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PubMed1991

The use of semantic knowledge in Alzheimer's disease: evidence for impairments of attention.

The ability to access and use semantic knowledge was examined in 25 patients with Alzheimer's disease (AD) and 31 healthy controls. The AD patients exhibited intact automatic activation of semantic relationships on a semantic priming task. They also made active use of category information on a recognition memory test. However, the patients did not use semantic information to generate expectations on the semantic priming task and were inconsistent in their use of category information on the memory test. It is suggested that attentional deficits underlie the observed impairments and may in fact help explain the patterns of performance across tasks differing widely in their overt cognitive demands.

Adult↗

PubMed2005

Linking semantic priming effect in functional MRI and event-related potentials.

The aim of this study is to examine the neural substrates involved in semantic priming using a combined event-related functional magnetic resonance imaging (fMRI) and event-related potentials (ERP) study. Twelve subjects were instructed to judge whether the presented target word was a real word or a nonword. Under the related condition, target words were preceded by a semantically related prime word. On the other hand, under the unrelated condition, prime words did not have semantic relatedness with the target word. The reaction time for reaching a judgment was longer under the unrelated condition than under the related condition, indicating that the recognition of target words is promoted by semantic priming under the related condition. In the fMRI results, we found reduced activity in the dorsal and ventral left inferior frontal gyrus, the anterior cingulate, and left superior temporal cortex for related versus unrelated conditions (i.e., the repetition suppression effect). ERP analysis revealed that the amplitude of the N400 component was reduced under the related condition compared with the unrelated condition (i.e., the N400 priming effect). Correlation analysis between the BOLD repetition suppression effect and the N400 priming effect decomposed by independent component analysis (ICA) across subjects showed significant correlation in the left superior temporal gyrus. This finding is consistent with the recent MEG data suggesting that the source of N400 is judged to be the bilateral superior temporal lobe. We discussed this finding herein in relation to the modulation of access to the phonological representation caused by semantic priming.

Adult↗

PubMed2005

Stimulus pacing affects the activation of the medial temporal lobe during a semantic classification task: an fMRI study.

Our purpose was to explore the influence of stimulus pacing in blocked functional MRI studies on the activation pattern elicited by a semantic retrieval task. Twenty-two participants performed both a fixed-paced and a self-paced functional MR imaging experiment in which a semantic categorization (animal/object) task was contrasted with a perceptual (small/capital letter string) categorization task. Group and single-subject ROI analyses were computed. In both the fixed-paced and self-paced experiments, semantic categorization contrasted with perceptual categorization elicited a cerebral network generally accepted to be involved in semantic processing comprising left inferior prefrontal, left lateral temporal, paracingular and right cerebellar areas. Our main finding was that the self-paced stimulus modality additionally yielded significant activation in the medial temporal lobe (MTL) structures including the hippocampus and the parahippocampal gyrus. More elaborative single-subject ROI analyses revealed MTL activation in 86% of the subjects for the self-paced design, but only in 21% of subjects for the fixed-paced design. The discussion focuses on possible explanations for this finding. We conclude that a self-paced as opposed to a fixed-paced semantic retrieval paradigm is able to detect significant MTL activation in groups as well as in single-subjects. This offers opportunities for the application of such a design in clinical practice.

Adult↗

PubMed2005

Declarative memory impairments in Alzheimer's disease and semantic dementia.

Semantic dementia (SD) and Alzheimer's disease (AD) are both disorders in which early pathology affects the temporal lobe yet they produce distinct syndromes of declarative memory impairment-loss of established semantic knowledge with relatively preserved episodic memory in the former and the converse in the latter. Groups with mild SD and mild AD who showed a double dissociation in these two aspects of declarative memory were studied-the SD group's episodic memory and the AD group's semantic knowledge each being comparable to controls. Positron emission tomography and volumetric magnetic resonance imaging were used to map deficits in regional cerebral metabolic rate and mesial temporal lobe (MTL) atrophy, respectively. Episodic memory impairment in AD was associated with dysfunction of an integrated network (mesial temporal lobe, mamillary bodies, dorso-mesial thalamus and posterior cingulate). Semantic memory impairment in SD was associated with bilateral rostral temporal lobe hypometabolism. The SD group had comparable MTL atrophy and hypometabolism to that found in AD but the remainder of their limbic-diencephalic network was preserved suggesting that the latter explains their ability to acquire new episodic memories. The results challenge the view that amnesia in early AD can be explained by the degree of MTL damage alone while showing that semantic impairment can occur with damage restricted to the rostral temporal lobes.

