main effects were also found, with aMCI recalling less than controls.

CONCLUSION

Consolidation was impaired in aMCI as compared with controls. Our findings indicate that aMCI-related performance typically measured at 30 min underestimates aMCI-associated memory deficits. This is the first study to isolate consolidation by controlling for initial learning differences and using a protracted delay period to target consolidation in an aMCI sample.

Human amnesia and the medial temporal lobe illuminated by neuropsychological and neurohistological findings for patient E.P

We present neurohistological information for a case of bilateral, symmetrical damage to the medial temporal lobe and well-documented memory impairment. E.P. developed profound memory impairment at age 70 y and then was studied for 14 y He had no capacity for learning facts and events and had retrograde amnesia covering several decades. He also had a modest impairment of semantic knowledge. Neurohistological analysis revealed bilaterally symmetrical lesions of the medial temporal lobe that eliminated the temporal pole, the amygdala, the entorhinal cortex, the hippocampus, the perirhinal cortex, and rostral parahippocampal cortex. The lesion also extended laterally to involve the fusiform gyrus substantially. Last, the superior, inferior, and middle temporal gyri were atrophic, and subjacent white matter was gliotic. Several considerations indicate that E.P.'s severe memory impairment was caused by his medial temporal lesions, whereas his impaired semantic knowledge was caused by lateral temporal damage. His lateral temporal damage also may have contributed to his extensive retrograde amnesia. The findings illuminate the anatomical relationship between memory, perception, and semantic knowledge.

Serial position functions following selective hippocampal lesions in monkeys: effects of delays and interference

We examined the role of the hippocampus in list-memory processing. Three rhesus monkeys that had extensive experience in this task and had demonstrated full abstract-concept learning and excellent list memory performance (Katz et al., 2002; Wright et al., 2003) received bilateral neurotoxic hippocampal lesions and were re-tested in the serial list memory task. Effects of delays on memory performance were measured in all monkeys, whereas the effects of proactive interference were assessed in only one. Despite a slight change in performance of one of the three animals during re-learning of the same/different task, selective hippocampal damage had little or no effects on list memory accuracy. In addition, the hippocampal damage did not impact serial list position functions (SPFs) but slightly altered the dynamic of the SPF curves. Finally, even more remarkable was that accurate memory performance of one animal remained intact despite the use of small set size of 8 items that created high proactive interference across lists thereby eliminating any use of familiarity judgments to support performance. Together the findings indicate that, with short list items and extensive training in the task (i.e., reference memory), monkeys with selective hippocampal lesions may be able to use alternative memory processes (i.e., working memory) that are mediated by structures other than the hippocampus.

Focal retrograde amnesia and the attribution of causality: An exceptionally critical view

A detailed critique of the literature on focal retrograde amnesia is provided. Some of the cases commonly cited in this literature had, in fact, severely impaired anterograde memory, most often involving visuospatial material. Other cases showed poor anterograde memory in more moderate or subtle form, begging the question of whether "like" had really been compared with "like" across the retrograde and anterograde domains: there may be alternative explanations for the observed patterns of performance. One suggestion is that these patients suffer an impairment of long-term consolidation, an attractive hypothesis but one which requires much more rigorous testing than has occurred to date and which implies that the underlying problem is not specific to retrograde memory. Moreover, within the literature on cases of focal retrograde amnesia, differing patterns of performance on tests of autobiographical memory or remote semantic knowledge have been reported, and sometimes these may have reflected factors other than the sites of lesions. Many of the most convincing cases in this literature have been those in whom there was an initially severe anterograde amnesia as well as an extensive retrograde loss: in these cases, the critical issue is what determines differential patterns of recovery across these domains-it is likely that both physiological and psychological factors are important. A second, somewhat different, group are patients with semantic dementia, who show a pronounced recency effect in remote memory but, in these cases, the most parsimonious explanation may be in terms of predominantly semantic/linguistic and/or strategic factors. A third group are those with transient epileptic amnesia but, in these cases, the memory gaps may reflect past (anterograde) ictal activity. A fourth group are those in whom psychogenic factors may well be relevant. Although it is difficult to "prove" psychological causation, the logical difficulties in attributing causation where brain lesions are either very subtle or multiple have been considerably underestimated in the neuropsychological literature. Given these problems, in uncertain or equivocal cases, it is as critical to present the relevant psychological data for the reader to evaluate as it is to provide the pertinent memory test scores: this is underemphasised in many of the studies reviewed. Publication of cases in the absence of such data may lead to faulty clinical, neuropsychological, and cognitive conclusions. Abbreviations : AA: anterograde amnesia; AMI: Autobiographical Memory Interview; PTA: posttraumatic amnesia; RA: retrograde amnesia; RMT: Recognition Memory Test; TEA: transient epileptic amnesia; TGA: transient global amnesia; WMS: Wechsler Memory Scale.

