The impairment of verbal semantic memory: a single case study

Phonological Short-term Memory and the Learning of Novel Words: The Effect of Phonological Similarity and Item Length

The investigation of a patient with a selective impairment of phonological short-term memory has recently provided evidence that this system may be involved in long-term learning of novel words, for which a pre-existing semantic representation is not available (Baddeley, Papagno, & Vallar, 1988). The present series of experiments in normal subjects explored this hypothesis. We assessed the effects of phonological similarity and item length, which reflect the operation of the phonological short-term store and the rehearsal component of verbal memory, upon paired associate long-term learning of auditorily presented words and non-words. Phonological similarity affected the learning of novel words more than known words (Experiment 1); when a delay was interposed between presentation and recall, the disruptive effect was confined to novel words (Experiment 2). Also word length disrupted the learning of novel words, but not of known words (Experiment 3). These results tie in with neuropsychological evidence to suggest a role for phonological short-term memory in the learning of new words, and they have developmental implications for the study of language acquisition.

Past, present, and prospects: Reflections 40 years on from the selective impairment of semantic memory (Warrington, 1975)

We summarize the main findings and conclusions of Warrington's (1975) paper, The Selective Impairment of Semantic memory, a neuropsychological paper that described three cases with degenerative neurological conditions [Warrington, E. K. (1975). The selective impairment of semantic memory. The Quarterly Journal of Experimental Psychology, 27, 635–657]. We consider the developments that have followed from its publication and give a selective overview of the field in 2014. The initial impact of the paper was on neuropsychological investigations of semantic loss followed some 14 years later by the identification of Semantic Dementia (the condition shown by the original cases) as a distinctive form of degenerative disease with unique clinical and pathological characteristics. We discuss the distinction between disorders of semantic storage and refractory semantic access, the evidence for category- and modality-specific impairments of semantics, and the light that has been shed on the structure and organization of semantic memory. Finally we consider the relationship between semantic memory and the skills of reading and writing, phonological processing, and autobiographical memory.

Précis of From neuropsychology to mental structure

Neuropsychological results are increasingly cited in cognitive theories although their methodology has been severely criticised. The book argues for an eclectic approach but particularly stresses the use of single-case studies. A range of potential artifacts exists when inferences are made from such studies to the organisation of normal function – for example, resource differences among tasks, premorbid individual differences, and reorganisation of function. The use of “strong” and “classical” dissociations minimises potential artifacts. The theoretical convergence between findings from fields where cognitive neuropsychology is well developed and those from the normal literature strongly suggests that the potential artifacts are not critical. The fields examined in detail in this respect are short-term memory, reading, writing, the organisation of input and output speech systems, and visual perception. Functional dissociation data suggest that not only are input systems organised modularly, but so are central systems. This conclusion is supported by findings on impairment of knowledge, visual attention, supervisory functions, memory, and consciousness.

Comprehension of concrete and abstract words in semantic dementia

The vast majority of brain-injured patients with semantic impairment have better comprehension of concrete than abstract words. In contrast, several patients with semantic dementia (SD), who show circumscribed atrophy of the anterior temporal lobes bilaterally, have been reported to show reverse imageability effects, that is, relative preservation of abstract knowledge. Although these reports largely concern individual patients, some researchers have recently proposed that superior comprehension of abstract concepts is a characteristic feature of SD. This would imply that the anterior temporal lobes are particularly crucial for processing sensory aspects of semantic knowledge, which are associated with concrete not abstract concepts. However, functional neuroimaging studies of healthy participants do not unequivocally predict reverse imageability effects in SD because the temporal poles sometimes show greater activation for more abstract concepts. The authors examined a case-series of 11 SD patients on a synonym judgment test that orthogonally varied the frequency and imageability of the items. All patients had higher success rates for more imageable as well as more frequent words, suggesting that (1) the anterior temporal lobes underpin semantic knowledge for both concrete and abstract concepts, (2) more imageable items--perhaps because of their richer multimodal representations--are typically more robust in the face of global semantic degradation and (3) reverse imageability effects are not a characteristic feature of SD.

