From long-term to short-term: Distinct neural networks underlying semantic knowledge and its recruitment in working memory

Although numerous studies suggest that working memory (WM) and semantic long-term knowledge interact, the nature and underlying neural mechanisms of this intervention remain poorly understood. Using functional magnetic resonance imaging (fMRI), this study investigated the extent to which neural markers of semantic knowledge in long-term memory (LTM) are activated during the WM maintenance stage in 32 young adults. First, the multivariate neural patterns associated with four semantic categories were determined via an implicit semantic activation task. Next, the participants maintained words - the names of the four semantic categories implicitly activated in the first task - in a verbal WM task. Multi-voxel pattern analyses showed reliable neural decoding of the four semantic categories in the implicit semantic activation and the verbal WM tasks. Critically, however, no between-task classification of semantic categories was observed. Searchlight analyses showed that for the WM task, semantic category information could be decoded in anterior temporal areas associated with abstract semantic category knowledge. In the implicit semantic activation task, semantic category information was decoded in superior temporal, occipital and frontal cortices associated with domain-specific semantic feature representations. These results indicate that item-level semantic activation during verbal WM involves shallow rather than deep semantic information.

Clinical and structural disconnectome evaluation in a case of optic aphasia

Optic Aphasia (OA) and Associative Visual Agnosia (AVA) are neuropsychological disorders characterized by impaired naming on visual presentation. From a cognitive point of view, while stimulus identification is largely unimpaired in OA (where access to semantic knowledge is still possible), in AVA it is not. OA has been linked with right hemianopia and disconnection of the occipital right-hemisphere (RH) visual processing from the left hemisphere (LH) language areas.In this paper, we describe the case of AA, an 81-year-old housewife suffering from a deficit in naming visually presented stimuli after left occipital lesion and damage to the interhemispheric splenial pathway. AA has been tested through a set of tasks assessing different levels of visual object processing. We discuss behavioral performance as well as the pattern of lesion and disconnection in relation to a neurocognitive model adapted from Luzzatti and colleagues (1998). Despite the complexity of the neuropsychological picture, behavioral data suggest that semantic access from visual input is possible, while a lesion-based structural disconnectome investigation demonstrated the splenial involvement.Altogether, neuropsychological and neuroanatomical findings support the assumption of visuo-verbal callosal disconnection compatible with a diagnosis of OA.

The relationship between semantics, phonology, and naming performance in aphasia: a structural equation modeling approach

The exploration of naming error patterns in aphasia provides insights into the cognitive processes underlying naming performance. We investigated how semantic and phonological abilities correlate and how they influence naming performance in aphasia. Data from 296 individuals with aphasia, drawn from the Moss Aphasia Psycholinguistics Project Database, were analyzed using a structural equation model. The model incorporated latent variables for semantics and phonology and manifest variables for naming accuracy and error patterns. There was a moderate positive correlation between semantics and phonology after controlling for overall aphasia severity. Both semantic and phonological abilities influenced naming accuracy. Semantic abilities negatively related to semantic, mixed, unrelated errors, and no responses. Interestingly, phonology positively affected semantic errors. Additionally, phonological abilities negatively related to each of phonological and neologism errors. These results highlight the role of semantic and phonological skills on naming performance in aphasia and reveal a relationship between these cognitive processes.

A Standardized Set of 380 Pictures for Lebanese Arabic: Norms for Name Agreement, Conceptual Familiarity, Imageability, and Subjective Frequency

Research on language processing requires language-specific norms of pictorial and linguistic experimental stimuli across different psycholinguistic variables. Such normative data have not yet been collected for Lebanese Arabic (LA), an Arabic dialect. Arabic languages are characterized by diglossia: while modern standard Arabic is their common means of formal communication, Arabic dialects are the medium of oral communication within each community. This claims for specific dialectal norms. Thus, the main goal of the present study was to collect normative LA data for 380 pictures taken from Cykowicz, Friedman, Rothstein, and Snodgrass (The Quarterly Journal of Experimental Psychology Section A 50(3):560-585; including the 260 pictures of Snodgrass & Vanderwart in Journal of experimental psychology: Human learning and memory 6(2):174-215, 1980) using a sample of 248 native LA speakers. Norms are reported for name agreement, conceptual familiarity, imageability and subjective frequency, together with word length in number of letters and syllables. We compared the obtained norms with the normative data of other Arabic dialects (Levantine, Tunisian and Gulf Arabic) and with English, French and Spanish. Results showed the distinction of LA from the other Arabic dialects. This provides support of specific dialectal Arabic norms and will allow researchers to rigorously select the stimuli to investigate language processing in LA-speaking populations.