A touch screen-automated cognitive test battery reveals impaired attention, memory abnormalities, and increased response inhibition in the TgCRND8 mouse model of Alzheimer's disease

Transgenic mouse models of Alzheimer's disease (AD) with abundant β-amyloid develop memory impairments. However, multiple nonmnemonic cognitive domains such as attention and executive control are also compromised early in AD individuals, but have not been routinely assessed in animal models. Here, we assessed the cognitive abilities of TgCRND8 mice—a widely used model of β-amyloid pathology—with a touch screen-based automated test battery. The test battery comprises highly translatable tests of multiple cognitive constructs impaired in human AD, such as memory, attention, and response control, as well as appropriate control tasks. We found that familial AD mutations affect not only memory, but also cause significant alterations of sustained attention and behavioral flexibility. Because changes in attention and response inhibition may affect performance on tests of other cognitive abilities including memory, our findings have important consequences for the assessment of disease mechanisms and therapeutics in animal models of AD. A more comprehensive phenotyping with specialized, multicomponent cognitive test batteries for mice might significantly advance translation from preclinical mouse studies to the clinic.

Perirhinal cortex lesions uncover subsidiary systems in the rat for the detection of novel and familiar objects

The present study compared the impact of perirhinal cortex lesions on tests of object recognition. Object recognition was tested directly by looking at the preferential exploration of novel objects over simultaneously presented familiar objects. Object recognition was also tested indirectly by presenting just novel objects or just familiar objects, and recording exploration levels. Rats with perirhinal cortex lesions were severely impaired at discriminating a novel object from a simultaneously presented familiar object (direct test), yet displayed normal levels of exploration to novel objects presented on their own and showed normal declines in exploration times for familiar objects that were repeatedly presented (indirect tests). This effective reduction in the exploration of familiar objects after perirhinal cortex lesions points to the sparing of some recognition mechanisms. This possibility led us to determine whether rats with perirhinal cortex lesions can overcome their preferential exploration deficits when given multiple object familiarisation trials prior to that same (familiar) object being paired with a novel object. It was found that after multiple familiarisation trials, objects could now successfully be recognised as familiar by rats with perirhinal cortex lesions, both following a 90-min delay (the longest delay tested) and when object recognition was tested in the dark after familiarisation trials in the light. These latter findings reveal: (i) the presumed recruitment of other regions to solve recognition memory problems in the absence of perirhinal cortex tissue; and (ii) that these additional recognition mechanisms require more familiarisation trials than perirhinal-based recognition mechanisms.

Profound retroactive interference in anterograde amnesia: What interferes?

OBJECTIVE

Anterograde amnesia is characterized by a profound inability to retain new information. Recent research suggests that at least some of this severe memory impairment may be the product of retroactive interference. What exactly interferes with memory in amnesic patients, however, remains unknown. The aim of the present study was to examine whether or not postlearning material which is highly dissimilar from the material to be remembered would interfere with amnesic patients' memory.

METHOD

Prose retention was tested in 10 densely amnesic patients and 10 controls following a 10 minute delay period, which was either unfilled (minimal interference) or filled with a tone detection task in which participants were required to listen for piano notes (nonspecific interference).