Alzheimer Disease↗

PubMed1999

Gender affects word retrieval of certain categories in semantic fluency tasks.

Recent studies suggest that gender influences phonetically-cued fluency and some semantic memory tasks. In this study we analysed the effect of demographic variables on semantic fluency tasks. The semantic categories considered were: animals, fruits, tools and vehicles. The influence of age and education was common to all the categories considered and seems a general characteristic of the semantic fluency task. Gender had a significant effect only with fruits and tools, but a diverging role: females fared better with fruits and males with tools. We discuss whether the source of the gender effect should be located at the level of the semantic representation of each category or at the level of item recall in the short time (one minute) granted for the task.

Adolescent↗

PubMed1990

Where do semantic errors come from?

We report the performance of two brain-damaged subjects (RGB and HW) whose frequent errors in spoken production are nearly always semantically related to the target word. Both subjects show similar, high rates of these "semantic" errors in oral naming and oral reading; yet neither subject makes semantic errors in comparable written tasks. Further, results of a variety of lexical tasks with the same stimuli demonstrate unimpaired comprehension of printed or spoken words, including those that are orally produced as semantic errors. These patterns of performance are interpreted as resulting from damage to the phonological output lexicon. The postulated deficit is contrasted to the hypothesis of impairment to the lexical-semantic component, required to explain performance by brain-damaged subjects described elsewhere who make seemingly identical types of oral production errors to those of RGB and HW, but, in addition, make comparable errors in writing and comprehension tasks.

Brain Damage, Chronic↗

PubMed1998

ERPs, semantic processing and age.

ERPs (N400, LPC and CNV) were elicited in two sets of subjects grouped according to age (young vs. elderly) using a word-pair category matching paradigm. Each prime consisted of a Japanese noun (constructed from two to four characters of the Hiragana) followed by one Chinese character (Kanji) as the target, this latter representing one of five semantic categories. There were two equally probable target conditions: match or mismatch. Each target was preceded by a prime, either belonging to, or not belonging to, the same semantic category. The subjects were required to respond with a specified button press to the given target according to the condition. We found RTs to be longer in the elderly subjects and under the mismatch condition. N400 amplitude was reduced in the elderly subjects under the mismatch condition and there was no difference between match and mismatch response, which were similar in amplitude to that under match condition for the young subjects. In addition, the CNV amplitudes were larger in the elderly subjects. These results suggested that functional changes in semantic processing through aging (larger semantic networks and diffuse semantic activation) were the cause of this N400 reduction, attributing a subsidiary role to attentional disturbance. We also discuss the importance of taking age-related changes into consideration in clinical studies.

Adult↗

PubMed2002

Magnetoencephalography evidence of semantic violations in auditory sentence comprehension by native speakers of Japanese.

The present study examined whether both vowel-shortening and vowel-lengthening violations would elicit a semantic field component in native speakers of Japanese, and whether such a component would also be observed in phonemic violations. Stimuli contained semantically correct and incorrect versions of two types of vowel duration (shortening and lengthening), and semantically deviant words with phonemic violations. Five hundred Japanese sentences were aurally presented to each subject, while neuromagnetic fields were recorded using a dual 37-channel gradiometer system. A prominent magnetic field component, peaking at around 400 ms after the onset of the target word, was elicited by vowel-shortening and phonemic violations in the left hemisphere only, but not vowel-lengthening violations. This semantic field component was labeled the 'SeF-M400'. The results suggest that auditory semantic mechanism is more sensitive to vowel-shortening violations than to vowel-lengthening violations.

Adult↗

PubMed2001

An ERD mapping study of the neurocognitive processes involved in the perceptual and semantic analysis of environmental sounds and words.

The aim of this paper was to investigate and compare the EEG mechanisms underlying the perceptual and semantic processes involved in environmental and language sounds perception by manipulating the degree of identification of sounds and using the ERD (event-related desynchronization) method in healthy subjects. Four types of stimuli were analyzed: meaningful environmental sounds, meaningless sounds, words and non-words. We report many similarities in the ERDs and ERSs (event-related synchronizations) patterns among all stimuli, with: (i) similar time-course of ERDs and ERSs between meaningful environmental sounds and words, and between meaningless sounds and non-words; (ii) similar topography of the maximal ERDs for meaningful environmental sounds, words and non-words; and (iii) same right posterior ERSs for all four stimuli. However, differences were also observed: (i) in time-course, with earlier ERSs for meaningless than meaningful stimuli, whether environmental or verbal; and (ii) in topography, with ERDs predominating in left and right hemisphere channels for meaningful and meaningless environmental sounds, respectively; (iii) ERSs predominating in the left temporal channel for non-words and in the left posterior and right frontal channels for meaningless sounds. The results of this study suggest that meaningful stimuli involve greater and longer-lasting semantic processes than meaningless stimuli, while the occurrence of ERSs for the latter points to the possible involvement of an inhibitory processing of semantic representations. Finally, the findings concerning the comparison between verbal and non verbal stimuli suggest the involvement of left-lateralized phonological and semantic processes for the former, and more distributed neurocognitive processes in both hemispheres for the latter although with predominant left laterality for their semantic processing.