Body-object interaction effect in word recognition and its relationship with screen time in Chinese children

A sample of 144 s- and 150 fourth-grade Chinese children was recruited to investigate the influence of body-object interactions (BOIs) on word recognition, i.e., how easily they could interact physically with each word's referent. The moderation on this relationship of children's screen time for entertainment purposes (i.e., the viewing or use of any device with a screen) was also examined. In a lexical decision task, the children were asked to judge whether each item was a real Chinese word. Each real word was assigned a BOI rating score. Model analysis showed that the BOI rating was a significant predictor of the children's word recognition performances. The children recognized the words with higher BOI ratings at higher accuracy rates and higher response speeds more than the words with lower BOI ratings, showing a BOI effect. These results suggest an involvement of sensorimotor information in processing concepts. As well, the results showed a moderating effect of screen time on the BOI effect. With the increase of screen time, the BOI effect was reduced in terms of response speed. Moreover, the influence of the screen time on the BOI effect was larger in the second graders than in the fourth graders.

Evidence for a deep, distributed and dynamic code for animacy in human ventral anterior temporal cortex

How does the human brain encode semantic information about objects? This paper reconciles two seemingly contradictory views. The first proposes that local neural populations independently encode semantic features; the second, that semantic representations arise as a dynamic distributed code that changes radically with stimulus processing. Combining simulations with a well-known neural network model of semantic memory, multivariate pattern classification, and human electrocorticography, we find that both views are partially correct: information about the animacy of a depicted stimulus is distributed across ventral temporal cortex in a dynamic code possessing feature-like elements posteriorly but with elements that change rapidly and nonlinearly in anterior regions. This pattern is consistent with the view that anterior temporal lobes serve as a deep cross-modal ‘hub’ in an interactive semantic network, and more generally suggests that tertiary association cortices may adopt dynamic distributed codes difficult to detect with common brain imaging methods.

On Simulating Neural Damage in Connectionist Networks

A key strength of connectionist modelling is its ability to simulate both intact cognition and the behavioural effects of neural damage. We survey the literature, showing that models have been damaged in a variety of ways, e.g. by removing connections, by adding noise to connection weights, by scaling weights, by removing units and by adding noise to unit activations. While these different implementations of damage have often been assumed to be behaviourally equivalent, some theorists have made aetiological claims that rest on nonequivalence. They suggest that related deficits with different aetiologies might be accounted for by different forms of damage within a single model. We present two case studies that explore the effects of different forms of damage in two influential connectionist models, each of which has been applied to explain neuropsychological deficits. Our results indicate that the effect of simulated damage can indeed be sensitive to the way in which damage is implemented, particularly when the environment comprises subsets of items that differ in their statistical properties, but such effects are sensitive to relatively subtle aspects of the model's training environment. We argue that, as a consequence, substantial methodological care is required if aetiological claims about simulated neural damage are to be justified, and conclude more generally that implementation assumptions, including those concerning simulated damage, must be fully explored when evaluating models of neurological deficits, both to avoid over-extending the explanatory power of specific implementations and to ensure that reported results are replicable.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (10.1007/s42113-020-00081-z) contains supplementary material, which is available to authorized users.