RESULTS

A significant nonspecific retroactive interference effect was observed in the amnesic patients (p < 0.004): Whereas 7 out of the 10 amnesic patients were able to recall some prose material following the unfilled delay period, only 1 of them was able to recall any material after the tone detection delay.

CONCLUSIONS

The data reveal that some amnesic patients have the capacity to retain new material for much longer than usual but that apparently any new postlearning information profoundly interferes with such retention. This nonspecific retroactive interference effect deviates from the item-specific interference effect that is typically assessed in clinical practice and which is frequently observed in patients with executive impairment. We hypothesize that these interference effects are qualitatively different, occurring during distinct memory processes, namely retrieval (item-specific interference) and consolidation (nonspecific interference).

Signal detection with criterion noise: applications to recognition memory

A tacit but fundamental assumption of the theory of signal detection is that criterion placement is a noise-free process. This article challenges that assumption on theoretical and empirical grounds and presents the noisy decision theory of signal detection (ND-TSD). Generalized equations for the isosensitivity function and for measures of discrimination incorporating criterion variability are derived, and the model's relationship with extant models of decision making in discrimination tasks is examined. An experiment evaluating recognition memory for ensembles of word stimuli revealed that criterion noise is not trivial in magnitude and contributes substantially to variance in the slope of the isosensitivity function. The authors discuss how ND-TSD can help explain a number of current and historical puzzles in recognition memory, including the inconsistent relationship between manipulations of learning and the isosensitivity function's slope, the lack of invariance of the slope with manipulations of bias or payoffs, the effects of aging on the decision-making process in recognition, and the nature of responding in remember-know decision tasks. ND-TSD poses novel, theoretically meaningful constraints on theories of recognition and decision making more generally, and provides a mechanism for rapprochement between theories of decision making that employ deterministic response rules and those that postulate probabilistic response rules.

EPS Mid-Career Award 2006. Understanding anterograde amnesia: disconnections and hidden lesions

Three emerging strands of evidence are helping to resolve the causes of the anterograde amnesia associated with damage to the diencephalon. First, new anatomical studies have refined our understanding of the links between diencephalic and temporal brain regions associated with amnesia. These studies direct attention to the limited numbers of routes linking the two regions. Second, neuropsychological studies of patients with colloid cysts confirm the importance of at least one of these routes, the fornix, for episodic memory. By combining these anatomical and neuropsychological data strong evidence emerges for the view that damage to hippocampal-mammillary body-anterior thalamic interactions is sufficient to induce amnesia. A third development is the possibility that the retrosplenial cortex provides an integrating link in this functional system. Furthermore, recent evidence indicates that the retrosplenial cortex may suffer "covert" pathology (i.e., it is functionally lesioned) following damage to the anterior thalamic nuclei or hippocampus. This shared indirect "lesion" effect on the retrosplenial cortex not only broadens our concept of the neural basis of amnesia but may also help to explain the many similarities between temporal lobe and diencephalic amnesia.

Episodic memory and neuroimaging of hippocampus and fornix in chronic schizophrenia

A group of 44 patients with schizophrenia and 43 age-matched controls completed psychometrically-matched tasks of recall and recognition. The patients showed similarly depressed scores across both recall and recognition matched tasks, independent of their reduced IQ and executive functioning scores. In addition, reduced memory scores correlated in the expected direction with magnetic resonance imaging (MRI) of the hippocampus and diffusion tension imaging (DTI) of the fornix for subsets of both patients and controls that had available these structural imaging measures. Reduced executive functioning also correlated with lower fornix integrity for the patient subset. However, increased hippocampal volume correlated, in the negative direction, with lower scores for executive functioning and IQ in the control subset. Implications of these results are discussed.

