Anterograde but not retrograde memory loss following combined mammillary body and medial thalamic lesions

Time to put the mammillothalamic pathway into context

The medial diencephalon, in particular the mammillary bodies and anterior thalamic nuclei, has long been linked to memory and amnesia. The mammillary bodies provide a dense input into the anterior thalamic nuclei, via the mammillothalamic tract. In both animal models, and in patients, lesions of the mammillary bodies, mammillothalamic tract and anterior thalamic nuclei all produce severe impairments in temporal and contextual memory, yet it is uncertain why these regions are critical. Mounting evidence from electrophysiological and neural imaging studies suggests that mammillothalamic projections exercise considerable distal influence over thalamo-cortical and hippocampo-cortical interactions. Here, we outline how damage to the mammillary body-anterior thalamic axis, in both patients and animal models, disrupts behavioural performance on tasks that relate to contextual ("where") and temporal ("when") processing. Focusing on the medial mammillary nuclei as a possible 'theta-generator' (through their interconnections with the ventral tegmental nucleus of Gudden) we discuss how the mammillary body-anterior thalamic pathway may contribute to the mechanisms via which the hippocampus and neocortex encode representations of experience.

Practical Ways of Improving Memory Storage and Retrieval Problems in Patients with Head Injuries

Memory disorders resulting from head injuries can be very distressing to both patients and carers. Understanding memory processes can be helpful in selecting the best memory strategies for particular patients. These strategies are discussed together with definitions of amnesic conditions and memory aids. The important message from using such memory strategies is that patients learning how to improve their poor memory have a far greater chance of improvement in the long term than those who simply rehearse items to be memorised.

The importance of mammillary body efferents for recency memory: towards a better understanding of diencephalic amnesia

Despite being historically one of the first brain regions linked to memory loss, there remains controversy over the core features of diencephalic amnesia as well as the critical site for amnesia to occur. The mammillary bodies and thalamus appear to be the primary locus of pathology in the cases of diencephalic amnesia, but the picture is complicated by the lack of patients with circumscribed damage. Impaired temporal memory is a consistent neuropsychological finding in Korsakoff syndrome patients, but again, it is unclear whether this deficit is attributable to pathology within the diencephalon or concomitant frontal lobe dysfunction. To address these issues, we used an animal model of diencephalic amnesia and examined the effect of mammillothalamic tract lesions on tests of recency memory. The mammillothalamic tract lesions severely disrupted recency judgements involving multiple items but left intact both recency and familiarity judgements for single items. Subsequently, we used disconnection procedures to assess whether this deficit reflects the indirect involvement of the prefrontal cortex. Crossed-lesion rats, with unilateral lesions of the mammillothalamic tract and medial prefrontal cortex in contralateral hemispheres, were unimpaired on the same recency tests. These results provide the first evidence for the selective importance of mammillary body efferents for recency memory. Moreover, this contribution to recency memory is independent of the prefrontal cortex. More broadly, these findings identify how specific diencephalic structures are vital for key elements of event memory.

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.

MRI atlas of the human hypothalamus

Gaining new insights into the anatomy of the human hypothalamus is crucial for the development of new treatment strategies involving functional stereotactic neurosurgery. Here, using anatomical comparisons between histology and magnetic resonance images of the human hypothalamus in the coronal plane, we show that discrete gray and white hypothalamic structures are consistently identifiable by MRI. Macroscopic and microscopic images were used to precisely annotate the MRI sequences realized in the coronal plane in twenty healthy volunteers. MRI was performed on a 1.5 T scanner, using a protocol including T1-weighted 3D fast field echo, T1-weighted inversion-recovery, turbo spin echo and T2-weighted 2D fast field echo imaging. For each gray matter structure as well as for white matter bundles, the different MRI sequences were analyzed in comparison to each other. The anterior commissure and the fornix were often identifiable, while the mammillothalamic tract was more difficult to spot. Qualitative analyses showed that MRI could also highlight finer structures such as the paraventricular nucleus, the ventromedial nucleus of the hypothalamus and the infundibular (arcuate) nucleus, brain nuclei that play key roles in the regulation of food intake and energy homeostasis. The posterior hypothalamic area, a target for deep brain stimulation in the treatment of cluster headaches, was readily identified, as was the lateral hypothalamic area, which similar to the aforementioned hypothalamic nuclei, could be a putative target for deep brain stimulation in the treatment of obesity. Finally, each of the identified structures was mapped to Montreal Neurological Institute (MNI) space.

