Nature of personal semantic memory: evidence from Alzheimer’s disease

BackgroundModified Dead‐Alive Test (M‐DAT), which was developed and validated by Ozel‐Kizil et. al. (2018) evaluates semantic and episodic memory together. The original form was developed by Kapur et al. (1989), however it was not adequately studied.MethodM‐DAT consists of 45 names of celebrities who had died in remote past (15), died in the last five years (15) and who are still alive (15) and participants are asked whether they are alive or dead. Correct number of items concerning celebrities who died in remote past gives the semantic memory performance, while half of the the sum of correct number of items concerning celebrities who are still alive and had died in the last five years is accepted as episodic memory performance. Semantic and episodic memory performances of patients with DSM‐5 major neurocognitive disorder due to Alzheimer’s Disease‐MND‐AD (n=69), patients with DSM‐5 minor neurocognitive disorder‐MiND (n=27) who were admitted to geriatric psychiatry clinic and volunteered healthy controls‐HC (n=29) were compared. The groups were compared either by one‐way ANOVA or Kruskal‐Wallis tests and Bonferroni or Mann Whitney U tests was used for post‐hoc analysis.ResultMND‐AD group was older and less educated. Also MND‐AD group had lower MMSE scores while MiND and HCs had similar scores. MND‐AD had lower semantic and episodic scores than MiND and HC, also the scores of MiND group were lower than HCs. Both M‐DAT semantic and episodic scores were positively correlated with education in the whole sample (R=.53, p<0.001; R=.43, p<0.001). Although semantic memory performances were higher than episodic memory performances in all groups, two scores were positively correlated (R=.71, p<0.001).ConclusionThe results of the study suggested a semantic memory impairment as well as episodic memory deficit in patients with major and minor neurocognitive disorder. Although previous studies have conflicting results, semantic memory deficit early in Alzheimer’s disease and in mild cognitive impairment was also reported (3). M‐DAT is an alternative for the assessment of semantic memory that is usually evaluated by verbal fluency tests. Evaluating episodic and semantic memory together is an important advantage however M‐DAT is affected by education and the items require updating.

As part of a unifying theory of autism, Ben Shalom (2009) proposed that while procedural, perceptual and semantic memory functions are intact in Autism Spectrum Disorder (ASD), the more integrative level of episodic memory is impaired. According to Ben Shalom, this reduced integration may be due to the reduced function of the medial prefrontal cortex (mPFC), which may also explain the reduced integration found in motor, sensory-perceptual and emotional processes in ASD. The present review examines this hypothesis, by focusing on evidence regarding autobiographical memory (AM) episodes in ASD—arguably the highest form of memory integration processes. Most research on memory in ASD thus far has focused on memory for experimentally-presented stimuli (Lind, 2010; Boucher et al., 2012). The present paper builds on this literature to examine the rich evidence that has recently accumulated from in-depth, systematic studies of AM in ASD—memories of personally-related events that are naturalistically accumulated over a person's lifetime. Of note, research on AM is limited in its focus on memories that cannot be as readily verified (but see Bruck et al., 2007), and in its reliance on high-functioning verbal individuals. Nonetheless, studies of AM provide us with an unparalleled perspective on the naturalistic process of memory integration in ASD. Specifically, this review aims to determine how well memory episodes are integrated in ASD; which elements become integrated and which do not; whether the ability to form integrated, episodic memories relates to other cognitive and emotional capacities; and how this pattern of integration changes over time. Semantic and episodic autobiographical memory (AM) The declarative memory system comprises semantic and episodic components. Semantic memories are memories of timeless, de-contextualized facts. Episodic memory refers to personal events recollected in the context of a particular time and place, with some reference to oneself as a participant in the episode (Tulving, 2002). Thus, episodic memories involve two functions: the ability to bind different perceptual elements; and, in humans, the ability to perceive of oneself within this context. On a neurobiological level, episodic memory storage and retrieval are thought to involve the interaction of cortical association areas, in which basic sensory information regarding what occurred and where is stored; the hippocampus, which binds these elements into cohesive memories of individual events; and the mPFC, which further contextualizes these events into schemas, such as the self (Preston and Eichenbaum, 2013). AM refers to memory for information pertaining to the self; and while it is often viewed as overlapping with episodic memory, the two are not synonymous (Gilboa, 2004). Episodic memory is a memory system, while AM is a type of content (Gardiner, 2008). Thus, episodic memory functions can encompass both AM and simple phenomena that do not necessarily represent self-relevant information (e.g., source memory). At the same time, AM in fact comprises of both semantic and episodic knowledge (e.g., semantic knowledge of one's date of birth, alongside an episodic memory of one's last birthday). In children with ASD, both semantic and episodic AM is reduced (Bruck et al., 2007; Bon et al., 2012; Goddard et al., 2014), though by adulthood, adults with ASD show a spared memory for semantic AM, alongside reduced episodic AM (Klein et al., 1999; Crane and Goddard, 2008). These studies suggest that as semantic AM may grow in ASD, episodic AM impairments are pervasive. These results fit with the general memory profile in ASD, viz., spared semantic memory alongside difficulties with episodic memory, which is found across experimental studies (Boucher and Bowler, 2008). The present review concerns itself primarily with episodic AM in ASD, though semantic memory will be discussed as it relates to the content of autobiographical narratives.