Posterior parahippocampal place learning in H.M

In a previous experiment with patients who had undergone unilateral temporal thermocoagulation lesions to alleviate intractable epilepsy, we demonstrated that the right parahippocampal cortex was critical for the performance of a spatial memory task (Bohbot et al. (1998) Neuropsychologia 36:1217-1238). Based on this evidence, we predicted that H.M., whose caudal parahippocampal cortex was structurally intact (Corkin et al. (1997) J Neurosci 17:3964-3979), would be able to learn the spatial memory task. This task was designed to be a human analogue of the Morris water maze in that it measured participants' ability to learn the location of a target, which was an invisible weight sensor placed under a carpet (Bohbot et al. (1998) Neuropsychologia 36:1217-1238). H.M. was first tested with the sensor under a small carpet (162 cm x 150 cm). Then, interspersed with the first sensor location, he was tested with the sensor in a second location, covered by a larger carpet (250 cm x 210 cm). He found the second target location in a direct path on only 10% of the trials. In contrast, when tested on the first sensor location, he walked directly toward the center of the testing area in 19/35 trials and from there found the sensor in a direct path on 15 of the 19 trials (80%). The number of direct hits at the first target location was significantly greater than chance (P < 0.0005). An analysis of H.M.'s paths showed that they were characteristic of fast learning, and that he did not rely on egocentric, short-term, or working memory strategies to learn the task. H.M's ability to locate the sensor is remarkable given his severe amnesia and his inability to explicitly recollect the testing episode. These findings underscore the role of the parahippocampal cortex in spatial memory.

Sex differences in face recognition memory in patients with temporal lobe epilepsy, patients with generalized epilepsy, and healthy controls

The influence of sex on face recognition memory was studied in 49 patients with temporal lobe epilepsy, 20 patients with generalized epilepsy, and 32 healthy controls. After learning 20 faces, serially presented for 5 seconds each, subjects had to recognize the 20 among 40 faces (including 20 new faces) immediately and 24 hours later. Women had better face recognition than men, with no significant differences between groups. Women's advantage was due mainly to superior delayed recognition. Taken together, the results suggest that sex has a similar impact on face recognition in patients with epilepsy and healthy controls, and that testing delayed face recognition raises sensitivity for sex differences. The influence of sex on face recognition in patients with epilepsy should be acknowledged when evaluating individuals or comparing groups.

Memory hazard functions: A vehicle for theory development and test

A framework is developed to rigorously test an entire class of memory retention functions by examining hazard properties. Evidence is provided that the memory hazard function is not monotonically decreasing. Yet most of the proposals for retention functions, which have emerged from the psychological literature, imply that memory hazard is monotonically decreasing over the entire temporal range. Furthermore, the few remaining proposals, that do not have monotonically decreasing hazard, have difficulty fitting data over both short-term and long-term intervals. A new 2-trace hazard model is developed that successfully circumvents these difficulties. This new model is used to account for the size of memory span and the time course of proactive and retroactive interference effects. The model can fit the retention characteristics of H. M., the famous amnesic patient, as well as normal experimental participants. The model is also used to account for the time course of the misinformation effect.

Rate of acquisition, adult age, and basic cognitive abilities predict forgetting: New views on a classic problem

Rate of forgetting is putatively invariant across individuals, sharing few associations with individual-differences variables known to influence encoding and retrieval. This classic topic in learning and memory was revisited using a novel statistical application, multilevel modeling, to examine whether (a) slopes of forgetting varied across individuals and (b) observed individual differences in forgetting shared systematic relations with adult age, learning speed, and cognitive ability. Participants (N = 136) received mnemonic training prior to memorizing 4-digit numbers to perfection, and retention was tested immediately after training and after 30 min, 24 hr, 7 weeks, and 8 months. Slower rate of learning to criterion, older age, and poorer cognitive performance predicted accelerated forgetting with associations most pronounced within 24 hr from baseline. Observed correlates of differential forgetting slopes are similar to those previously found to affect encoding, suggesting continuity rather than asymmetry of prediction for these memory processes.