The chicken with four legs: a case of semantic amnesia and cryptogenic epilepsy

Here we describe the unusual condition of selective semantic amnesia related to cryptogenic temporal lobe epilepsy. XY, an adult male patient, presented with partial seizures and disabling dysnomia. Neuropsychological tests revealed seriously impaired semantic memory. Electroencephalography documented ictal epileptic abnormalities in the left temporal lobe. Positron emission tomography showed reduced metabolism in the temporoparietal regions, but the results of magnetic resonance imaging and spectroscopy were normal. After 11 years, neuropsychological tests showed selective impairment of semantic memory and computed tomography provided normal results. This case shows that semantic memory may be selectively and lastingly altered, highlighting a distinction between semantic amnesia and global amnesia. Moreover, it is unique in that it occurred without evidence of gross temporal lobe pathology. The pathophysiological pattern of epileptic abnormalities in the left temporoparietal cortex supports the role played by dysfunctional neuronal networks (as provoked by focal epileptic discharges) in determining selective semantic amnesia.

Refractory effects in stroke aphasia: A consequence of poor semantic control

This study examined the full range of effects associated with "semantic access impairment" - namely, refractory variables (semantic relatedness, speed of presentation and item repetition), inconsistency, the absence of frequency effects and facilitation by cues - in a series of stroke patients with multimodal semantically impairment. By investigating all of these factors in a group of patients who were not specifically selected to show "access" effects, we were able to establish (1) whether this pattern is a common consequence of infarcts that produce semantic impairment and (2) if these symptoms co-occur. All of the patients showed effects of cueing and an absence of frequency effects in comprehension. Patients whose brain damage included the left inferior prefrontal cortex (LIPC) also showed marked effects of refractory variables; in contrast, two patients with temporal-parietal but not frontal lesions were less sensitive to these variables. Parallel results were obtained for cyclical naming and word-picture matching tasks suggesting that the LIPC plays a role in semantic selection as well as lexical retrieval. Rapid presentation and item repetition is likely to have increased the selection demands in both of these tasks in a similar fashion. Unlike patients with classical "semantic access impairment", our semantically impaired stroke patients showed significant test-retest consistency, indicating that their difficulties did not result from an unpredictable failure of semantic access--instead, their deficits were interpreted as arising from failures of semantic control.

Degraded Semantic Knowledge And Accurate Object Use

In the present paper we report the performance on object use and on semantic tasks of two patients, D.L. with probable semantic dementia, and A.M. with an atypical onset of dementia of Alzheimer, assessed twice two years apart. In particular, we investigated whether the patients' ability to use objects degraded as a function of their semantic knowledge about those objects. Results from the two assessments in 2002 and in 2004 confirmed that both patients had a selective loss of the lexical-semantic knowledge, despite a relative preservation of the other cognitive abilities including object use. This pattern of results suggests that semantic knowledge is not necessarily involved in the correct use of objects.

Semantic impairment in stroke aphasia versus semantic dementia: a case-series comparison

Different neuropsychological populations implicate diverse cortical regions in semantic memory: semantic dementia (SD) is characterized by atrophy of the anterior temporal lobes whilst poor comprehension in stroke aphasia is associated with prefrontal or temporal-parietal infarcts. This study employed a case-series design to compare SD and comprehension-impaired stroke aphasic patients directly on the same battery of semantic tests. Although the two groups obtained broadly equivalent scores, they showed qualitatively different semantic deficits. The SD group showed strong correlations between different semantic tasks--regardless of input/output modality--and substantial consistency when a set of items was assessed several times. They were also highly sensitive to frequency/familiarity and made coordinate and superordinate semantic errors in picture naming. These findings support the notion that amodal semantic representations degrade in SD. The stroke aphasia group also showed multimodal deficits and consistency across different input modalities, but inconsistent performance on tasks requiring different types of semantic processing. They were insensitive to familiarity/frequency--instead, tests of semantic association were influenced by the ease with which relevant semantic relationships could be identified and distractors rejected. In addition, the aphasic patients made associative semantic errors in picture naming that SD patients did not make. The aphasic patients' picture naming performance improved considerably with phonemic cues suggesting that these patients retained knowledge that could not be accessed without contextual support. We propose that semantic cognition is supported by two interacting principal components: (i) a set of amodal representations (which progressively degrade in SD) and (ii) executive processes that help to direct and control semantic activation in a task-appropriate fashion (which are dysfunctional in comprehension-impaired stroke aphasic patients).