Distinct neuroanatomical bases of episodic and semantic memory performance in Alzheimer’s disease

Structural stability and change in semantic and episodic memory performance as well as interindividual differences in 5-year changes in these constructs are examined within a sample of older adults (age rangeT1 = 60-80; n = 361). Interindividual differences in change were limited but significant. Stability coefficients were higher for semantic memory (.95) than for episodic memory (.87). Changes in episodic and semantic memory performance were strongly associated (r =.68). Across time, variances and covariances increased, and a tendency toward dedifferentiation in terms of increasing correlations was found. Chronological age was related to both level and change, but gender and education were only related to level of memory performance. Collectively, these results depict relatively high degrees of structural stability and stability of interindividual differences in declarative memory in old age.

Episodic and semantic memory in children with mesial temporal sclerosis

One of the most common distinctions in long-term memory is that between semantic (i.e., general world knowledge) and episodic (i.e., recollection of contextually specific events from one's past). However, emerging cognitive neuroscience data suggest a surprisingly large overlap between the neural correlates of semantic and episodic memory. Moreover, personal semantic memories (i.e., knowledge about the self and one's life) have been studied little and do not easily fit into the standard semantic-episodic dichotomy. Here, we used fMRI to record brain activity while 48 participants verified statements concerning general facts, autobiographical facts, repeated events, and unique events. In multivariate analysis, all four types of memory involved activity within a common network bilaterally (e.g., frontal pole, paracingulate gyrus, medial frontal cortex, middle/superior temporal gyrus, precuneus, posterior cingulate, angular gyrus) and some areas of the medial temporal lobe. Yet the four memory types differentially engaged this network, increasing in activity from general to autobiographical facts, from autobiographical facts to repeated events, and from repeated to unique events. Our data are compatible with a component process model, in which declarative memory types rely on different weightings of the same elementary processes, such as perceptual imagery, spatial features, and self-reflection.

The ERP correlates of self-knowledge: Are assessments of one’s past, present, and future traits closer to semantic or episodic memory?

Semantic, episodic, and autobiographical memory in a postmeningitic amnesic patient

The relationship between episodic and semantic memory systems has long been debated. Some authors argue that episodic memory is contingent on semantic memory (Tulving 1984), while others postulate that both systems are independent since they can be selectively damaged (Squire 1987). The interaction between these memory systems is particularly important in the elderly, since the dissociation of episodic and semantic memory defects characterize different aging-related pathologies. Here, we investigated the interaction between semantic knowledge and episodic memory processes associated with faces in elderly subjects using an experimental paradigm where the semantic encoding of famous and unknown faces was compared to their episodic recognition. Results showed that the level of semantic awareness of items affected the recognition of those items in the episodic memory task. Event-related magnetic fields confirmed this interaction between episodic and semantic memory: ERFs related to the old/new effect during the episodic task were markedly different for famous and unknown faces. The old/new effect for famous faces involved sustained activities maximal over right temporal sensors, showing a spatio-temporal pattern partly similar to that found for famous versus unknown faces during the semantic task. By contrast, an old/new effect for unknown faces was observed on left parieto-occipital sensors. These findings suggest that the episodic memory for famous faces activated the retrieval of stored semantic information, whereas it was based on items' perceptual features for unknown faces. Overall, our results show that semantic information interfered markedly with episodic memory processes and suggested that the neural substrates of these two memory systems overlap.

Pathological tau is suggested to play a role in cognitive deterioration in the preclinical phase of Alzheimer’s disease. We investigated cross-sectional associations of tau burden with episodic and semantic memory performance in older adults without dementia. A systematic search in MEDLINE (via PubMed), PsychINFO, and Embase resulted in 24 eligible studies for meta-analysis. Tau burden was assessed using CSF, PET, or histopathological measures. All studies evaluated associations of tau with episodic memory: weighted effect sizes were -0.46 (95 % CI [-0.73; -0.20], p < .001) for episodic composite scores, -0.19 ([-0.36; -0.03], p = .024) for delayed word list recall, and -0.05 ([-0.14; 0.04], p = .257) for logical memory. Fourteen studies evaluated associations of tau with semantic memory: weighted effect sizes were -0.28 ([-0.52; -0.04], p = .023) for semantic composite scores, -0.06 ([-0.16; 0.03], p = .194) for semantic fluency, and 0.06 ([-0.06; 0.18], p = .319) for picture naming. Our findings indicate that tau burden related to both episodic and semantic memory impairment in older individuals without a diagnosis of mild cognitive impairment or manifest dementia, with episodic composite scores showing the strongest association with tau burden. Future potential lies in developing more sensitive scores to detect this subtle cognitive impairment, which could contribute to early identification of individuals in the preclinical phase of Alzheimer’s disease, thereby improving early diagnosis and timely intervention.