Neural Population Dynamics and Cognitive Function

Representations in the brain are encoded as patterns of activity of large populations of neurons. The science of population encoded representations, also known as parallel distributed processing (PDP), achieves neurological verisimilitude and has been able to account for a large number of cognitive phenomena in normal people, including reaction times (and reading latencies), stimulus recognition, the effect of stimulus salience on attention, perceptual invariance, simultaneous egocentric and allocentric visual processing, top-down/bottom-up processing, language errors, the effect of statistical regularities of experience, frequency, and age of acquisition, instantiation of rules and symbols, content addressable memory and the capacity for pattern completion, preservation of function in the face of noisy or distorted input, inference, parallel constraint satisfaction, the binding problem and gamma coherence, principles of hippocampal function, the location of knowledge in the brain, limitations in the scope and depth of knowledge acquired through experience, and Piagetian stages of cognitive development. PDP studies have been able to provide a coherent account for impairment in a variety of language functions resulting from stroke or dementia in a large number of languages and the phenomenon of graceful degradation observed in such studies. They have also made important contributions to our understanding of attention (including hemispatial neglect), emotional function, executive function, motor planning, visual processing, decision making, and neuroeconomics. The relationship of neural network population dynamics to electroencephalographic rhythms is starting to emerge. Nevertheless, PDP approaches have scarcely penetrated major areas of study of cognition, including neuropsychology and cognitive neuropsychology, as well as much of cognitive psychology. This article attempts to provide an overview of PDP principles and applications that addresses a broader audience.

Modeling early lexico-semantic network development: Perceptual features matter most

What aspects of word meaning are important in early word learning and lexico-semantic network development? Adult lexico-semantic systems flexibly encode multiple types of semantic features, including functional, perceptual, taxonomic, and encyclopedic. However, various theoretical accounts of lexical development differ on whether and how these semantic properties of word meanings are initially encoded into young children's emerging lexico-semantic networks. Whereas some accounts highlight the importance of early perceptual versus conceptual properties, others posit that thematic or functional aspects of word meaning are primary relative to taxonomic knowledge. We seek to shed light on these debates with 2 modeling studies that explore patterns in early word learning using a large database of early vocabulary in 5,450 children, and a newly developed set of semantic features of early acquired nouns. In Study 1, we ask whether semantic properties of early acquired words relate to order in which these words are typically learned; Study 2 models normative lexico-semantic noun-feature network development compared to random network growth. Both studies provide converging evidence that perceptual properties of word meanings play a key role in early word learning and lexico-semantic network development. The findings lend support to theoretical accounts of language learning that highlight the importance of the child's perceptual experience. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Incremental learning of perceptual and conceptual representations and the puzzle of neural repetition suppression

Incremental learning models of long-term perceptual and conceptual knowledge hold that neural representations are gradually acquired over many individual experiences via Hebbian-like activity-dependent synaptic plasticity across cortical connections of the brain. In such models, variation in task relevance of information, anatomic constraints, and the statistics of sensory inputs and motor outputs lead to qualitative alterations in the nature of representations that are acquired. Here, the proposal that behavioral repetition priming and neural repetition suppression effects are empirical markers of incremental learning in the cortex is discussed, and research results that both support and challenge this position are reviewed. Discussion is focused on a recent fMRI-adaptation study from our laboratory that shows decoupling of experience-dependent changes in neural tuning, priming, and repetition suppression, with representational changes that appear to work counter to the explicit task demands. Finally, critical experiments that may help to clarify and resolve current challenges are outlined.

Cohesive and coherent connected speech deficits in mild stroke

Spoken language production theories and lesion studies highlight several important prelinguistic conceptual preparation processes involved in the production of cohesive and coherent connected speech. Cohesion and coherence broadly connect sentences with preceding ideas and the overall topic. Broader cognitive mechanisms may mediate these processes. This study aims to investigate (1) whether stroke patients without aphasia exhibit impairments in cohesion and coherence in connected speech, and (2) the role of attention and executive functions in the production of connected speech. Eighteen stroke patients (8 right hemisphere stroke [RHS]; 6 left [LHS]) and 21 healthy controls completed two self-generated narrative tasks to elicit connected speech. A multi-level analysis of within and between-sentence processing ability was conducted. Cohesion and coherence impairments were found in the stroke group, particularly RHS patients, relative to controls. In the whole stroke group, better performance on the Hayling Test of executive function, which taps verbal initiation/suppression, was related to fewer propositional repetitions and global coherence errors. Better performance on attention tasks was related to fewer propositional repetitions, and decreased global coherence errors. In the RHS group, aspects of cohesive and coherent speech were associated with better performance on attention tasks. Better Hayling Test scores were related to more cohesive and coherent speech in RHS patients, and more coherent speech in LHS patients. Thus, we documented connected speech deficits in a heterogeneous stroke group without prominent aphasia. Our results suggest that broader cognitive processes may play a role in producing connected speech at the early conceptual preparation stage.