Adult↗

PubMed1997

Event-related desynchronization in the alpha band and the processing of semantic information.

The hypothesis was tested whether event-related power shifts in the upper alpha band are specifically related to semantic memory processes. In Expt. 1 subjects had to judge whether pairs of sequentially presented words (W1-W2) were semantically congruent. In the following experiments subjects were presented the W1 words of Expt. 1 and were asked to perform a free association task in Expt. 2 and a cued recall task in Expt. 3. It is assumed that semantic memory demands dominate in Expt. 1, whereas working memory demands dominate in Expt. 3 and that Expt. 2 takes an intermediate position with respect to both types of task demands. A significant task-related power change that responds selectively to semantic processing demands was found for the upper alpha band and over the left side of the scalp. The lower alpha band, on the other hand, most likely reflects unspecific processing demands such as attention. A more general interpretation of these findings is that different cognitive processes such as semantic memory, perceptual encoding and attentional processes are reflected by band power changes in different and rather narrow frequency bands over localized regions in the brain.

Adult↗

PubMed1998

A spatio-temporal comparison of semantic and episodic cued recall and recognition using event-related brain potentials.

The event-related brain potential (ERP) was used to spatially and temporally map the brain areas active as a function of type of recall (semantic vs. episodic) and episodic retrieval mode (recall vs. recognition) while difficulty of episodic recall was manipulated. ERPs were recorded from 32 scalp sites in 12 subjects, along with behavioral accuracy and recall speed. The results revealed that different but overlapping patterns of ERP activity were elicited during semantic and episodic recall. Recall of both types of information was characterized by ERP activity over left inferior frontal, central, bilateral temporal and posterior inferior brain areas. Compared to semantic recall, episodic recall elicited more activity over the frontal poles and right frontal scalp. Different but overlapping patterns of ERP activity were also found as a function of episodic retrieval mode. While episodic recall and recognition showed similar activity over the frontal poles and central scalp, there was no left inferior frontal activity elicited during recognition and no large, topographically widespread, late positive component (LPC) elicited when the same words were recalled. Manipulation of episodic recall difficulty and analysis of trials when recall failed indicated that these task (i.e., episodic vs. semantic) and retrieval mode (recall vs. recognition) differences in ERP activity were not likely to be due to differences in task difficulty. The results are discussed in terms of processes that the ERP activity may reflect and their similarity to results of PET studies of semantic and episodic retrieval.

Adult↗

PubMed2000

A comparison of rCBF patterns during letter and semantic fluency.

To evaluate the functional neuroanatomies underlying letter and category fluency, 18 normal controls were studied with oxygen-15 water regional cerebral blood flow positron emission tomography. Three counterbalanced conditions each consisted of 6 trials (45 s each): letter fluency (generating words when cued with a particular letter), semantic fluency (generating words when cued with a particular category), and a control condition (generating days of the week and months of the year). Relative to the control, participants activated similar brain regions during both fluency tasks, including the anterior cingulate, left prefrontal regions, thalamus, and cerebellum; reductions were found in parietal and temporal regions. In a direct comparison of the 2 fluency tasks, inferior frontal cortex and temporoparietal cortex (hypothesized to participate in a phonologic loop for accessing word pronunciation) were activated more during letter than semantic fluency, whereas left temporal cortex (associated with access to semantic storage) was activated more during semantic than letter fluency. This study identifies subtle differences in the neural networks underlying letter and semantic fluency that may underlie the dissociation of these abilities in patients.

Adult↗

PubMedDate not supplied

Brain potentials during reading reflect word expectancy and semantic association.