Determinants of capacious memory: a process-dissociation and experiential approach

The aim of this study is to delineate some important circumstances where exceptionally good memory performance, or capacious memory occurs. A further aim is to study memory processes involved in this memory phenomenon. In a first experiment, participants looked through two series of pictures differing in number and were evaluated in two-alternative forced-choice and yes-no recognition memory tasks combined in a process-dissociation procedure. Moreover, participants were asked to provide remember and know responses to tap recollective experience. The results as to forced-choice recognition task accuracy and according to process-dissociation procedure estimates were replicated in a second experiment with a more intrinsic contextual manipulation, and in a third, forgetting experiment. In addition to replicating previous findings, the results show (a) that capacious memory is associated with strong feelings of recollection; and (b) that familiarity (in terms of the process-dissociation framework) contributes to this phenomenon.

Evidence for semantic learning in profound amnesia: an investigation with patient H.M

Until recently, it seemed unlikely that any semantic knowledge could be acquired following extensive bilateral damage to the medial temporal lobes (MTL). Although recent studies have demonstrated some semantic learning in amnesic patients, questions remain regarding the limits of this capacity and the extent to which it relies on those patients' residual MTL function. The present study examined whether detailed, semantic memory could be acquired by a patient with no functioning hippocampus. We used cued recall and forced-choice recognition tasks to investigate whether the patient H.M. had acquired knowledge of people who became famous after the onset of his amnesia. Results revealed that, with first names provided as cues, he was able to recall the corresponding famous last name for 12 of 35 postoperatively famous personalities. This number nearly doubled when semantic cues were added, suggesting that his knowledge of the names was not limited to perceptual information, but was incorporated in a semantic network capable of supporting explicit recall. In forced-choice recognition, H.M. discriminated 87% of postmorbid famous names from foils. Critically, he was able to provide uniquely identifying semantic facts for one-third of these recognized names, describing John Glenn, for example, as "the first rocketeer" and Lee Harvey Oswald as a man who "assassinated the president." Although H.M.'s semantic learning was clearly impaired, the results provide robust, unambiguous evidence that some new semantic learning can be supported by structures beyond the hippocampus proper.

Contribution of recollection and familiarity judgements to rate of forgetting in organic amnesia

The present study sought to determine whether the rate of forgetting in amnesic and control participants varied as the relative contribution of familiarity judgements (or implicit memory) was manipulated. In Experiment 1, rates of forgetting were measured in two recognition conditions in which the relative contribution of familiarity judgements and recollection had been manipulated. No significant group by condition or group by condition by delay effects were found. In Experiment 2, we compared word recognition and word recall tasks on the assumption that this would produce a larger difference in the role of familiarity judgements versus recollection. In this case, we did obtain a significant difference in forgetting rates, the amnesic patients forgetting faster than the healthy subjects in the recall condition. In summary, amnesic patients showed faster forgetting on recall than recognition, which we have attributed to the absence of the opportunity for familiarity judgments in recall testing. We concluded that amnesic patients show a primary deficit in the acquisition of new information (with associated or secondary retrieval deficits), and that, in addition, they show a deficit in long-term retention detectable only on recall ('high recollection') testing.

What's new with the amnesic patient H.M.?

H.M. became amnesic in 1953. Since that time, nearly 100 investigators, first at the Montreal Neurological Institute and since 1966 at the Massachusetts Institute of Technology, have participated in studying him. We all understand the rare opportunity we have had to work with him, and we are grateful for his dedication to research. He has taught us a great deal about the cognitive and neural organization of memory. We are in his debt.

Time sequence and types of memory deficits after experimental status epilepticus

P35 rats subjected to kainate induced status epilepticus (SE) subsequently underwent four consecutive series of the Morris Water Maze. They demonstrated, compared with controls, an early (P46-49), and subsequent (P60-63) disturbance in acquisition, but not in long term retention, of spatial memory. They eventually achieved performance levels similar to those of controls (P74-77, P91-94). These data support the hypothesis that acquisition, but not long term retention, of spatial material is impaired in this model of temporal lobe epilepsy (TLE), probably due to the hippocampal injury that occurs after SE.

