Biomedical subjects

E A Stamatakis

Publications and source records attributed to E A Stamatakis.

7 recordsLinked to original sources

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

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↗

PubMed2001

Spatial normalization of lesioned HMPAO-SPECT images.

We investigated the effect of nonlinear alignment on SPECT images with lesions. Linear alignment produces reliable results but the introduction of nonlinear methods can improve matching by accounting for global brain shape. We examined the hypothesis that nonlinear alignment can introduce unwanted image distortions when lesions are present. We set out to quantify possible distortions by constructing artificial lesions in order to obtain images with controllable characteristics. We examined the use of basis functions (in SPM96 and SPM99) and other nonlinear models (in AIR3.08) designed to achieve optimum alignment between image and template. We found that the use of models with high degrees of nonlinearity will result in unwanted deformations and that the safest way to align images with lesions is to use 12-point linear affine transformations. Masking was examined as a remedy to distortions caused by nonlinear methodologies and produced significantly improved results.

Artifacts↗

PubMed2000

Analysis of HMPAO SPECT scans in head injury using Statistical Parametric Mapping.

The paper examines the ability of Statistical Parametric Mapping (SPM) to contribute towards the quantitative analysis of HMPAO SPECT images containing lesions. A validation study is described in which SPECT images were created that contained synthetic lesions and were analysed with SPM. The study established a set of guidelines concerning the alignment, smoothing, and statistical analysis of images. These were then applied to analysis of SPECT scans from head injured patients. A demonstration is given of the use of SPM to identify localised blood flow abnormalities associated with cognitive deficits after head injury. Correlations between blood flow abnormalities and a test of visual memory are illustrated.

Journal Article↗

PubMed1999

Validation of statistical parametric mapping (SPM) in assessing cerebral lesions: A simulation study.

Simulated abnormalities were introduced in a normal SPECT with known and controllable characteristics (abnormality size and depth) in an attempt to provide validation for the analysis of SPECT lesion studies using SPM. Two simulations were carried out. The first determined the minimum hypoperfusion depth detectable using SPM by altering mean local intensity while keeping the size of the lesion constant. This was done by changing the mean local intensity in percentile increments of 10 down to -100 and up to 50. The second simulation determined the cluster size that SPM can detect by keeping the mean intensity of the lesion constant while altering its size from 4 voxels to 63,000 voxels in a total brain volume of 300, 000 voxels. Both simulations determined which method of normalization is most appropriate, what level of grey matter thresholding should be used, and at what statistical probability peak threshold (u) the results should be determined. Proportional scaling was found to be the most appropriate normalization method. ANCOVA was useful where very large abnormalities were present and normalization external to SPM was not available. In those cases, ANCOVA was used in conjunction with measurement of an unaffected part of the brain (in this case medial occipital lobe). For better results statistical probability peak threshold was set to p(u) = 0. 01 and grey matter threshold was set to a value below 0.5. SPM produced best results when the abnormality represented a decrease of about -50% from the normal or more and detected other decreases in an acceptable manner.

Analysis of Variance↗