34,238 research outputs found
Taking both sides: do unilateral anterior temporal lobe lesions disrupt semantic memory?
No full textThe most selective disorder of central conceptual knowledge arises in semantic dementia, a degenerative condition associated
with bilateral atrophy of the inferior and polar regions of the temporal lobes. Likewise, semantic impairment in both herpes
simplex virus encephalitis and Alzheimer’s disease is typically associated with bilateral, anterior temporal pathology. These
findings suggest that conceptual representations are supported via an interconnected, bilateral, anterior temporal network
and that it may take damage to both sides to produce an unequivocal deficit of central semantic memory. We tested and
supported this hypothesis by investigating a case series of 20 patients with unilateral temporal damage (following vascular
accident or resection for tumour or epilepsy), utilizing a test battery that is sensitive to semantic impairment in semantic
dementia. Only 1/20 of the cases, with a unilateral left lesion, exhibited even a mild impairment on the receptive semantic
measures. On the expressive semantic tests of naming and fluency, average performance was worse in the left- than
right-unilateral cases, but even in this domain, only one left-lesion case had scores consistently more than two standard
deviations below control means. These results fit with recent parallel explorations of semantic function using repetitive
transcranial magnetic stimulation as well as functional imaging in stroke aphasic and neurologically intact
participants. The evidence suggests that both left and right anterior temporal lobe regions contribute to the representation of
semantic memory and together may form a relatively damage-resistant, robust system for this critical aspect of higher cognition
The relationship between phonological and morphological deficits in Broca's aphasia: further evidence from errors in verb inflection
No full textA previous study of 10 patients with Broca’s aphasia demonstrated that the advantage for producing the past tense of irregular over regular verbs exhibited by these patients was eliminated when the two sets of past-tense forms were matched for phonological complexity (Bird, Lambon Ralph, Seidenberg, McClelland, & Patterson, 2003). The interpretation given was that a generalised phonological impairment was central to the patients’ language deficits, including their poor performance on regular past tense verbs. The current paper provides further evidence in favour of this hypothesis, on the basis of a detailed analysis of the errors produced by these same 10 patients in reading, repetition, and sentence completion for a large number of regular, irregular, and nonce verbs. The patients’ predominant error types in all tasks and for all verb types were close and distant phonologically related responses. The balance between close and distant errors varied along three continua: the severity of the patient (more distant errors produced by the more severely impaired patients); the difficulty of the task (more distant errors in sentence completion > reading > repetition); the difficulty of the item (more distant errors for novel word forms than real verbs). A position analysis for these phonologically related errors revealed that vowels were most likely to be preserved and that consonant onsets and offsets were equally likely to be incorrect. Critically, the patients’ errors exhibited a strong tendency to simplify the phonological form of the target. These results are consistent with the notion that the patients’ relatively greater difficulty with regular past tenses reflects a phonological impairment that is sensitive to the complexity of spoken forms
Semantic memory is impaired in both dementia with Lewy Bodies (DLB) and dementia of Alzheimer's type (DAT): a comparative neuropsychological study and literature review
No full textOBJECTIVE---To test the hypothesis that semantic impairment is present in both patients with dementia with Lewy bodies (DLB) and those with dementia of Alzheimer's type (DAT).
METHODS---A comprehensive battery of neuropsychological tasks designed to assess semantic memory, visuoperceptual function, verbal fluency, and recognition memory was given to groups of patients with DLB (n=10), DAT (n=10) matched pairwise for age and mini mental state examination (MMSE), and age matched normal controls (n=15).
RESULTS---Both DLB and DAT groups exhibited impaired performance across the range of tasks designed to assess semantic memory. Whereas patients with DAT showed equivalent comprehension of written words and picture stimuli, patients with DLB demonstrated more severe semantic deficits for pictures than words. As in previous studies, patients with DLB but not those with DAT were found to have impaired visuoperceptual functioning. Letter and category fluency were equally reduced for the patients with DLB whereas performance on letter fluency was significantly better in the DAT group. Recognition memory for faces and words was impaired in both groups.