Compensating for cognitive deficits following brain injury

Two of the most important goals of rehabilitation are to (a) reduce everyday consequences of impaired cognitive functioning (disabilities) and (b) reduce the level of handicap (the extent to which these problems prevent successful return to society). One of the ways by which we can achieve these goals is to enable people to compensate for their cognitive deficits. This paper (i) describes a theoretical framework for understanding compensatory behavior, (ii) discusses different forms of compensation, (iii) considers compensation for several cognitive disorders, and (iv) presents suggestions for predicting which patients will find it easy to compensate and which require more intensive and focused rehabilitation.

Profound amnesia after damage to the medial temporal lobe: A neuroanatomical and neuropsychological profile of patient E. P

E. P. became profoundly amnesic in 1992 after viral encephalitis, which damaged his medial temporal lobe bilaterally. Because of the rarity of such patients, we have performed a detailed neuroanatomical analysis of E. P.'s lesion using magnetic resonance imaging, and we have assessed his cognitive abilities with a wide range of neuropsychological tests. Finally, we have compared and contrasted the findings for E. P. with the noted amnesic patient H.M, whose surgical lesion is strikingly similar to E. P.'s lesion.

Long-term spatial memory in rats with hippocampal lesions

In animal models of human amnesia, using lesion methods, it has been difficult to establish the role played by the hippocampus in the formation of long-term spatial knowledge. For example, lesions sustained after acquisition have generally produced a flat retrograde amnesia for spatial information. These results have not made it possible to dissociate the participation of the hippocampus in retrieval/performance processes from its participation in consolidation/retention. The present study was designed to investigate if electrolytic hippocampal lesions made before training lead to a deficit in the long-term retention of spatial knowledge when the rats show equal performance levels during the acquisition. Results show that lesioned rats learn a place response just as well as the control rats when, during the training, an intramaze cue orients the animal in its navigation towards the goal arm. One day after reaching criterion, lesioned and control rats remember the task perfectly during a transfer test in which the intramaze signal used previously is not present. However, 24 days later, the hippocampal animals manifest a profound deficit in the retention of the spatial information. When the spatial task learned during the acquisition phase requires only the use of a guidance strategy, control and lesioned animals show the same level of performance during the training phase and the same degree of retention during the retraining phase 24 days after criterion. Taken together, these results suggest that the hippocampus plays a crucial role in long-term retention of allocentric spatial information.

An evaluation of the concurrent discrimination task as a measure of habit learning: performance of amnesic subjects

Habit learning has been defined as an association between a stimulus and a response that develops slowly and automatically through repeated reinforcement. Concurrent discrimination (CD) learning, in which subjects learn to choose the rewarded objects in a series of pairs, is believed to be an example of habit learning in monkeys. Studies of human amnesic subjects, however, have produced equivocal results, revealing impaired or absent learning on the same CD tasks that monkeys with medial temporal-lobe (MTL) lesions learn normally. One possible explanation for impaired performance in human amnesic subjects is that, unlike monkeys, human subjects use explicit memory to solve CD problems. To test this hypothesis, we administered a 10-object pair CD learning task to two amnesic subjects, HM and PN, and normal control subjects (NCS). Both amnesic subjects have severe anterograde amnesia with little ability to form explicit memories. On the CD task, they demonstrated little or no learning and acquired no explicit knowledge of the task procedures or reward contingencies. In contrast, NCS learned the task quickly and easily using explicit memory strategies. These results suggest that CD tasks cannot be learned by habit in human subjects, and emphasize the discrepancies between the human and monkey literature on habit learning.

Criterion-based validity of an intracarotid amobarbital recognition-memory protocol

PURPOSE

We tested whether the behavioral components of an Intracarotid Amobarbital Protocol (IAP) had criterion validity. It was hypothesized that a recognition-memory test designed for intracarotid injections and used to predict the risk of global amnesia before an elective temporal lobectomy should also identify persons who are severely amnesic due to other neurologic causes. Divergent validity predicts that speech tasks would be unaffected by amnesia. Test-retest reliability also was measured.

METHODS

Fifteen persons with severe amnesia were administered four alternate forms of a yes/no recognition-memory protocol and a speech protocol. No drug injection occurred. Standardized neuropsychological tests were used to divide the amnesic group into those with Global Amnesia (i.e., retain no ongoing memories), Severe Amnesia (i.e., memory impaired), and Amnesia Plus (severe amnesia plus other neuropsychologic deficits).