Knowing 7 x 8, but not the meaning of 'elephant': evidence for the dissociation between numerical and non-numerical semantic knowledge

Patients affected by semantic dementia (SD) and other severe cognitive deficits may show preserved numerical skills, including the retrieval of multiplication facts from long-term memory. No studies so far specifically investigated the network of arithmetic facts in semantic dementia. Thus, it is unknown whether preserved multiplication in SD truly reflects intact semantic knowledge or preserved retrieval of verbal sequences (just as the recitation of rhymes or poems). In the present study a patient (SG) with SD underwent an extensive assessment of number processing and calculation abilities. In particular, multiplication knowledge was investigated through a series of computerised tasks (production task, multiple-choice task, number bisection task with multiplicative triplets, number-matching task). SG demonstrated excellent performance in all number processing and calculation tasks. In computerised tasks tapping multiplication fact knowledge, SG was as accurate and fast as healthy controls. Analyses on individual regression slopes indicated that SG's reaction time effects (problem-size effect, problem-difficulty effect, interference effects, and facilitation effect) were comparable to those found in controls in each task. These results add new evidence to the independence of numerical knowledge from other semantic information and provide further insight into the organisation of stored arithmetic knowledge.

The natural history of late-stage "pure" semantic dementia

Relatively little is known about the neuropsychological profile of late-stage semantic dementia. This article provides a detailed assessment of patient MK who, despite her very severe semantic impairments, remained cooperative to testing and, unusually, did not show additional behavioral/personality changes. Although MK's initial presentation was typical of semantic dementia (SD), her performance began to deviate from the normal pattern. She developed impairments of single word repetition and regular word reading, and began to produce phonological errors in picture naming and spontaneous speech. These deficits might suggest that late-stage SD includes an independent disorder of phonology. An alternative possibility, however, is that phonological processing cannot proceed normally in the face of profound semantic degradation. A series of experiments supported the latter explanation of MK's deficits. In picture naming, MK showed little effect of progressive phonological cueing, did not reveal an increased sensitivity to word length or phonological complexity and continued to show a high degree of item-specific consistency in both accuracy and errors: she tended to produce the same erroneous phonemes for each item. She remained sensitive to the effects of phonological similarity in immediate serial recall. Letter substitution errors in regular word reading were more common for lower frequency letters (e.g., Q, Z). These letters also produced more item errors in immediate serial recall, suggesting that a frequency-graded loss of letter knowledge, rather than separate orthographic and phonological deficits, accounted for the deficits in both of these tasks. These findings are discussed in terms of theories that posit strong interactivity between phonology and semantics.

Partial knowledge of abstract words in patients with cortical degenerative conditions

Partial knowledge is a common but rarely studied consequence of damage to conceptual representations and is characterized by the retained ability to retrieve crude, superordinate information but not specific, detailed information about a conceptual entity. Previous studies have described partial knowledge for concrete items particularly following semantic dementia (SD). The present study was designed to investigate the occurrence of partial knowledge effects in the conceptual domain of abstract words. A novel 3-level synonym comprehension test was administered to 9 patients with SD, 20 patients with probable Alzheimer's disease (AD), and 40 healthy control subjects. All subject groups showed weaker performance on tasks requiring a fine specification of word meaning compared with those for which a broad sense of meaning or valence was necessary. However, this gradient of partial knowledge was significantly greater for SD and AD subjects than for controls. These results demonstrate that partial knowledge is a general property of a degraded knowledge base and is not restricted to the concrete word domain. It constitutes a normal phenomenon that is exacerbated in the context of neurodegenerative disease.