The neuroelectric activity of the human brain that accompanies linguistic processing can be studied through recordings of event-related potentials (e.r.p. components) from the scalp. The e.r.ps triggered by verbal stimuli have been related to several different aspects of language processing. For example, the N400 component, peaking around 400 ms post-stimulus, appears to be a sensitive indicator of the semantic relationship between a word and the context in which it occurs. Words that complete sentences in a nonsensical fashion elicit much larger N400 waves than do semantically appropriate words or non-semantic irregularities in a text. In the present study, e.r.ps were recorded in response to words that completed meaningful sentences. The amplitude of the N400 component of the e.r.p. was found to be an inverse function of the subject's expectancy for the terminal word as measured by its 'Cloze probability'. In addition, unexpected words that were semantically related to highly expected words elicited lower N400 amplitudes. These findings suggest N400 may reflect processes of semantic priming or activation.

Brain↗

PubMed2001

Generalized dissociative amnesia: episodic, semantic and procedural memories lost and found.

OBJECTIVE: This review tests Ribot's classic twofold categorization of generalized amnesia (GA) into Type I, total loss of episodic memory, and Type II, additional more or less extensive loss of semantic and/or procedural memory. It also explores his law of regression, according to which, cast in modern terms, recovery of lost procedural and semantic memories precedes recovery of episodic memory, as well as reported aetiological factors. METHOD: Clinically and formally assessed cases of GA, published since 1845, were surveyed and further analysed. RESULTS: Over and above authentic episodic memory loss, cases differed widely in the extent of impairment of semantic and procedural memory. Recovery of semantic and procedural memory often preceded recovery of episodic memory. This particularly applied to authenticated trauma memories. To an extent, lost memories affected current functioning, and in some cases were associated with alternating dissociative personalities. Severe memory distortions upon memory recovery were not reported. Most cases were trauma or stress related, while in some cases the aetiology remained unknown. CONCLUSIONS: Contrary to the view expressed in DSM-IV, which states that dissociative amnesia pertains to an inability to recall personal information, GA may also involve loss and recovery of semantic and procedural memories. Since the loss of various memory types in GA is dimensional rather than categorical, Ribot's typological distinction does not hold. Some of the reviewed cases suggest a trauma-related aetiology. Generalized amnesia of varying degrees of severity can involve delayed retrieval of trauma memories, as well as the loss and delayed retrieval of the premorbid personality.

Amnesia↗

PubMed2006

Dissociated developmental trajectories for semantic and phonological false memories.

False recognition following presentation of semantically related and phonologically related word lists was evaluated in 8-, 11-, and 13-year-olds. Children heard lists of words that were either semantic (e.g., bed, rest, wake ...) or phonological associates (e.g., pole, bowl, hole ...) of a critical unpresented word (e.g., sleep, roll), respectively. A semantic false memory was defined as false recognition of a semantically related but unpresented word. A phonological false memory was defined as false recognition of a phonologically related but unpresented word. False memories in the two tasks showed opposite developmental trends, increasing with age for semantic relatedness and decreasing with age for phonological relatedness.

Adolescent↗

PubMed2005

What happens to semantic memory when formal thought disorder remits? Revisiting a case study.

INTRODUCTION: Laws, Kondel, and McKenna (1999) previously reported a case study analysis of a schizophrenic patient (TC) with severe formal thought disorder (FTD). Examining consistency across item and modality of input, Laws et al. documented an impairment of access to semantic knowledge in TC. METHOD: Following substantial improvement in his FTD, we readministered the same extensive battery of neuropsychological tests tapping semantic memory functioning. RESULTS: Whilst TC's naming remained relatively good, it also became more consistent across both time and modality. Tasks tapping language comprehension and understanding of semantic association revealed some significant improvements. Nevertheless, TC showed a residual propensity to verify false information. CONCLUSION: Improvement in FTD in schizophrenia was accompanied by a better and more stable semantic memory performance in TC. The findings are consistent with, and expand upon the original suggestion that thought disorder reflects disorganised access to semantic memory.

Journal Article↗

PubMed2006

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.

Aged↗

PubMed1995

Distributed representations of semantic knowledge in the brain.

Category-specific language impairments have been postulated to require the existence of an explicit category organization within semantic memory. However, it may be possible to demonstrate analytically that this is not necessary. We hypothesize that category-specific organization can emerge from perceptual, functional, and associative feature information about objects that is maintained in order to process language. In this paper, we conduct several experiments to test the computational validity of this hypothesis. Physical objects were encoded in terms of semantic features, based on basic perceptual and motor modalities and higher level knowledge of function, for use in artificial neural networks. Mathematical methods were used to analyse the encodings and the neural networks. The results demonstrate the emergence of semantic categories in the networks, although such information was not preprogrammed. We conclude that category-specific language organization can emerge from the inherent nature of semantic features themselves, and does not require special internal categorical organization of semantic memory.

Brain↗