RESULTS

Two persons with Global Amnesia obtained scores at or below chance (i.e., failed) on the memory protocol. Unexpectedly, 12 of 13 severely amnesic persons obtained near-perfect memory scores. Amnesia had no impact on the speech protocol. Pass/Fail outcomes were highly correlated across all four sets.

CONCLUSIONS

A four-item IAP memory protocol showed good reliability and criterion validity in identifying the rare condition of Global Amnesia, but it was insensitive to other disabling, severe amnesic disorders. This IAP memory protocol might have validity in predicting a postsurgical Global Amnesic disorder, but it did not identify and therefore could not predict other more common severe amnesic disorders.

Forgetting rates in neuropsychiatric disorders

OBJECTIVE

Previous studies have attributed accelerated forgetting rates on recognition memory tasks to temporal lobe pathology, but findings in some patient groups may have been attributable to metabolic disruption. Findings in psychiatric disorders such as schizophrenia are conflicting. The purpose of the present study was to compare forgetting rates in patients with confusional states (post-electroconvulsive therapy (post-ECT), delirium), with those obtained in schizophrenic patients (with putative temporal lobe pathology), non-ECT depressed patients, and healthy controls. The findings could also be compared with previous reports in patients with head injury, focal structural lesions, and Alzheimer's dementia.

METHODS

Two studies employed a picture recognition task to examine forgetting rates, the first between delays of 1 minute, 15 minutes, and 30 minutes, and the second between delays of 10 minutes, 2 hours, and 24 hours.

RESULTS

There were no significant differences in forgetting rates between 1 minute and 30 minutes, but the ECT group showed accelerated forgetting between 10 minutes and 2 hours compared with healthy controls, associated with a rapid decline in "hit rate". This was not attributable to differential changes in either depression or severity of memory impairment. There were no differences in forgetting rates across the other subject groups.

CONCLUSION

Post-ECT confusional state patients (similarly to "within post-traumatic amnesia" patients with head injury) show accelerated forgetting on a recognition memory task and, in this, they contrast with patients who have focal structural lesions or widespread cortical atrophy. Accelerated forgetting may reflect the effect of disrupted cerebral metabolism on either "consolidation" or memory "binding" processes.

Images of medial temporal lobe functions in human learning and memory

Investigations of the neural basis of mammalian memory have focused more often on the medial temporal lobe (MTL) than on any other brain region. In humans, the amnesic syndrome revealed the essential importance of the multiple structures located in the MTL system for declarative memory (the remembrance of events and facts). Other neural systems mediate procedural forms of memory, including delay eyeblink conditioning, which depends on the cerebellum, and cognitive skill learning, which depends on the striatum. We review three functional imaging studies that reveal different patterns of MTL activation associated with declarative and procedural memory tasks. One study shows separate MTL activations during the encoding or retrieval of declarative memories. A second study shows MTL activation that occurs in parallel with cerebellum-dependent delay eyeblink conditioning, but does not appear to influence that form of procedural memory. A third study reveals suppression of the MTL during striatum-dependent cognitive skill learning. These studies provide images of MTL activations that are correlated with, independent from, or antagonistic to memory performance.

Do amnesics forget colours pathologically fast?

We tested amnesic and control subjects on a task which required the recognition of single, difficult to name colours, after delays ranging from 7 seconds to 120 seconds after performance of the two subject groups had been matched at the shortest delay by giving the amnesic patients longer study time. The amnesic patients showed abnormally fast forgetting over the two minute period. Furthermore, a subgroup of nine subjects with presumed damage to midline diencephalic structures (Korsakoff's syndrome) were found to forget as fast as a group of six subjects with presumed medial temporal lobe damage (herpes simplex encephalitis). These results contrast both with studies using the Huppert and Piercy procedure and those using the Brown-Peterson task, none of which have shown convincing evidence of accelerated forgetting in medial temporal lobe or diencephalic amnesia.