Object identification leads to a conceptual broadening of object representations in lateral prefrontal cortex

Recent experience identifying objects leads to later improvements in both speed and accuracy ("repetition priming"), along with simultaneous reductions of neural activity ("repetition suppression"). A popular interpretation of these joint behavioral and neural phenomena is that object representations become perceptually "sharper" with stimulus repetition, eliminating cells that are poorly stimulus-selective and responsive and reducing support for competing representations downstream. Here, we test this hypothesis in an fMRI-adaptation experiment using pictures of objects. Prior to fMRI, participants repeatedly named a set of object pictures. During fMRI, participants viewed adaptation sequences composed of rapidly repeated objects (3-6 repetitions over several seconds) that were either named previously or that were new for the fMRI session, followed by single "deviant" object pictures used to measure recovery from adaptation and that shared a relationship to the adapted picture (a different exemplar of the same object, a conceptual associate, or an unrelated picture). Effects of adaptation and recovery were found throughout visually responsive brain regions. Occipitotemporal cortical regions displayed repetition suppression to previously named relative to new adapters but failed to exhibit pronounced changes in neural tuning. In contrast, changes in the slope of the recovery curves were found in the left lateral prefrontal cortex: Greater residual adaptation was observed to exemplar stimuli and conceptual associates following previously named adapting stimuli, consistent with greater rather than reduced neural overlap among representations of conceptually related objects. Furthermore, this change in neural tuning was directly related to the proportion of conceptual errors made by participants in the naming sessions pre- and post-fMRI, establishing that the experience-dependent conceptual broadening of object representations seen in fMRI is also manifest in behavior. In a follow-up behavioral experiment, we further show that recent naming experience leads to greater semantic priming when using the previously named pictures as briefly presented primes. Taken together, our results fail to support perceptual sharpening as the primary mediator between repetition suppression and behavioral priming at durations typically used to study priming and instead highlight an experience-dependent broadening of conceptual representations. We suggest that alternative mechanisms, such as increases in neural synchronization, are more promising in explaining priming in the face of repetition suppression.

A normative database and determinants of lexical retrieval for 186 Arabic nouns: effects of psycholinguistic and morpho-syntactic variables on naming latency

Research into lexical retrieval requires pictorial stimuli standardised for key psycholinguistic variables. Such databases exist in a number of languages but not in Arabic. In addition there are few studies of the effects of psycholinguistic and morpho-syntactic variables on Arabic lexical retrieval. The current study identified a set of culturally and linguistically appropriate concept labels, and corresponding photographic representations for Levantine Arabic. The set included masculine and feminine nouns, nouns from both types of plural formation (sound and broken), and both rational and irrational nouns. Levantine Arabic speakers provided norms for visual complexity, imageability, age of acquisition, naming latency and name agreement. This delivered a normative database for a set of 186 Arabic nouns. The effects of the morpho-syntactic and the psycholinguistic variables on lexical retrieval were explored using the database. Imageability and age of acquisition were the only significant determinants of successful lexical retrieval in Arabic. None of the other variables, including all the linguistic variables, had any effect on production time. The normative database is available for the use of clinicians and researchers in the Arab world in the domains of speech and language pathology, neurolinguistics and psycholinguistics. The database and the photographic representations will be soon available for free download from the first author's personal webpage or via email.

Spelling-to-sound correspondences affect acronym recognition processes

A large body of research has examined the factors that affect the speed with which words are recognized in lexical decision tasks. Nothing has yet been reported concerning the important factors in differentiating acronyms (e.g., BBC, HIV, NASA) from nonwords. It appears that this task poses little problem for skilled readers, in spite of the fact that acronyms have uncommon, even illegal, spellings in English. We used regression techniques to examine the role of a number of lexical and nonlexical variables known to be important in word processing in relation to lexical decision for acronym targets. Findings indicated that acronym recognition is affected by age of acquisition and imageability. In a departure from findings in word recognition, acronym recognition was not affected by frequency. Lexical decision responses for acronyms were also affected by the relationship between spelling and sound-a pattern not usually observed in word recognition. We argue that the complexity of acronym recognition means that the process draws phonological information in addition to semantics.