Mammillary bodies and fornix fibers are injured in heart failure

Cognitive abnormalities, including memory deficits, are common in heart failure (HF). Brain structures, including the hippocampus, fornix, and thalamus participate in memory processing, and most show structural injury and functional deficits in HF. The mammillary bodies and fornix play essential roles in spatial and working memory processing, interact with other structures, and may also be injured in HF. We assessed mammillary body volumes and cross-sectional fornix areas in 17 HF and 50 control subjects using high-resolution T1-weighted magnetic resonance images. Mammillary body volumes and fornix cross-sectional areas were significantly reduced bilaterally in HF, and these differences remained after controlling age, gender, and intracranial volume. Mammillary body and fornix injury may contribute to the compromised spatial and working memory deficits in HF. Pathological processes eliciting the damage may include injury accompanying hypoxic/ischemic processes in pathologic HF perfusion and breathing, and thiamine deficiency accompanying diuretic use and nutritional mal-absorption in the condition.

[An unusual case of anterior communicating artery syndrome]

We describe the case of a 51-year-old man who developed acute partial anterior communicating artery syndrome (ACoAS) due to a nonruptured aneurysm of that artery. The patient presented with anterograde memory deficits, particularly impaired delayed recall, whereas his declarative learning and retrograde memory were relatively spared. Full ACoAS is usually associated with confabulations and personality change, which did not present in the case reported here. However, the patient presented with the flat affect and reduced drive typical of frontal lobe disorder. Clinical and neuropsychological assessment largely ruled out other causal factors involved in the symptomatology. Upon follow-up 2 years after onset of the AcoAS, the patient's neuropsychological performance had remained stable, yet his affective resonance and, in part, his spatial orientation had improved. We conclude that malformations of the intracranial arterial system ought to be taken into account as differential diagnosis of acute memory disorders.

Bilateral dorsolateral thalamic lesions disrupts conscious recollection

In an earlier study we disputed the claim that the mediodorsal thalamic nucleus is critical for familiarity. We reported patient (QX) who showed a severe deficit in conscious recollection, and behavioural problems (disinhibition, emotional lability) with relative sparing of familiarity-aware memory following a left mediodorsal thalamic lesion. More recent MR imaging has revealed that QX's lesions are more extensive than previously reported and involve both dorsolateral thalamic nuclei, and whilst there is evidence of left mediodorsal thalamic damage, it is not the main focus of damage. This paper reports a full analysis of QX's thalamic pathology alongside a more detailed investigation of his recognition memory, using yes/no and forced-choice procedures, and executive function. The results revealed impairments in yes/no recognition and conscious recollection rates of famous, artist and unknown names. In addition to the previously noted behavioural disinhibition and emotional lability, a deficit in spontaneous planning ability was evident on the Zoo Map Test (subtest of the Bahavioural Assessment of the Dysexecutive Syndrome). Forced-choice recognition, familiarity estimates and remote memory showed higher levels of preservation. The findings indicate that the dorsolateral thalamus is part of the extended hippocampal circuit which is causally critical only for recall and conscious recollection of complex associations rather than for the more automatic processes linked with novelty detection.

