Biomedical subjects

L K Tyler

Publications and source records attributed to L K Tyler.

At least 19 recordsLinked to original sources

PubMed2005

The basal ganglia and rule-governed language use: evidence from vascular and degenerative conditions.

The Declarative/Procedural Model of Pinker, Ullman and colleagues claims that the basal ganglia are part of a fronto-striatal procedural memory system which applies grammatical rules to combine morphemes (the smallest meaningful units in language) into complex words (e.g. talk-ed, talk-ing). We tested this claim by investigating whether striatal damage or loss of its dopaminergic innervation is reliably associated with selective regular past tense deficits in patients with subcortical cerebrovascular damage, Parkinson's disease or Huntington's disease. We focused on past tense morphology since this allows us to contrast the regular past tense (jump-jumped), which is rule-based, with the irregular past tense (sleep-slept), which is not. We used elicitation and priming tasks to test patients' ability to comprehend and produce inflected forms. We found no evidence of a consistent association between striatal dysfunction and selective impairment of regular past tense morphology, suggesting that the basal ganglia are not essential for processing the regular past tense as a sequence of morphemes, either in comprehension or production, in contrast to the claims of the Declarative/Procedural Model. All patient groups showed normal activation of semantic and morphological representations in comprehension, despite difficulties suppressing semantically appropriate alternatives when trying to inflect novel verbs. This is consistent with previous reports that striatal dysfunction spares automatic activation of linguistic information, but disrupts later language processes that require inhibition of competing alternatives.

Adult↗

PubMed2004

Anteromedial temporal cortex supports fine-grained differentiation among objects.

Patients with damage to left anteromedial temporal cortex often show a striking deficit: they fail to recognize animals and other living things. This failure of recognition presents an important challenge to theories of the neural representation of conceptual knowledge. Here we propose that this lesion-behaviour association arises because polymodal neurons in anteromedial temporal cortex integrate simple features into complex feature conjunctions, providing the neural infrastructure for differentiating among objects.

Adult↗

PubMed2004

Unitary vs multiple semantics: PET studies of word and picture processing.

In this paper we examine a central issue in cognitive neuroscience: are there separate conceptual representations associated with different input modalities (e.g., Paivio, 1971, 1986; Warrington & Shallice, 1984) or do inputs from different modalities converge on to the same set of representations (e.g., Caramazza, Hillis, Rapp, & Romani, 1990; Lambon Ralph, Graham, Patterson, & Hodges, 1999; Rapp, Hillis, & Caramazza, 1993)? We present an analysis of four PET studies (three semantic categorisation tasks and one lexical decision task), two of which employ words as stimuli and two of which employ pictures. Using conjunction analyses, we found robust semantic activation, common to both input modalities in anterior and medial aspects of the left fusiform gyrus, left parahippocampal and perirhinal cortices, and left inferior frontal gyrus (BA 47). There were modality-specific activations in both temporal poles (words) and occipitotemporal cortices (pictures). We propose that the temporal poles are involved in processing both words and pictures, but their engagement might be primarily determined by the level of specificity at which an object is processed. Activation in posterior temporal regions associated with picture processing most likely reflects intermediate, pre-semantic stages of visual processing. Our data are most consistent with a hierarchically structured, unitary system of semantic representations for both verbal and visual modalities, subserved by anterior regions of the inferior temporal cortex.

Adult↗

PubMed2004

Neural processing of nouns and verbs: the role of inflectional morphology.