CONCLUSIONS---Semantic impairment is not limited to patients with DAT. Patients with DLB exhibit particular problems when required to access meaning from pictures that is most likely to arise from a combination of semantic and visuoperceptual impairments
The degraded concept representation system in semantic dementia:damage to pan-modal hub, then visual spoke
No full textThe core clinical feature of semantic dementia is a progressive yet selective degradation of conceptual knowledge. Understanding the cognitive and neuroanatomical basis for this deficit is a key challenge for both clinical and basic science. Some researchers attribute the deficit to damage to pan-modal conceptual representations that are independent of any particular sensory-motor modality and are represented in the ventrolateral anterior temporal lobes. Others claim that damage to modality-specific visual feature representations in the occipitotemporal 'ventral stream' is responsible. In the present study, we tested the hypothesis that concept degradation in semantic dementia involves a combination of these pan-modal and modality-specific elements. We investigated factors influencing knowledge of object concepts by analysing 43 sets of picture-naming data from patients with semantic dementia. We found a strong influence of two pan-modal factors: highly familiar and typical items were named more accurately than less familiar/atypical items at all stages of the disorder. Items associated with rich sensory-motor information were also named more successfully at all stages, and this effect was present for sound/motion knowledge and tactile/action knowledge when these modalities were studied separately. However, there was no advantage for items rich in visual colour/form characteristics; instead, this factor had an increasingly negative impact in the later stages of the disorder. We propose that these results are best explained by a combination of (i) degradation of modality-independent conceptual representations, which is present throughout the disorder and is a consequence of atrophy focused on the ventrolateral anterior temporal lobes; and (ii) a later additional deficit for concepts that depend heavily on visual colour/form information, caused by the spreading of atrophy to posterior ventral temporal regions specialized for representing this information. This explanation is consistent with a graded hub-and-spoke model of conceptual knowledge, in which there is a gradual convergence of information along the temporal lobes, with visual attributes represented in the posterior cortex giving way to pan-modal representations in the anterior areas.</p
Distinct patterns of olfactory impairment in Alzheimer's disease, semantic dementia, frontotemporal dementia and corticobasal degeneration
No full textPerformance on tests of odour discrimination, naming, and matching was compared in patients with four distinct forms of neurodegenerative
disease: Alzheimer’s disease (AD), semantic dementia (SD), frontotemporal dementia (FTD), and corticobasal degeneration (CBD). TheSDpatients
were found to have a severe impairment of identification from olfaction despite having normal discrimination, consistent with the multimodal
semantic impairment characteristic of this patient group. The AD patients’ poor odour discrimination suggests that a perceptual impairment is the
root of their poor odour identification. Mild impairments in odour identification observed in FTD and CBD are consistent with their generalised
executive dysfunction. The findings illustrate that breakdown in olfaction can occur at a perceptual or semantic level, analogous to the distinction
between apperceptive and associative forms of deficit in the visual and auditory modalities. The findings add further insights into the nature of
the semantic deficit in SD by exploring a hitherto neglected modality and may have relevance in explaining the altered eating habits commonly
associated with SD
Semantic memory is an a modal dynamic system: Evidence from the interaction of naming and object use in semantic dementia
No full textClassic neurological accounts and some contemporary theories of semantic memory assume that
concepts are acquired through a learning process that draws together information experienced in each of
our verbal and nonverbal modalities. These accounts embody three critical assumptions: semantic
representations are amodal; the mapping between surface form and meaning varies for different
modalities; and the representations are dynamic. The influence of these three factors was revealed in
data collected over a 4-year longitudinal period in two patients with semantic dementia. Semantic
assessment revealed a parallel decline in verbal and nonverbal aspects of conceptual knowledge,
reflecting a gradual degradation of a single amodal semantic system. As expected, when the patients’
semantic impairment was mild, they presented with profound anomia but relatively preserved object
use. Over time, performance on all semantic tasks including object use declined. High item-by-item
consistency across these tasks was observed in all testing sessions. The impact of dynamic semantic
representations was revealed by a striking clinical finding. Although unable to name many of the objects
in isolation, their performance was significantly facilitated if they were asked to name while they
demonstrated the use of each object. These results are discussed in the context of contemporary models
of semantic memory
Remembering 'zeal' but not 'thing':reverse frequency effects as a consequence of deregulated semantic processing