Macaque monkey retrosplenial cortex: II. Cortical afferents

We investigated the cortical afferents of the retrosplenial cortex and the adjacent posterior cingulate cortex (area 23) in the macaque monkey by using the retrograde tracers Fast blue and Diamidino yellow. We quantitatively analyzed the distribution of labeled neurons throughout the cortical mantle. Injections involving the retrosplenial cortex resulted in labeled neurons within the retrosplenial cortex and in areas 23 and 31 (approximately 78% of the total labeled cells). In the remainder of the cortex, the heaviest projections originated in the hippocampal formation, including the entorhinal cortex, subiculum, presubiculum, and parasubiculum. The parahippocampal and perirhinal cortices also contained many labeled neurons, as did the prefrontal cortex, mainly in areas 46, 9, 10, and 11, and the occipital cortex, mainly area V2. Injections in area 23 also resulted in numerous labeled cells in the posterior cingulate and retrosplenial regions (approximately 67% of total labeled cells). As in the retrosplenial cortex, injections of area 23 led to many labeled neurons in the frontal cortex, although most of these cells were in areas 9 and 46. Larger numbers of retrogradely labeled cells were also distributed more widely in the posterior parietal cortex, including areas 7a, 7m, LIP, and DP. There were some labeled cells in the parahippocampal cortex. These connections are consistent with the retrosplenial cortex acting as an interface between the working memory functions in the prefrontal areas and the long-term memory encoding in the medial temporal lobe. The posterior cingulate cortex, in contrast, may be more highly associated with visuospatial functions.

Retrograde amnesia and the volume of critical brain structures

There are many controversies concerning the structural basis of retrograde amnesia (RA). One view is that memories are held briefly within a medial temporal store ("hippocampal complex") before being "consolidated" or reorganised within temporal neocortex and/or networks more widely distributed within the cerebral cortex. An alternative view is that the medial temporal lobes are always involved in the storage and retrieval (reactivation) of autobiographical memories (multiple trace theory). The present study used quantitative magnetic resonance imaging (MRI) in 40 patients with focal pathology/volume loss in different sites, to examine the correlates of impairment on three different measures of RA. The findings supported the view that widespread neural networks are involved in the storage and retrieval of autobiographical and other remote memories. Brain volume measures in critical structures could account for 60% of variance on autobiographical memory measures (for incidents and facts) in diencephalic patients and for 60-68% of variance in patients with frontal lesions. Significant correlations with medial temporal lobe volume were found only in the diencephalic group, in whom they were thought to reflect thalamic changes, but not in patients with herpes encephalitis or hypoxia in whom the temporal lobes were particularly implicated. The latter finding fails to support one of the main predictions of multiple trace theory, as presently expounded.

Two cases of functional focal retrograde amnesia with impairment of object use

We report on two patients, TH and KN, with focal retrograde amnesia (FRA). Their memory loss regarding life events extended to their whole lives, whereas they could acquire and retain new information. They also showed prominent deficits in production and comprehension of common words. In addition, at least in the testing situation, they were impaired in their recognition and use of familiar objects. Although both cases of FRA followed an episode that can cause brain pathology, MRI revealed no structural abnormality in either patient. Stressful situations preceding the onset were evident in KN, but not in TH. We discuss their impairments of object knowledge from a neuropsychological perspective, and we interpret the etiology of their condition as a functional rather than a psychogenic amnesia.

Right medial thalamic lesion causes isolated retrograde amnesia

Pervasive retrograde amnesia without anterograde memory impairment has rarely been described as a consequence of circumscribed brain damage. We report this phenomenon in a 33 yr-old, right-handed man (JG) in association with the extension in the right thalamus of a previously small, bilateral thalamic lesion. JG presented with a dense amnesia for autobiographical material more than a few years old, with some sparing of recent memories. Furthermore, he was completely unable to recognise famous people or world events. Many other aspects of semantic knowledge were intact and there was no evidence of general intellectual impairment, executive dysfunction or loss of visual imagery. Magnetic resonance imaging revealed an acute lesion in the right thalamus and two small, symmetrical, bilateral non-acute thalamic lesions. Follow-up neuropsychological assessment indicated a stable pattern of impaired retrograde and spared anterograde memory over 18 months and psychiatric assessments yielded no evidence of confabulation, malingering or other symptoms to suggest psychogenic amnesia. JG's profile indicates that the division of declarative memory into just two categories - episodic and semantic - is inadequate. Rather, his case adds to the growing body evidence to suggest that world knowledge pertaining to people and events is stored or accessed similarly to autobiographical information and differently from other types of more general factual knowledge. We hypothesize that the right mediodorsal thalamic nucleus and immediately surrounding regions comprise the central processing mechanism referred to by McClelland (Revue Neurologique, 150 (1994) 570) and Markowitsch (Brain Research Review, 21 (1995) 117) as responsible for inducing and co-ordinating the recall of these sorts of cortically stored memory engrams.