Characteristics of Verbal Semantic Impairment in Left Hemisphere Epilepsy

Fifty-two patients with partial epilepsy of left (n=30) or right (n=22) hemisphere origin were compared with 23 healthy subjects to explore the characteristics and mechanisms of verbal semantic deficits. Picture Naming, Picture Pointing, and the Semantic Questionnaire assessed semantic retrieval, comprehension, and judgment, respectively. In comparison with the controls and right hemisphere patients, the left hemisphere patients showed impairments on Picture Naming and the Semantic Questionnaire. On Picture Naming, the left hemisphere patients made significant omissions and intracategorical errors; on the Semantic Questionnaire, they made errors at superordinate and subordinate levels of information, they made more errors in relation to living than nonliving things, and there were significant associations between their Picture Naming and Semantic Questionnaire scores. In this population, the mixed profiles of semantic deficits suggests the coexistence of altered retrieval and information loss.

Remembering and forgetting of semantic knowledge in amnesia: a 16-year follow-up investigation of RFR

We report our long-term follow-up investigations of RFR, a post-encephalitic case of very grave anterograde and retrograde amnesia. We also describe the results of quantitative neuroimaging of his brain injury that showed bilateral and severe reduction in the hippocampal formation and medial temporal structures with sparing of left lateral/posterior and right posterior temporal cortex. We established that RFR had a persistent severe anterograde and retrograde amnesia for personal and public events. His personal semantic knowledge was relatively spared for the retrograde period. There was a modest and global reduction in RFR's vocabulary for words acquired in adulthood before he became amnesic but there was no evidence of any retrograde gradient. His retrograde knowledge of people was also without any gradient. Remarkably, there had been no change in the extent of his semantic knowledge across a prolonged re-test interval indicating that the loss of semantic knowledge was stable and likely to have arisen at the time of his initial lesion. RFR also showed evidence of a limited but significant ability to acquire new word meanings and a more restricted capacity for learning about new celebrities. While he was able to demonstrate face and name familiarity for newly famous people, he was unable to provide much semantic detail. RFR's amnesia can be partially explained by contemporary theories that allow for parallel cortical and hippocampal memory systems but is difficult to reconcile in detail with any extant view.

Mechanisms underlying perseveration in aphasia: evidence from a single case study

Aphasic individuals often inappropriately and unintentionally repeat recent responses, errors termed recurrent perseverations. In a series of picture naming experiments, we investigated the impact of manipulating stimulus factors on the number of perseverations produced by an aphasic patient (E.B.) with markedly impaired naming skills. E.B. produced significantly more perseverations to pictures with low frequency names and following stimulus repetition. In contrast, semantic relatedness and presentation rate failed to influence perseveration. Our results are considered in the context of theories that relate recurrent perseverations to intact priming mechanisms [Brain 121 (1998) 1641; Aphasiology 12 (1998) 319; J. Exp. Psychol. Learn. Mem. Cogn. 19 (1993) 243]. We conclude that these theories can correctly predict some but not all aspects of E.B.'s perseverations. In particular, they failed to predict that: (1) perseverations often appeared to reflect the earlier sequential proximity of stimuli and responses; and (2) perseverations became less likely as more experimental trials intervened, a trend that did not interact with presentation rate. We review evidence relating recurrent perseverations to neuromodulatory deficits and we conclude that a theory of the functional role of neuromodulation in the cerebral cortex proposed by Hasselmo [Neural Netw. 7 (1994) 13] is capable of accounting for all aspects of E.B.'s perseverative behavior.

The impact of synaptic depression following brain damage: a connectionist account of "access/refractory" and "degraded-store" semantic impairments

Neuropsychological studies of patients with acquired semantic impairments have yielded two distinct and contrasting patterns of performance in a spoken-word/picture-matching task (Warrington & Cipolotti, 1996). Patients labeled access/refractory are strongly influenced by presentation rate, semantic relatedness of distractors, and repetition, yet they seem relatively unaffected by lexical frequency. Degraded-store patients, on the other hand, are strongly affected by lexical frequency but are less affected by presentation rate, semantic relatedness, or repetition. Our account of these patterns of performance is based on the distinction between two different types of neurological damage: (1) damage to neuromodulatory systems that function to amplify neural signals while suppressing normal refractory-like effects and (2) damage to connections between groups of neurons that encode semantic information and are sensitive to frequency/familiarity. We present a connectionist model that learns to map spoken-word input to semantic representations and that incorporates a particular form of neural refractoriness referred to as synaptic depression, as well as a simple form of neuromodulation. We show that the model is capable of accounting for the contrasting patterns of semantic impairment under these two different forms of damage and, furthermore, demonstrate how it is capable of handling several documented cases that are exceptions to the basic patterns of impairment. Several predictions and limitations of the present model are discussed.