Dissociations of nouns and verbs following brain damage have been interpreted as evidence for distinct neural substrates underlying different aspects of the language system. Some neuroimaging studies have supported this claim by finding neural differentiation for nouns and verbs [Brain 122 (1999) 2337] while others have argued against neural specialisation [Brain 119 (1996) 159; Brain 124 (2001) 1619]. We suggest that one reason why these inconsistencies may have arisen is because the morphological structure of nouns and verbs has been ignored. In an event-related functional magnetic resonance imaging (fMRI) study we test the hypothesis that the neural processing of nouns and verbs differs when they are inflected. We contrasted the processing of regularly inflected nouns (dogs) with regularly inflected verbs (hitting), and found that the LIFG was more strongly activated in processing regularly inflected verbs compared to regularly inflected nouns. Moreover, regions of LIFG that were more active in the fMRI study for inflected verbs partially overlapped with the lesions in patients who have particular problems with verb morphology. Taken together with previous studies, these results suggest that noun and verb stems do not differ in terms of their representation, but when verbs are morphologically complex they differentially engage those neural systems which are involved in processes of morpho-phonology and syntax.

Adult↗

PubMed2004

Processing objects at different levels of specificity.

How objects are represented and processed in the brain is a central topic in cognitive neuroscience. Previous studies have shown that knowledge of objects is represented in a feature-based distributed neural system primarily involving occipital and temporal cortical regions. Research with nonhuman primates suggest that these features are structured in a hierarchical system with posterior neurons in the inferior temporal cortex representing simple features and anterior neurons in the perirhinal cortex representing complex conjunctions of features (Bussey & Saksida, 2002; Murray & Bussey, 1999). On this account, the perirhinal cortex plays a crucial role in object identification by integrating information from different sensory systems into more complex polymodal feature conjunctions. We tested the implications of these claims for human object processing in an event-related fMRI study in which we presented colored pictures of common objects for 19 subjects to name at two levels of specificity - basic and domain. We reasoned that domain-level naming requires access to a coarser-grained representation of objects, thus involving only posterior regions of the inferior temporal cortex. In contrast, basic-level naming requires finer-grained discrimination to differentiate between similar objects, and thus should involve anterior temporal regions, including the perirhinal cortex. We found that object processing always activated the fusiform gyrus bilaterally, irrespective of the task, whereas the perirhinal cortex was only activated when the task required finer-grained discriminations. These results suggest that the same kind of hierarchical structure, which has been proposed for object processing in the monkey temporal cortex, functions in the human.

Adult↗

PubMed2003

Objects and their actions: evidence for a neurally distributed semantic system.

An influential model of conceptual knowledge claims that objects are represented in a distributed network of cortical areas that store information about different types of attributes, such as form, colour, and motion (A. Martin et al., 2000, in: The Cognitive Neurosciences, 2nd ed., MIT Press, Cambridge). Two specific claims of this account are that (a) the motions and actions associated with objects (along with other attributes) are automatically activated whenever the object concept is evoked and (b) topographically distinct neural regions are responsible for motion/action attributes pertaining to objects in the categories of tools and animals. We used fMRI to examine the neural activation associated with conceptual processing of nouns referring to animals and tools and for verbs referring to tool-associated actions (e.g., drilling, painting) and biological actions (e.g., walking, jumping). We found that object names and their associated actions activated the same set of neural regions (left fusiform gyrus, superior and middle temporal cortex) consistent with the claim that word tool and animal concepts implicitly activate the actions associated with them. However, there was no evidence of category specificity for either objects or actions, with essentially the same activations for the form and motion attributes of both living and nonliving categories.

Adult↗

PubMed2002

Overcoming confounds of stimulus blocking: an event-related fMRI design of semantic processing.

The way in which meaning is represented and processed in the brain is a key issue in cognitive neuroscience, which can be usefully addressed by functional imaging techniques. In contrast to previous imaging studies of semantic knowledge, which have primarily used blocked designs, in this study we use an event-related fMRI (erfMRI) design, which has the advantage of enabling events to be presented pseudorandomly, thus reducing strategic processes and enabling more direct comparison with psychological behavioral studies. We used a semantic categorization task in which events were words representing either artifact or natural kinds concepts. Significant areas of activation for semantic processing included inferior frontal lobe bilaterally (BA 47) and left temporal regions, both inferior (BA 36 and 20) and middle (BA 21). These are areas that have been identified in previous neuroimaging studies of semantic knowledge. However, there were no significant differences between artifact and natural kinds concepts. These results are consistent with our previous imaging studies using blocked designs and suggest that conceptual knowledge is represented in a unitary, distributed neural system undifferentiated by domain of knowledge. These findings demonstrate that event-related designs can generate activations that are similar to those seen in blocked designs investigating semantics and, moreover, offer a greater capacity for interpretation free from the confounds of block effects.