The loss of episodic memories in retrograde amnesia: single-case and group studies

Retrograde amnesia in neurological disorders is a perplexing and fascinating research topic. The severity of retrograde amnesia is not well correlated with that of anterograde amnesia, and there can be disproportionate impairments of either. Within retrograde amnesia, there are various dissociations which have been claimed-for example, between the more autobiographical (episodic) and more semantic components of memory. However, the associations of different types of retrograde amnesia are also important, and clarification of these issues is confounded by the fact that retrograde amnesia seems to be particularly vulnerable to psychogenic factors. Large frontal and temporal lobe lesions have been postulated as critical in producing retrograde amnesia. Theories of retrograde amnesia have encompassed storage versus access disruption, physiological processes of 'consolidation', the progressive transformation of episodic memories into a more 'semantic' form, and multiple-trace theory. Single-case investigations, group studies and various forms of neuroimaging can all contribute to the resolution of these controversies.

Distribution of estrogen receptor beta (ERbeta) mRNA in hypothalamus, midbrain and temporal lobe of spayed macaque: continued expression with hormone replacement

This study used in situ hybridization (ISH) to examine the distribution of estrogen receptor beta (ERbeta) mRNA in hypothalamic, limbic, and midbrain regions of monkey brain and its regulation by estrogen (E) and progesterone (P). Monkey-specific ERbeta cDNAs were developed with human primers and reverse transcription and polymerase chain reaction (RT-PCR) using mRNA extracted from a rhesus monkey prostate gland. ERbeta 5' (262 bases) and 3' (205 bases) riboprobes were used in combination for ISH. Ovariectomized and hysterectomized (spayed) pigtail macaques (Macaca nemestrina; four per treatment group) were either untreated spayed-controls, treated with E (28 days), or treated with E plus P (14 days E+14 days E and P). Dense ERbeta hybridization signal was seen in the preoptic area, paraventricular nucleus, and ventromedial nucleus of the hypothalamus; the substantia nigra, caudal linear, dorsal raphe, and pontine nuclei of the midbrain; the dentate gyrus, CA1, CA2, CA3, CA4, and the prosubiculum/subiculum areas of the hippocampus. Expression in the suprachiasmatic region, supraoptic nucleus, arcuate nucleus, and amygdala was less intense. Image analysis of the dense areas showed no significant difference in the hybridization signal in individual regions of the hypothalamus, midbrain, or hippocampus between any of the treatment groups. However, P treatment decreased overall ERbeta signal in the hypothalamus and hippocampus when several different subregions were combined. The localization of ERbeta in monkey brain by ISH is in general agreement with that previously described in rodents. The presence of monkey ERbeta mRNA in brain regions that lack ERalpha should help to clarify the molecular mechanisms by which E acts in the central nervous system to influence hormone secretion, mood disorders, cognition, and neuroprotection.