What lies beneath: a comparison of reading aloud in pure alexia and semantic dementia

Exaggerated effects of word length upon reading-aloud performance define pure alexia, but have also been observed in semantic dementia. Some researchers have proposed a reading-specific account, whereby performance in these two disorders reflects the same cause: impaired orthographic processing. In contrast, according to the primary systems view of acquired reading disorders, pure alexia results from a basic visual processing deficit, whereas degraded semantic knowledge undermines reading performance in semantic dementia. To explore the source of reading deficits in these two disorders, we compared the reading performance of 10 pure alexic and 10 semantic dementia patients, matched in terms of overall severity of reading deficit. The results revealed comparable frequency effects on reading accuracy, but weaker effects of regularity in pure alexia than in semantic dementia. Analysis of error types revealed a higher rate of letter-based errors and a lower rate of regularization responses in pure alexia than in semantic dementia. Error responses were most often words in pure alexia but most often nonwords in semantic dementia. Although all patients made some letter substitution errors, these were characterized by visual similarity in pure alexia and phonological similarity in semantic dementia. Overall, the data indicate that the reading deficits in pure alexia and semantic dementia arise from impairments of visual processing and knowledge of word meaning, respectively. The locus and mechanisms of these impairments are placed within the context of current connectionist models of reading.

Single Case Studies as Seeds

Single case studies are research studies of single participants. They explore new ideas and can suggest extensions in methods and for treatment (Yin, 1984). In this article the case study refers only to conditions observed and is limited to what was observed from these comparisons between normal function and patterns of specific conditions. This does not reflect the theory of a formal case study but rather it is an attempt to show the case study and simple computer modelling as learning tools in a complex environment. Critics of case study methodology cite small case numbers as not having grounds for establishing reliability or generalization of findings and that the intensity of exposure needed for thorough case studies could bias the research findings subtracting from researcher objectivity. Early computer models were used to simulate the function of the brain and provide partial answers. They provided insight into the understanding of complex function. In recent history, computer models and case studies have been cast aside in favor of live brain imaging and complex biochemical reactions. It is good to remember that these tools brought us to the place of knowledge we enjoy today and have enlarged diagnostic and treatment choices. They are still valuable and inexpensive methods that can impact the imaginations of neuroscientists and kindle their passions to solve the complexities of the human mind one problem at a time. Greater rigor can be easily maintained by adopting a format whereby a patient would be assessed by a fully standardized neuropsychological battery and the performance then compared to large sample of normative data. The formal study is important for generalization of findings across conditions and can be applied once normative data has been collected as a basis for comparison.

Won't get fooled again: An event-related potential study of task and repetition effects on the semantic processing of items without semantics

A growing body of evidence suggests that semantic access is obligatory. Several studies have demonstrated that brain activity associated with semantic processing, measured in the N400 component of the event-related brain potential (ERP), is elicited even by meaningless, orthographically illegal strings, suggesting that semantic access is not gated by lexicality. However, the downstream consequences of that activity vary by item type, exemplified by the typical finding that N400 activity is reduced by repetition for words and pronounceable nonwords but not for illegal strings. We propose that this lack of repetition effect for illegal strings is caused not by lack of contact with semantics, but by the unrefined nature of that contact under conditions in which illegal strings can be readily categorised as task-irrelevant. To test this, we collected ERPs from participants performing a modified Lexical Decision Task, in which the presence of orthographically illegal acronyms rendered meaningless illegal strings more difficult lures than normal. Confirming our hypothesis, under these conditions illegal strings elicited robust N400 repetition effects, quantitatively and qualitatively similar to those elicited by words, pseudowords, and acronyms.

Is recurrent perseveration a product of deafferented functional systems with otherwise normal post-activation decay rates?

Recent work in neuropsychology, clinical aphasiology and neuropharmacology have presented evidence that the causative substrates of recurrent perseveration in adults with aphasia are more recondite and subject to distinct interpretations than originally thought. This article will discuss and evaluate how various proposals from theory, from the clinic and from drug therapy interact and compete in the search for a cause or causes of recurrent perseveration.