Spared numerical abilities in a case of semantic dementia

We report a case study of a patient (IH) with a progressive impairment of semantic memory affecting all categories of knowledge apart from numbers. Pictorial material was better understood than words, but was still severely impaired. The selective preservation of nearly all aspects of numerical knowledge suggested that this domain might have different neuropsychological status from other aspects of semantic memory.

Semantic amnesia without dementia: documentation of a case

We described the case of a patient affected by a progressive semantic memory disorder associated with prevalent temporal lobe atrophy. This deficit seems to be "pure" in the sense that it has not been found to overlap with other cognitive deficits (intellectual, linguistic, perceptual, visuo-spatial etc.) for a long time. Furthermore, despite his impaired semantic knowledge, the autobiographical memory of the patient was largely intact. This case therefore represents a form of "semantic amnesia" without dementia, and supports the hypothesis that there is a partial distinction between "semantic" and "episodic" memory.

Surface dyslexia in nonfluent progressive aphasia

This article presents the case of a 59-year-old male, JH, with a 6-year history of primary progressive aphasia (PPA), a disorder characterized by isolated language deterioration with relative preservation of other cognitive abilities. JH also shows typical features of surface dyslexia, a reading disorder exemplified by the selective preservation of phonological reading. One recent theory is that surface dyslexia in individuals with PPA results from a loss of semantic knowledge. In this paper we consider an additional possibility and present data supporting the notion that surface dyslexia may also arise from the malfunction in the links between semantic representations and phonology. JH has remarkably preserved lexical semantic knowledge when assessed on tasks that do not require verbal output. Further, item-by-item comparisons of his oral reading and comprehension ability show no significant correspondence between his reading and semantic knowledge. These findings lead us to conclude that, in JH's case, the surface dyslexia is attributable not to a semantic deficit per se but rather to the inability to access phonological information from semantics. JH's language profile is considered in relation to potential sources of surface dyslexia and other cases of progressive aphasia.

A new case of retrograde amnesia with abnormal forgetting rate

The classic amnesic syndrome is characterised by a severe anterograde amnesia and a less important retrograde amnesia with sparing of the semantic component. We report the case of a patient who showed a global amnesic syndrome following a mild head injury. Initially, amnesia was both anterograde and retrograde, and also involved semantic knowledge. Two years later the picture had changed remarkably. The retrograde deficit for autobiographical events was still total, while semantic memory had recovered to a large extent. Learning had also greatly improved, but only if assessed after a short delay; abnormally rapid forgetting rate were found at longer intervals. This pattern of impairment does not lend itself to an easy interpretation. However, the hypothesis of a consolidation deficit may be advanced.

Autobiographical experience and word meaning

The role of current personal experience in understanding of word meaning was investigated in a patient, WM, who suffers from semantic dementia. The study was prompted by the observation that WM, despite being severely impaired on formal tests of word comprehension and naming, retained a range of vocabulary pertaining to her daily life. If autobiographical experience has a general facilitatory effect, then this should affect which concepts are retained and which lost, but not influence the quality of that conceptual knowledge. Conversely, if personal autobiography has a direct role in investing concepts with meaning, then WM's understanding of nominal terms that she uses spontaneously in conversation ought not to be normal, but should be constrained by the autobiographical context in which she uses those terms. WM could define nouns and noun phrases drawn from her conversational vocabulary, but her definitions had a markedly autobiographical quality. Moreover, WM was extremely impaired in her ability to define new noun phrases, constructed by combining words from her conversational vocabulary (e.g. "dog licence", constructed from "driving licence" and "dog"; "oil field" constructed from "oil" and "field"). It was concluded that WM does not have normal conceptual understanding of nouns and noun phrases that she uses appropriately in conversation. Her understanding is narrow and autobiographically constrained. The findings, which suggest an interactive relationship between autobiographical and semantic memory, have implications for understanding of the progressive breakdown of semantic knowledge.