Hypothalamopontine projections in the rat: anterograde axonal transport studies utilizing light and electron microscopy

Projections to the basilar pontine nuclei (BPN) from a variety of hypothalamic nuclei were traced in the rat utilizing the anterograde transport of biotinylated dextran amine. Light microscopy revealed that the lateral hypothalamic area (LH), the posterior hypothalamic area (PH), and the medial and lateral mammillary nuclei (MMN and LMN) are the four major hypothalamic nuclei that give rise to labeled fibers and terminals reaching the rostral medial and dorsomedial BPN subdivisions. Hypothalamopontine fibers extended caudally through the pontine tegmentum dorsal to the nucleus reticularis tegmenti pontis and then coursed ventrally from the main descending bundle toward the ipsilateral basilar pontine gray. Some hypothalamopontine fibers crossed the midline in the tegmental area just dorsal to the pontine gray to terminate in the contralateral BPN. Electron microscopy revealed that the ultrastructural features of synaptic boutons formed by axons arising in the LH, PH, MMN, and LMN are similar to one another. All labeled hypothalamopontine axon terminals contained round synaptic vesicles and formed asymmetric synaptic junctions with dendritic shafts as well as dendritic appendages, and occasionally with neuronal somata. Some labeled boutons formed the central axon terminal in a glomerular synaptic complex. In summary, the present findings indicate that the hypothalamus projects predominantly to the rostral medial and dorsomedial portions of the BPN which, in turn, provide input to the paraflocculus and vermis of the cerebellum. Since the hypothalamic projection zones in the BPN also receive cerebral cortical input, including limbic-related cortex, the hypothalamopontine system might serve to integrate autonomic or limbic-related functions with movement or somatic motor-related activity. Alternatively, since the cerebellum also receives direct input from the hypothalamus, the BPN may function to provide additional somatic and visceral inputs that are used by the cerebellum to perform the integrative function.

Thiamin deficiency and Korsakoff's syndrome: failure to find memory impairments following nonalcoholic Wernicke's encephalopathy

Prolonged alcohol consumption is associated with a variety of neuropsychiatric conditions, including the dense amnesic disorder known as Korsakoff's syndrome. Korsakoff's syndrome is frequently diagnosed in alcoholics after an episode of acute thiamin deficiency. The accepted view within the medical literature is that the etiology of this disorder lies in thiamin deficiency or Wernicke's encephalopathy. However, examination of the published reports of pure thiamin deficiency unaccompanied by chronic and excessive consumption of alcohol shows that, in this group of patients, the rate of progression to Korsakoff's syndrome is low. This result suggests that the memory impairments associated with alcohol-related brain damage cannot be attributed to thiamin deficiency alone. The etiology of alcohol-related cognitive impairments such as Korsakoff's syndrome is still poorly understood but several lines of evidence suggest multiple causal factors interact to produce deficits in performance. Animal models that manipulate only a single putative etiological factor are unlikely to elucidate the multiple influences that lead to Korsakoff's syndrome. A study of the natural history of alcohol-related brain damage is needed that will allow an assessment of individual risk factors and their interactions.

Efferent connections of the anteromedial nucleus of the thalamus of the rat

The projections from the anteromedial nucleus of the thalamus (AM) were investigated using anterograde and retrograde tracing techniques. AM projects to nearly the entire rostrocaudal extent of limbic cortex and to visual cortex. Anteriorly, AM projects to medial orbital, frontal polar, precentral agranular, and infraradiata cortices. Posteriorly, AM projects to retrosplenial granular, entorhinal, perirhinal and presubicular cortices, and to the subiculum. Further, AM projects to visual cortical area 18b, and to the lateral and basolateral nuclei of the amygdala. AM projections are topographically organized, i.e., projections to different cortical areas arise from distinct parts of AM. The neurons projecting to rostral infraradiata cortex (IRalpha) are more caudally located in AM than the neurons projecting to caudal infraradiata cortex (IRbeta). The neuronal cell bodies that project to the terminal field in area 18b are located primarily in ventral and lateral parts of AM, whereas neurons projecting to perirhinal cortex and amygdala are more medially located in AM. Injections into the most caudal, medial part of AM (i.e., the interanteromedial [IAM] nucleus) label terminals in the rostral precentral agranular, caudal IRbeta, and caudal perirhinal cortices. Whereas most AM axons terminate in layers I and V-VI, exceptions to this pattern include area 18b (axons and terminals in layers I and IV-V), the retrosplenial granular cortex (axons and terminals in layers I and V), and the presubicular, perirhinal, and entorhinal cortices (axons and terminals predominantly in layer V). Together, these findings suggest that AM influences a widespread area of limbic cortex.