Refractoriness and the healthy brain: a behavioural study on semantic access

While many behavioural studies on refractory phenomena in lexical/semantic access have focused on the mechanisms involved in the oral production of names, comprehension tasks have been almost exclusively used in neuropsychological studies on brain damaged patients. We report the results of two experiments on healthy participants conducted by means of speeded word to picture matching tasks. They assess the effects of the same variables examined in the study of refractory access dysphasic patients: semantic distance and word frequency (experiment 1) and presentation rate and serial position effects (experiment 2). Semantic access patients usually show little effect of word frequency but a large semantic distance effect. However, critical in characterising the syndrome as 'refractory', effects of presentation rate and serial position should also be present. The experiments involved the use of a deadline response procedure. The critical manipulation was the absence of a Response Stimulus Interval (RSI) in the fast presentation rate conditions; slower presentation rates involved 1 s RSI. With these manipulations the typical behavioural pattern of performance provided by semantic access dysphasic patients was reproduced. Semantic distance effects were more powerful than word frequency effects (experiment1). Presentation rate effects were found and, most important for a "refractory" account of the effects, a serial position effect was obtained (experiment 2). These results provide the first evidence of such a broad range of refractory effects at the same time in comprehension tasks in healthy subjects and support a purely semantic account for the locus of refractoriness. Moreover, error analysis showed a predominance of perseverative errors with subsequent representations of the same target, supporting a failure of cognitive control mechanisms as the cause of refractory behaviour. The findings are discussed in the light of current models of lexical and semantic processing.

When does less yield more? The impact of severity upon implicit recognition in pure alexia

Pure alexia (PA) is characterised by strong effects of word length on reading times and is sometimes accompanied by an overt letter-by-letter (LBL) reading strategy. Past studies have reported "implicit recognition" in some individual PA patients. This is a striking finding because such patients are able to perform semantic classification and lexical decision at above chance levels even when the exposure duration is short enough to prevent explicit identification. In an attempt to determine the prevalence of this "implicit recognition" effect, we assessed semantic categorisation and lexical decision performance using limited exposure durations in 10 PA cases. The majority of the patients showed above chance accuracy in semantic categorisation and lexical decision. Performance on the lexical decision test was influenced by frequency and imageability. In addition, we found that the extent to which patients showed evidence of "implicit recognition" in both tasks was inversely related to the severity of their reading disorder. This result is consistent with hypotheses which suggest that this effect does not constitute an implicit form of unique word identification but is a reflection of the degree of partial activation within the word recognition system. These results also go some way towards explaining the individual variation in the presence of this effect observed across previous case-study investigations in the literature.

Is the 'naming' deficit in dyslexia a misnomer?

We report a study that investigated the widely held belief that naming-speed deficits in developmental dyslexia reflect impaired access to lexical-phonological codes. To investigate this issue, we compared adult dyslexic and adult non-dyslexic readers' performance when naming and semantically categorizing arrays of objects. Dyslexic readers yielded slower response latencies than non-dyslexic readers when naming objects, but a subsequent comparison of object-naming and object-categorization tasks showed that the apparent 'naming' deficit could be attributed to a more general difficulty in retrieving information - either phonological or semantic - from the visual stimulus. Our findings suggest that although visual-phonological connections may be crucial in explaining naming-speed performance they do not fully characterise dyslexic readers' naming-speed impairments.

Neuropsychological inference with an interactive brain: A critique of the “locality” assumption

When cognitive neuropsychologists make inferences about the functional architecture of the normal mind from selective cognitive impairments they generally assume that the effects of brain damage are local, that is, that the nondamaged components of the architecture continue to function as they did before the damage. This assumption follows from the view that the components of the functional architecture are modular, in the sense of being informationally encapsulated. In this target article it is argued that this “locality” assumption is probably not correct in general. Inferences about the functional architecture can nevertheless be made from neuropsychological data with an alternative set of assumptions, according to which human information processing is graded, distributed, and interactive. These claims are supported by three examples of neuropsychological dissociations and a comparison of the inferences obtained from these impairments with and without the locality assumption. The three dissociations are: selective impairments in knowledge of living things, disengagment of visual attention, and overt face recognition. In all three cases, the neuropsychological phenomena lead to more plausible inferences about the normal functional architecture when the locality assumption is abandoned. Also discussed are the relations between the locality assumption in neuropsychology and broader issues, including Fodor's modularity hypothesis and the choice between top-down and bottom-up research approaches.

