Biomedical subjects

Geoffrey W Stuart

Publications and source records attributed to Geoffrey W Stuart.

9 recordsLinked to original sources

PubMed2004

Changes in effective connectivity models in the presence of task-correlated motion: an fMRI study.

We investigated the effects of motion-correction strategy and time course selection method when structural equation modeling is applied to fMRI data in the presence of task-correlated motion. Three motion-correction methods were employed for a group of 12 subjects performing an orthographic lexical retrieval task: (1) a rigid body realignment as implemented in SPM99, (2) a rigid body realignment combined with the inclusion of motion parameters in the statistical model, and (3) the FLIRT motion correction followed by an ICA analysis aiming to identify and remove the motion-related components and the ghosting artifacts. For each motion correction, the time courses of the activated regions were selected in three ways: (1) using the voxels with the highest Z scores, (2) using the average across all the statistically significant voxels in the region of interest, and (3) using a within-region, across-subjects, singular value decomposition. The resulting models of effective connectivity were markedly different, although the activation pattern was not substantially altered by the motion-correction method. Higher values for the path coefficients were obtained for the models fitted to the covariance matrices based on the average time courses than for the covariance matrices based on a single voxel time course. Our results suggest caution with the interpretation of task-induced changes in effective connectivity since, for higher-order cognitive brain functions, multiple models can be fitted to a given data set and these models cannot be rejected on an anatomical or cognitive basis. Hum. Brain Mapping 21:49-63, 2004.

Brain Mapping↗

PubMed2004

Individual differences in anterior cingulate/paracingulate morphology are related to executive functions in healthy males.

The neuropsychological correlates of inter-individual variations in cortical folding are poorly understood. Anterior cingulate (AC) cortex is one region characterized by considerable variability, particularly with respect to the paracingulate sulcus (PCS), which is present in only 30-60% of individuals and more commonly found in the left cerebral hemisphere. To investigate whether inter-individual differences in this PCS asymmetry are related to cognitive performance, we classified 30 healthy right-handed males as displaying either a leftward, rightward or symmetric pattern of folding based on the incidence and extent of the PCS in each hemisphere, and compared their performance on tasks engaging executive cognitive processes associated with frontal lobe function. We found that the more common leftward PCS asymmetry was associated with better performance across verbal and non-verbal executive tasks, but that PCS variability had no effect on tasks less dependent on executive functions. These results suggest that the leftward pattern of folding is associated with a non-specific performance advantage on cognitively demanding executive function tasks, possibly due to differences in functional interactions between AC/paracingulate cortex and connected frontal regions. It therefore appears that normal variations in brain morphology are associated with individual differences in cognitive abilities.

Adult↗

PubMed2004

Cultural consistency in Australian dental students from two different ethnic backgrounds.

The objective of this study was to examine the value orientations of dental students from different ethnic backgrounds studying in Australian dental schools. A ninety-eight-item questionnaire was used to collect the data, including fifteen subscales developed consistent with the Kluckhohn and Strodtbeck value orientation model. To compare the value orientation structure and to determine if any ethnic differences existed, a multivariate analysis of variance called profile analysis was performed on the fifteen value subscales. Of the 401 participants, 30.2 percent were Anglo-Australian and 44.9 percent were Asian. The remainder (24.9 percent) were "Other-background residents" or "Other-international students." This article presents data from Asian (n=184) and Anglo-Australian (n=124) background respondents. Mean age was 21.7 years (s.e. 0.35 years) among Anglo-Australian and 20.8 years (s.e. 0.17 years) among Asians. Asians born overseas represented 70.7 percent (n=130) of this group. Of Australia-born Asians (n=54), 90.6 percent were first-generation Australians. When comparing their value profile, we found a significant association between ethnicity and value orientation profile scores (p<0.05). Despite the significant overall result, the strength of the association (eta(2)=0.007) indicated that this effect was trivial relative to the unexplained variance in value orientation. The findings suggest that, behind an apparent ethnic diversity, a single distinctive value profile might exist to which the majority of dental students subscribe.

Adult↗

PubMed2003

Contrast sensitivity in subgroups of developmental dyslexia.

It has been proposed that developmental dyslexia is associated with a deficit in the magnocellular pathway of the visual system. Other research focuses upon the heterogeneous nature of developmental dyslexia, and evidence that subgroups of dyslexia may be identified based on selective deficits in specific component reading skills. This study tested the hypothesis that visual processing deficits may be present in different subgroups of developmental dyslexia by comparing the visual contrast sensitivity of three subgroups of dyslexic children (phonological, surface and mixed) and controls. The stimulus designed to measure magnocellular visual function was a low spatial frequency Gaussian blob, flickered sinusoidally at a temporal frequency of 8.33 Hz. The control stimulus, designed to measure parvocellular visual function, was a relatively high spatial frequency Gaussian windowed grating (8 c/deg) slowly ramped on and off. There were no significant differences between the groups of dyslexic and control children in contrast sensitivity to either stimulus. The findings do not support the existence of a magnocellular system deficit in dyslexia.