Retrograde amnesia: clinical and methodological caveats

Several clinical and methodological caveats are outlined as they pertain to retrograde amnesia research, and data relevant to these caveats are presented. Three caveats in particular are noted in relation to recently published cases of marked retrograde amnesia; (i) temporal lobe epilepsy may influence memory for news events; (ii) there may be additional, unsuspected pathology in cases of amnesia, such as those with cerebral hypoxia; (iii) degree of media exposure is closely related to performance on the types of news events memory tests that are commonly used in retrograde amnesia research.

Recovery of function processes in human amnesia: evidence from transient global amnesia

There are few clues as to the processes that underlie recovery of function from human amnesia. Evidence is offered from the perspective of a study of recovery of function during an episode of transient global amnesia (TGA) that occurred as a complication of a cerebral angiographic procedure being carried out in a neurosciences centre, and where there was therefore a unique opportunity to examine acute changes in memory function. This allowed us to conduct the first quantitative study where shrinkage of anterograde and retrograde memory loss was plotted at four separate intervals throughout the acute recovery process, and also 24 hr later. Recovery of retrograde amnesia preceded recovery from anterograde amnesia. Resolution of a naming deficit more closely paralleled recovery from retrograde amnesia rather than anterograde amnesia. Within retrograde amnesia for public events, there was a temporal gradient of memory loss, with more recent events affected to a greater degree than earlier events. Within anterograde amnesia, picture recognition memory preceded recovery of story recall memory. On the basis of these findings, and related observations in the published literature, it is proposed that recovery from some types of human amnesia, such as that associated with TGA, follows a 'lateral-to-medial' rule--lateral inferotemporal areas that play a major role in retrograde amnesia recover first from hypometabolism related to the TGA attack, followed by 'interface' areas such as the rhinal and parahippocampal cortices that are considered to have a role in both anterograde and retrograde memory functioning, with the last areas to recover physiological integrity being discrete limbic-diencephalic structures such as the hippocampus.

Novel decision making in patients with prefrontal or posterior brain damage

Patients with lesions of the frontal lobes experience greater difficulties in performing nonroutine tasks, but their ability to cope with novel situations has not been studied. We assessed novel and previously practiced decisions in normal subjects and patients with frontal or posterior brain damage using a unimanual two-choice response time test. Patients with frontal damage had a dramatic impairment on novel decision, whereas practiced decision was normal. Relative Judgment Theory analysis suggested that the basic disorder concerns the ability to create internal referents that are determined from instructions and are subsequently required to associate the current stimulus with the appropriate response. The results of the study suggest that the prefrontal cortex is critical for novel decision making and that it operates mainly at the stage of creation of internal referents that associate some subsequent event with the selection of appropriate processing. This basic disorder may underlie some behavioral changes such as apathy and difficulty on tests that are sensitive to frontal damage.

A study of recovery of memory function in a case of witnessed functional retrograde amnesia

We describe a case of transient functional retrograde amnesia that had features of a fugue state, and that was accompanied by loss of personal identity and significant autobiographical amnesia. Uniquely, we were able to gather information from witnesses who observed the episode from its onset, and we were also able to monitor the acute stages of recovery of memory function over the subsequent four-week period. The profile of memory loss was characterised by impaired performance on both autobiographical and public events memory tasks, in the context of normal anterograde memory test scores. Shrinkage of retrograde amnesia took place over a four-week period, with autobiographical and public events components of retrograde memory recovering at the same rate. We discuss the possible role of unconscious processes underlying the episode, as compared to conscious simulation. We argue that most cases of functional retrograde amnesia may represent a combination of conscious simulation and unconscious processes.