Neural regions essential for distinct cognitive processes underlying picture naming

We hypothesized that distinct cognitive processes underlying oral and written picture naming depend on intact function of different, but overlapping, regions of the left hemisphere cortex, such that the distribution of tissue dysfunction in various areas can predict the component of the naming process that is disrupted. To test this hypothesis, we evaluated 116 individuals within 24 h of acute ischaemic stroke using a battery of oral and written naming and other lexical tests, and with magnetic resonance diffusion and perfusion imaging to identify the areas of tissue dysfunction. Discriminant function analysis, using the degree of hypoperfusion in various Brodmann's areas--BA 22 (including Wernicke's area), BA 44 (part of Broca's area), BA 45 (part of Broca's area), BA 21 (inferior temporal cortex), BA 37 (posterior, inferior temporal/fusiform gyrus), BA 38 (anterior temporal cortex) and BA 39 (angular gyrus)--as discriminant variables, classified patients on the basis of the primary component of the naming process that was impaired (defined as visual, semantics, modality-independent lexical access, phonological word form, orthographic word form and motor speech by the pattern of performance and types of errors across lexical tasks). Additionally, linear regression analysis demonstrated that the areas contributing the most information to the identification of patients with particular levels of impairment in the naming process were largely consistent with evidence for the roles of these regions from functional imaging. This study provides evidence that the level of impairment in the naming process reflects the distribution of tissue dysfunction in particular regions of the left anterior, inferior and posterior middle/superior temporal cortex, posterior inferior frontal and inferior parietal cortex. While occipital cortex is also critical for picture naming, it is likely that bilateral occipital damage is necessary to disrupt visual recognition. These findings provide new evidence that a network of brain regions supports naming, but separate components of this network are differentially required for distinct cognitive processes or representations underlying the complex task of naming pictures.

Simulating single word processing in the classic aphasia syndromes based on the Wernicke-Lichtheim-Geschwind theory

The Wernicke-Lichtheim-Geschwind (WLG) theory of the neurobiological basis of language is of great historical importance, and it continues to exert a substantial influence on most contemporary theories of language in spite of its widely recognized limitations. Here, we suggest that neurobiologically grounded computational models based on the WLG theory can provide a deeper understanding of which of its features are plausible and where the theory fails. As a first step in this direction, we created a model of the interconnected left and right neocortical areas that are most relevant to the WLG theory, and used it to study visual-confrontation naming, auditory repetition, and auditory comprehension performance. No specific functionality is assigned a priori to model cortical regions, other than that implicitly present due to their locations in the cortical network and a higher learning rate in left hemisphere regions. Following learning, the model successfully simulates confrontation naming and word repetition, and acquires a unique internal representation in parietal regions for each named object. Simulated lesions to the language-dominant cortical regions produce patterns of single word processing impairment reminiscent of those postulated historically in the classic aphasia syndromes. These results indicate that WLG theory, instantiated as a simple interconnected network of model neocortical regions familiar to any neuropsychologist/neurologist, captures several fundamental "low-level" aspects of neurobiological word processing and their impairment in aphasia.

Lexical competition effects in aphasia: deactivation of lexical candidates in spoken word processing

Research has shown that Broca's and Wernicke's aphasic patients show different impairments in auditory lexical processing. The results of an experiment with form-overlapping primes showed an inhibitory effect of form-overlap for control adults and a weak inhibition trend for Broca's aphasic patients, but a facilitatory effect of form-overlap was found for Wernicke's aphasic participants. This suggests that Wernicke's aphasic patients are mainly impaired in suppression of once-activated word candidates and selection of one winning candidate, which may be related to their problems in auditory language comprehension.

Functional imaging and neuropsychology findings: how can they be linked?