No full textMore efficient processing of high frequency (HF) words is a ubiquitous finding in healthy individuals, yet frequency effects are often small or absent in stroke aphasia. We propose that some patients fail to show the expected frequency effect because processing of HF words places strong demands on semantic control and regulation processes, counteracting the usual effect. This may occur because HF words appear in a wide range of linguistic contexts, each associated with distinct semantic information. This theory predicts that in extreme circumstances, patients with impaired semantic control should show an outright reversal of the normal frequency effect. To test this prediction, we tested two patients with impaired semantic control with a delayed repetition task that emphasised activation of semantic representations. By alternating HF and low frequency (LF) trials, we demonstrated a significant repetition advantage for LF words, principally because of perseverative errors in which patients produced the previous LF response in place of the HF target. These errors indicated that HF words were more weakly activated than LF words. We suggest that when presented with no contextual information, patients generate a weak and unstable pattern of semantic activation for HF words because information relating to many possible contexts and interpretations is activated. In contrast, LF words are associated with more stable patterns of activation because similar semantic information is activated whenever they are encountered.</p
Semantic diversity:A measure of contextual variation in word meaning based on latent semantic analysis
No full textSemantic ambiguity is typically measured by summing the number of senses or dictionary definitions that a word has. Such measures are somewhat subjective and may not adequately capture the full extent of variation in word meaning, particularly for polysemous words that can be used in many different ways, with subtle shifts in meaning. Here, we describe an alternative, computationally derived measure of ambiguity based on the proposal that the meanings of words vary continuously as a function of their contexts. On this view, words that appear in a wide range of contexts on diverse topics are more variable in meaning than those that appear in a restricted set of similar contexts. To quantify this variation, we performed latent semantic analysis on a large text corpus to estimate the semantic similarities of different linguistic contexts. From these estimates, we calculated the degree to which the different contexts associated with a given word vary in their meanings. We term this quantity a word's semantic diversity (SemD). We suggest that this approach provides an objective way of quantifying the subtle, context-dependent variations in word meaning that are often present in language. We demonstrate that SemD is correlated with other measures of ambiguity and contextual variability, as well as with frequency and imageability. We also show that SemD is a strong predictor of performance in semantic judgments in healthy individuals and in patients with semantic deficits, accounting for unique variance beyond that of other predictors. SemD values for over 30,000 English words are provided as supplementary materials.</p
Shapes, scents and sounds:quantifying the full multi-sensory basis of conceptual knowledge
No full textContemporary neuroscience theories assume that concepts are formed through experience in multiple sensory-motor modalities. Quantifying the contribution of each modality to different object categories is critical to understanding the structure of the conceptual system and to explaining category-specific knowledge deficits. Verbal feature listing is typically used to elicit this information but has a number of drawbacks: sensory knowledge often cannot easily be translated into verbal features and many features are experienced in multiple modalities. Here, we employed a more direct approach in which subjects rated their knowledge of objects in each sensory-motor modality separately. Compared with these ratings, feature listing over-estimated the importance of visual form and functional knowledge and under-estimated the contributions of other sensory channels. An item's sensory rating proved to be a better predictor of lexical-semantic processing speed than the number of features it possessed, suggesting that ratings better capture the overall quantity of sensory information associated with a concept. Finally, the richer, multi-modal rating data not only replicated the sensory-functional distinction between animals and non-living things but also revealed novel distinctions between different types of artefact. Hierarchical cluster analyses indicated that mechanical devices (e.g., vehicles) were distinct from other non-living objects because they had strong sound and motion characteristics, making them more similar to animals in this respect. Taken together, the ratings align with neuroscience evidence in suggesting that a number of distinct sensory processing channels make important contributions to object knowledge. Multi-modal ratings for 160 objects are provided as supplementary materials.</p
Cooperative Legacy Project oral history interview with Ralph Schreurs
No full textSouth Dakota Farmers Union Communications Director, Chuck Groth, Cooperative Legacy Project interview with Ralph Scheurs, former general manager of Split Rock Telecommunications Cooperative and South Dakota telecommunications pionee
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