Brain↗

PubMed2002

The influence of explicit instructions and stimulus material on lateral frontal responses to an encoding task.

In this functional magnetic resonance imaging study, we explored the effects of both stimulus material and encoding task demands on activation in lateral prefrontal cortex (PFC). Two factors were manipulated: material type and task instructions. Subjects encoded words or abstract figures (factor 1: stimulus type) and were required to make either a meaning-based or a form-based (letter or shape) decision about each stimulus (factor 2: task instructions). Abstract figures engendered significantly higher levels of right PFC activity than did words. This effect was seen for meaning-based and form-based processing tasks and was significantly greater for the former. We did not observe a differential response of left lateral PFC to verbal and pictorial material. A double dissociation, however, was found within left PFC. A ventrolateral region (within left inferior frontal gyrus) showed the highest levels of activity when words were processed according to their meaning whereas activity in a more dorsolateral region (within left middle frontal gyrus) was greatest when words were processed according to their form (constituent letters). We have therefore observed a main effect of material type in producing lateralized activation of frontal lobes, although the strength of this effect is sensitive to the nature of the task that subjects are asked to perform. Left-side lateral PFC activity is also sensitive to task instructions but this effect was specific to verbal material. The complex patterns of frontal effect counsel against any simple dichotomy of frontal function at the level of either material or task type.

Adult↗

PubMed2001

Voxel-based morphometry of herpes simplex encephalitis.

Voxel-based morphometry (VBM) is a powerful tool for analyzing changes in gray or white matter density of the brain. By using an automated segmentation procedure and standardized parametric statistics it avoids biases inherent in operator-dependent morphological operations (J. Ashburner and K. J. Friston, 2000, NeuroImage 11, 805-821). Since its introduction in 1995, VBM has been used to examine anatomical changes in a variety of diseases associated with neurologic and psychiatric dysfunction. Given the power of this technique for discerning subtle anatomical changes, we wanted to assess its performance on brains with gross structural abnormalities. Such results could have implications regarding the difficulties to be faced when examining other types of distorted brains (e.g., brains with changes due to degenerative disease). This report describes the use of VBM for examining individual and group changes in gray matter concentration in five patients who had recovered from herpes simplex encephalitis (HSE) compared with age- and sex-matched controls. Because HSE tends to affect a specific set of brain regions we thought that this would (1) provide an opportunity to assess the anatomical face validity of VBM, (2) allow us to assess the problems of this technique when used on distorted brains, and (3) provide an in vivo demonstration of the gray matter changes due to HSE. We found that, despite problems in normalizing and segmenting these severely distorted brains, VBM was able to identify correctly a number of the regional gray matter abnormalities in HSE. The results, while consistent with the well-known histopathology of the disease, also demonstrate potential difficulties with this method.

Adult↗

PubMed2001

The neural representation of nouns and verbs: PET studies.

Neuropsychological studies of patients with selective deficits for nouns or verbs have been taken as evidence for the neural specialization of different word classes. Noun deficits are associated with lesions in anterior temporal regions while verb deficits arise from left inferior frontal lesions. However, neuroimaging studies do not unequivocally support this account, with only some studies supporting claims for regional specialization. We carried out two PET studies to determine whether there is any regional specialization for the processing of nouns and verbs. One study used the lexical decision task and the other used a more semantically demanding task, i.e. semantic categorization. We found robust activation of a semantic network extending from left inferior frontal cortex into the inferior temporal lobe, but no differences as a function of word class. We interpret these data within the framework of cognitive accounts in which conceptual knowledge is represented within a non-differentiated distributed system.