It is argued that in poorly understood domains functional imaging and neuropsychology findings on cognitive processes can be related only through functional models of normal cognition. The psychological concept of "resource" can, however, be simply extrapolated to functional imaging. It is then argued that double dissociations can have analogous inferential power for extrapolation to models of normal cognition in functional imaging as in neuropsychology. The argument is illustrated by the example of the control processes involved in functional episodic memory imaging of experiments.

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.

Dissociating 'what' and 'how' in visual form agnosia: a computational investigation

Patients with visual form agnosia exhibit a profound impairment in shape perception (what an object is) coupled with intact visuomotor functions (how to act on an object), demonstrating a dissociation between visual perception and action. How can these patients act on objects that they cannot perceive? Although two explanations of this 'what-how' dissociation have been offered, each explanation has shortcomings. A 'pathway information' account of the 'what-how' dissociation is presented in this paper. This account hypothesizes that 'where' and 'how' tasks require less information than 'what' tasks, thereby allowing 'where/how' to remain relatively spared in the face of neurological damage. Simulations with a neural network model test the predictions of the pathway information account. Following damage to an input layer common to the 'what' and 'where/how' pathways, the model performs object identification more poorly than spatial localization. Thus, the model offers a parsimonious explanation of differential 'what-how' performance in visual form agnosia. The simulation results are discussed in terms of their implications for visual form agnosia and other neuropsychological syndromes.

Dissociations in processing past tense morphology: neuropathology and behavioral studies

Neuropsychological research showing that the regular ("jump-jumped") and irregular ("drive/drove") past tense inflectional morphology can dissociate following brain damage has been important in testing claims about the cognitive and neural status of linguistic rules. These dissociations have been interpreted as evidence for two different computational systems-a rule-based system underlying the processing of regulars and the irregulars being individually listed in the mental lexicon. In contrast, connectionist accounts claim that these dissociations can be modeled within a single system. Combining behavioral data from patients with detailed information about their neuropathology can, in principle, provide strong constraints on accounts of the past tense. In this study, we tested five nonfluent aphasic patients, all of whom had extensive left hemisphere (LH) damage involving the left inferior frontal gyrus and underlying structures, and four patients with semantic deficits following herpes simplex encephalitis (HSE) who had extensive damage to the inferior temporal cortex. These patients were tested in experiments probing past tense processing. In a large priming study, the nonfluent patients showed no priming for the regular past tense but significant priming for the irregulars (whereas controls show priming for both). In contrast, the HSE patients showed significantly impaired performance for the irregulars in an elicitation task. These patterns of behavioral data and neuropathology suggest that two separable but interdependent systems underlie processing of the regular and irregular past tense.

Conceptual structure and the structure of concepts: a distributed account of category-specific deficits

We present a new account of the fine-grained structure of semantic categories derived from neuropsychological, behavioral, and developmental data. The account places theoretical emphasis on the functions of the referents of concepts. We claim (i) that the distinctiveness of functional features correlated with perceptual features varies across semantic domains; and (ii) that category structure emerges from the complex interaction of these variables. The representational assumptions that follow from these claims make strong predictions about what types of semantic information are preserved in patients showing category-specific deficits following brain damage. These claims are illustrated with a connectionist simulation which, when damaged, shows patterns of preservation of distinctive and shared functional and perceptual information which varies across semantic domains. The data model both dissociations between knowledge for artifacts and for living things and recent neuropsychological evidence concerning the robustness of functional information in the representation of concepts.

Selective preservation of naming from description and the "restricted preverbal message"

We report the case of a patient, LEW, who presents with modality-specific naming deficits. He is seriously impaired in naming pictures of both objects and actions. His naming to auditory verbal definitions and of actions carried out by the experimenter is, however, relatively well preserved. He has no visual perceptual deficits and his access to the semantics of pictures is as good as that to the semantics of spoken words. While LEW is not an optic aphasic patient, his pattern of performance is relevant to the debate that has taken place of the organization of the semantic system. We discuss his case from this perspective and argue that LEW's selective deficits support the multiple semantics position. We also argue that the "preverbal message" level in the speech production model of Levelt (1989) is the equivalent of "verbal semantics." We provide additional constraints and principles to the concept of the preverbal message and we term the system so constrained the "restricted preverbal message."