Child↗

PubMed2003

The overlay interference task and object-selective visual attention.

A series of experiments was carried out to examine object-based visual attention in an interference task. Observers were presented with two transparently overlapping equilateral triangles forming a "Star of David". One of these triangles was darker than the background, the other was lighter than the background. The observers were required to make a speeded choice response to the orientation of the darker triangle. The presence of the light triangle produced a robust interference effect that manifested as a slower response time. This effect was strongly modulated by the relative contrast of the target and distractor triangles. It was reduced when the light distractor triangle was separated in depth from the target triangle. Since the configuration rules out the possibility of 2-D spatial selection, it is concluded that object-based selection occurs in interference tasks and that the effectiveness of this selection is modulated by visual attributes that are not directly relevant to the task.

Adult↗

PubMed2003

Morphology of the anterior cingulate cortex in young men at ultra-high risk of developing a psychotic illness.

BACKGROUND: The anterior cingulate cortex (ACC) is consistently implicated in the pathophysiology of schizophrenia, and our own work has identified morphological anomalies in the ACC of people with this disorder. AIMS: To examine whether ACC morphological anomalies are present in a group at ultra-high risk of psychosis and whether such anomalies can be used to predict the subsequent development of a psychotic illness. METHOD: Magnetic resonance imaging of 75 healthy volunteers and 63 people at ultra-high risk of developing a psychotic disorder (all right-handed males) was used to examine ACC sulcal and gyral features. RESULTS: Compared with the controls, significantly fewer people in the ultra-high risk group had a well-developed left paracingulate sulcus and significantly more had an interrupted left cingulate sulcus. There was no difference between those who did (n=21) and did not (n=42) subsequently develop a psychotic illness. CONCLUSIONS: Although ACC anomalies are present in young people considered to be at ultra-high risk of psychosis, they do not identify individuals who subsequently make the transition to psychosis.

Adult↗

PubMed2002

Paracingulate morphologic differences in males with established schizophrenia: a magnetic resonance imaging morphometric study.

BACKGROUND: Our previous work on sulcal-gyral brain morphology in healthy volunteers revealed that males were characterized by greater cortical folding in the left versus right anterior cingulate cortex. Given the evidence showing an absence or reversal of normal anatomical asymmetries in patients with schizophrenia, the current study examined the anterior cingulate cortex sulcal-gyral patterns in patients with schizophrenia. METHODS: Using high-resolution magnetic resonance imaging, we examined anterior cingulate cortex surface morphology in a group of 55 patients with established schizophrenia and 75 healthy controls. All subjects were male and right-handed. Depending on the presence of a paracingulate sulcus and its antero-posterior extent, three types of anterior cingulate cortex sulcal patterns were identified: "prominent," "present," and "absent." Measures of overall cerebral hemispheric folding were used as independent variables and as covariates to ascertain the specificity of the findings to the anterior cingulate cortex. RESULTS: Examination of anterior cingulate cortex morphology showed that, compared with controls, patients with schizophrenia lacked the leftward anterior cingulate cortex sulcal asymmetry, which was explained by reduced folding in the left anterior cingulate cortex. These differences were over and above differences in cortical folding across the entire left hemisphere. CONCLUSIONS: These findings suggest that, in male patients with schizophrenia, there is a disturbance in the neurodevelopment of the left anterior cingulate cortex, as well as a more general aberration of left hemisphere development.

Adult↗

PubMed2002

Anterior cingulate activation during Stroop task performance: a PET to MRI coregistration study of individual patients with schizophrenia.

OBJECTIVE: The authors used single-subject functional imaging analyses to 1) corroborate the findings of anterior cingulate hypoperfusion during an attentional task in schizophrenia and 2) examine whether anterior cingulate activation is associated with underlying morphology. METHOD: Five healthy subjects and six patients with schizophrenia underwent positron emission tomography scanning while they performed the Stroop task. The medial-frontal lobes were masked out for analysis, and activation peaks were individually coregistered to each subject's magnetic resonance imaging scan. RESULTS: Healthy subjects showed activations in both limbic and paralimbic anterior cingulate regions. Patients with schizophrenia showed only paralimbic activations, and these were apparent only in patients having a paracingulate sulcus. CONCLUSIONS: These findings suggest that 1) patients with schizophrenia have limbic-anterior cingulate hypoperfusion during attentional tasks and 2) paralimbic activation is associated with underlying morphology.

Adult↗

PubMed2002

Simulation of the effects of global normalization procedures in functional MRI.

We report on differences in sensitivity and false-positive rate across five methods of global normalization using resting-state fMRI data embedded with simulated activation. These methods were grand mean session scaling, proportional scaling, ANCOVA, a masking method, and an orthogonalization method. We found that global normalization by proportional scaling and ANCOVA decreased the sensitivity of the statistical analysis and induced artifactual deactivation even when the correlation between the global signal and the experimental paradigm was relatively low. The masking method and the orthogonalization method performed better from this perspective but are both restricted to certain experimental conditions. Based on the results of these simulations, we offer practical guidelines for the choice of global normalization method least likely to bias the experimental results.

Algorithms↗