Adolescent↗

PubMed2000

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.

Brain↗

PubMed2000

Susceptibility-induced loss of signal: comparing PET and fMRI on a semantic task.

Functional magnetic resonance imaging (fMRI) has become a popular tool for investigations into the neural correlates of cognitive activity. One limitation of fMRI, however, is that it has difficulty imaging regions near tissue interfaces due to distortions from macroscopic susceptibility effects which become more severe at higher magnetic field strengths. This difficulty can be particularly problematic for language tasks that engage regions of the temporal lobes near the air-filled sinuses. This paper investigates susceptibility-induced signal loss in the temporal lobes and proposes that by defining a priori regions of interest and using the small-volume statistical correction of K. J. Worsley, S. Marrett, P. Neelin, A. C. Vandal, K. J. Friston, and A. C. Evans (1996, Hum. Brain Mapp. 4: 58-83), activations in these areas can sometimes be detected by increasing the statistical power of the analysis. We conducted two experiments, one with PET and the other with fMRI, using almost identical semantic categorization paradigms and comparable methods of analysis. There were areas of overlap as well as differences between the PET and fMRI results. One anticipated difference was a lack of activation in two regions in the temporal lobe on initial analyses in the fMRI data set. With a specific region of interest, however, activation in one of the regions was detected. These experiments demonstrate three points: first, even for almost identical cognitive tasks such as those in this study, PET and fMRI may not produce identical results; second, differences between the two methods due to macroscopic susceptibility artifacts in fMRI can be overcome with appropriate statistical corrections, but only partially; and third, new data acquisition paradigms are necessary to fully deal with susceptibility-induced signal loss if the sensitivity of the fMRI experiment to temporal lobe activations is to be enhanced.

Adult↗

PubMed2000

A progressive category-specific semantic deficit for non-living things.

We report a longitudinal study of a patient, ES, with a progressive degenerative disorder resulting from generalised cerebral atrophy. Across a range of tasks, ES showed a greater difficulty in recognising and naming artifacts than living things. This deficit for artifacts emerged over time, as she became more severely impaired. In one task, picture naming, there was a crossover from an initial deficit for living things to the later artifact deficit. All materials were carefully controlled to rule out potential confounding factors such as concept familiarity or age of acquisition. There was no evidence that ES's deficit for artifacts was associated with a greater loss of functional than visual information. The pattern of results are consistent with a recently proposed distributed connectionist model, in which a deficit for artifact concepts can emerge as the result of severe, general damage to semantic memory.

Aged↗

PubMed2000

The interaction of meaning and sound in spoken word recognition.

Models of spoken word recognition vary in the ways in which they capture the relationship between speech input and meaning. Modular accounts prohibit a word's meaning from affecting the computation of its form-based representation, whereas interactive models allow activation at the semantic level to affect phonological processing. We tested these competing hypotheses by manipulating word familiarity and imageability, using lexical decision and repetition tasks. Responses to high-imageability words were significantly faster than those to low-imageability words. Repetition latencies were also analyzed as a function of cohort variables, revealing a significant imageability effect only for words that were members of large cohorts, suggesting that when the mapping from phonology to semantics is difficult, semantic information can help the discrimination process. Thus, these data support interactive models of spoken word recognition.

Adult↗

PubMed1999

Why do Alzheimer patients have difficulty with pronouns? Working memory, semantics, and reference in comprehension and production in Alzheimer's disease.