Refractory semantics in global aphasia: on semantic organisation and the access-storage distinction in neuropsychology

A single case study is reported of a global aphasic patient, JM, with impaired access to semantic information which was particularly severe for the class of proper names. JM's ability to perform matching tasks with printed words and pictures to auditory words deteriorated when items were repeated, especially when the response-stimulus interval was short. Performance was also inconsistent across items. The effect of repeated testing on items generalised to other, previously untested members of the same category. Despite this, JM was able to access general semantic information about stimuli from the affected categories (e.g. to categorise boys' and girls' names), and showed good ability to access an input lexicon concerning these stimuli. There was also a close relationship between the categories affected when he was tested with pictures and printed words. We propose that JM's deficit can be attributed to his semantic system entering an abnormal refractory state once semantic access for a particular item has been achieved, and with this stage being isolated from the procedures providing access to stored lexical knowledge. Furthermore, the representations affected seem common to pictures and printed words. We discuss the implications of the results for understanding the nature of semantic representations in general and for proper names in particular, and for the distinction between access and storage deficits in neuropsychology.

Double dissociation without modularity: evidence from connectionist neuropsychology

Many theorists assume that the cognitive system is composed of a collection of encapsulated processing components or modules, each dedicated to performing a particular cognitive function. On this view, selective impairments of cognitive tasks following brain damage, as evidenced by double dissociations, are naturally interpreted in terms of the loss of particular processing components. By contrast, the current investigation examines in detail a double dissociation between concrete and abstract work reading after damage to a connectionist network that pronounces words via meaning and yet has no separable components (Plaut & Shallice, 1993). The functional specialization in the network that gives rise to the double dissociation is not transparently related to the network's structure, as modular theories assume. Furthermore, a consideration of the distribution of effects across quantitatively equivalent individual lesions in the network raises specific concerns about the interpretation of single-case studies. The findings underscore the necessity of relating neuropsychological data to cognitive theories in the context of specific computational assumptions about how the cognitive system operates normally and after damage.

How to differentiate retrieval from storage deficit: a stochastic approach to semantic memory modeling

Some of the methods currently used to differentiate retrieval from storage deficits in brain damaged patients are revisited. Indices based on association or agreement measures among the responses given by the subject on repeated trials, as well as direct estimates of storage and retrieval abilities based on crude percentages, are open to criticism. A simple Markov chain stochastic approach, which is free of the drawbacks of the other methods and which distinguishes storage from retrieval deficits more reliably and powerfully, is proposed.

Neural subsystems for object knowledge

Critical issues in the cognitive neuroscience of language are whether there are multiple systems for the representation of meaning, perhaps organized by processing system (such as vision or language), and whether further subsystems are distinguishable within these larger ones. We describe here a patient (K.R.) with cerebral damage whose pattern of acquired deficits offers direct evidence for a major division between visually based and language-based higher-level representations, and for processing subsystems within language. K.R. could not name animals regardless of the type of presentation (auditory or visual), but had no difficulty naming other living things and objects. When asked to describe verbally the physical attributes of animals (for example, 'what colour is an elephant?'), she was strikingly impaired. Nevertheless, she could distinguish the correct physical attributes of animals when they were presented visually (she could distinguish animals that were correctly coloured from those that were not). Her knowledge of input stimulus. To explain this selective deficit, these data mandate the existence of two distinct representations of such properties in normal individuals, one visually based and one language-based. Furthermore, these data establish that knowledge of physical attributes is strictly segregated from knowledge of other properties in the language system.

Actors but not scripts: the dissociation of people and events in retrograde amnesia

We describe our further investigations of the retrograde amnesia in a single case. R.F.R. became globally amnesic following an attack of Herpes Simplex Encephalitis. He could generate and recognize superordinate level information about the vast majority of proper names including the names of people but he was very impaired at giving information about what had "happened" to these same individuals. He could also provide detailed information about family friends but he could not recall salient major personal episodes in which these same individuals had been involved. Knowledge of people appears to be represented in a different way to that of events, even when a singular event has provided the main or only opportunity for learning about the individual.