Individuals with mild cognitive impairment (MCI) can often progress into Alzheimer’s Disease (AD). Research suggests that decline in episodic memory and semantic memory, as well as functional abilities, can be sensitive in predicting disease progression. This study aimed to (a) investigate episodic and semantic memory performance differences between AD and MCI, (b) determine if memory performance predicts observation-based activities of daily living (ADLs), and (c) explore whether semantic memory mediates the relationship between episodic memory and ADLs. Fifty-eight AD, 53 MCI, and 72 healthy control participants were administered the Rey-O, California Verbal Learning Test, Animal Fluency Test, Boston Naming Test, and Direct Assessment of Functional Status (DAFS). The results revealed, first, that AD participants performed significantly lower than the MCI participants across semantic memory and episodic memory tasks, with the exception of the Boston Naming Test. Second, hierarchical-stepwise regression analyses found that semantic memory significantly predicted DAFS orientation, communication, and financial skills in AD, but episodic memory predicted shopping skills. Furthermore, semantic memory significantly predicted DAFS transportation skills in AD and MCI. Third, within the overall sample, semantic memory mediated the relationship between episodic memory and ADLs. Taken together, the findings suggest decline in semantic memory (as measured by confrontational naming and category fluency) and episodic memory (as measured by list and complex visual design learning and recall) may lead to decline in different and specific aspects of functional abilities in AD and MCI.

Semantic cognition is supported by two interactive components: semantic representations and mechanisms that regulate retrieval (cf. ‘semantic control’). Neuropsychological studies have revealed a clear dissociation between semantic and episodic memory. This study explores if the same dissociation holds for control processes that act on episodic and semantic memory, or whether both types of long-term memory are supported by the same executive mechanisms. We addressed this question in a case-series of semantic aphasic patients who had difficulty retrieving both verbal and non-verbal conceptual information in an appropriate fashion following infarcts to left inferior frontal gyrus (LIFG). We observed parallel deficits in semantic and episodic memory: (i) the patients' difficulties extended beyond verbal materials to include picture tasks in both domains; (ii) both types of retrieval benefitted from cues designed to reduce the need for internal constraint; (iii) there was little impairment of both semantic and episodic tasks when control demands were minimised; (iv) there were similar effects of distractors across tasks. Episodic retrieval was highly susceptible to false memories elicited by semantically-related distractors, and confidence was inappropriately high in these circumstances. Semantic judgements were also prone to contamination from recent events. These findings demonstrate that patients with deregulated semantic cognition have comparable deficits in episodic retrieval. The results are consistent with a role for LIFG in resolving competition within both episodic and semantic memory, and also in biasing cognition towards task-relevant memory stores when episodic and semantic representations do not promote the same response.

Personal semantics: Is it distinct from episodic and semantic memory? An electrophysiological study of memory for autobiographical facts and repeated events in honor of Shlomo Bentin

Reduced levels of specific autobiographical memories in schizophrenia

This paper compares a patient, AB, who showed florid confabulation with two other patients, one of whom had bilateral frontal lesions, and the other bilateral temporal lobe pathology. AB' s confabulation followed a Wernicke episode, and she had a clinical diagnosis of an alcoholic Korsakoff syndrome, implying diencephalic pathology. However, cervical carcinoma was also found, and the persistence of her confabulation was attributed to metabolic or other non-metastatic complications of carcinoma, affecting cortical function. In the late stages of her illness, she also developed bilateral posterior parietal-occipital metastases. AB was assessed on three measures of confabulation-Dalla Barba' s 1993a Confabulation Battery, the Autobiographical Memory Interview, and an Informal Interview. She showed “spontaneous" confabulation extending across episodic, personal semantic, and general semantic memory. In this, she contrasted with the frontal lobe lesion patient, who showed confabulation only in personal semantic memory, and the temporal lobe lesion patient, who did notconfabulate. From an account of her confabulations given by her brother, as well as our own observations of her pattern of responses, three factors were identified as contributing to the confabulation- confusions in the context of memories, involving both time and place; a high rate of perseverations, particularly in semantic memory; and a tendency to respond indiscriminately to the immediate social and environmental context. It is concluded that frontal pathology and a disorder in contextmemory may be necessary, butnotsufficient, conditions for spontaneous confabulations to occur, and any theoretical description of confabulation may need to take account of a number of contributing factors.