Three experiments investigated the extent to which semantic and working-memory deficits contribute to Alzheimer patients' impairments in producing and comprehending referring expressions. In Experiment 1, the spontaneous speech of 11 patients with Alzheimer's disease (AD) contained a greater ratio of pronouns to full noun phrases than did the spontaneous speech produced by 9 healthy controls. Experiments 2 and 3 used a cross-modal naming methodology to compare reference comprehension in another group of 10 patients and 10 age-matched controls. In Experiment 2, patients were less sensitive than healthy controls to the grammatical information necessary for processing pronouns. In Experiment 3, patients were better able to remember referent information in short paragraphs when reference was maintained with full noun phrases rather than pronouns, but healthy controls showed the reverse pattern. Performance in all three experiments was linked to working memory performance but not to word finding difficulty. We discuss these findings in terms of a theory of reference processing, the Informational Load Hypothesis, which views referential impairments in AD as the consequence of normal discourse processing in the context of a working memory impairment.

Aged↗

PubMed1998

Sentence comprehension deficits in Alzheimer's disease: a comparison of off-line vs. on-line sentence processing.

Two studies explored whether sentence comprehension impairments in Alzheimer's disease (AD) are due to deficits in syntactic processing or memory. Study 1 used a picture-pointing sentence comprehension task to measure the final outcome of comprehension in an off-line fashion. It showed the comprehension of 30 patients with AD to be impaired, but suggested that the deficits could not be attributed solely to syntactic impairments. Study 2 investigated the effects of memory on sentence comprehension by comparing off-line (grammaticality judgment) with on-line (cross-modal naming) language processing in 11 AD and 9 control subjects. The results revealed impaired performance in the off-line task but normal performance in the on-line task using the same sentences. Performance on the off-line task correlated with independent measures of verbal working memory. These data are used to argue that sentence comprehension impairments are related to verbal working memory deficits in AD.

Aged↗

PubMed1998

Linguistic dissociations in Williams syndrome: evaluating receptive syntax in on-line and off-line tasks.

Williams syndrome (WS) is a neurodevelopmental disorder of genetic origin which results in relatively spared language in the face of serious non-verbal deficits. There is controversy, however, about how intact WS language abilities are. The discussion has focused on impairments of lexico-semantics and of morphological feature analysis, with the presumption that WS syntax is intact. We challenged this view and assessed WS receptive syntax by using two tasks testing various syntactic structures: an on-line word monitoring task and an off-line picture-pointing task. WS performance on the off-line task was generally poor. By contrast, their performance on the on-line task was far better and allowed us to ascertain precisely which aspects of WS receptive syntax are preserved and which are impaired. WS participants were sensitive to the violation of auxiliary markers and phrase structure rules but, unlike both the normal young and elderly controls, they did not show sensitivity to violations of subcategory constraints. The present study suggests that there exist dissociations within WS language which are not restricted to lexico-semantics or to morphological feature analysis, but which also invade their processing of certain syntactic structures. We conclude by arguing that WS syntax is not intact and that their language might turn out to be more like second language learning than normal acquisition.

Adolescent↗

PubMed1998

Going, going, gone...? Implicit and explicit test of conceptual knowledge in a longitudinal study of semantic dementia.

Patients suffering from semantic dementia provide important constraints on theories of the structure and organisation of semantic memory. In this article we report one such patient, AM, whose progressive deterioration of semantics enables us to address the much-debated issue of whether conceptual structure is hierarchically organised. The hierarchical account predicts that brain damage should impair lower levels of the hierarchy (property information) before affecting higher level (category) information (Warrington and Shallice, Q. J. Exp. Psychol. 1975, 27, 635-657). We evaluate this prediction by repeated testing of AM in two studies--a semantic priming task and a verification task--over an 18 month period, contrasting the progressive deterioration of properties (functional and perceptual) and category relations (category co-ordinates and category labels). Properties were preserved longer than category information, arguing against a hierarchical account of semantic memory. In addition, functional properties were most robust to brain damage, supporting our claim that functional information plays a special role in semantic representations (Durrant-Peatfield et al., Proc. 19th Ann. Conf. of the Cognitive Science Society. Erlbaum, Mahwah, NJ, 1997. pp. 193-198. Tyler et al., Cognitive Neuropsychol. 1997, 14, 511-545).

Anomia↗