Search PubMed⌕ Search

Biomedical subjects

J M Fletcher

Publications and source records attributed to J M Fletcher.

190 records · Page 11Linked to original sources
PubMed1976

Neuropsychologic, intellectual, and personality correlates of chronic marijuana use in native Costa Ricans.

The present cross-cultural study investigated the effects of chronic marijuana usage on higher adaptive brain functions and personality in native Costa Ricans. After extensive standardization in Costa Rica, a battery of neuropsychologic, intelligence, and personality tests was administered to two carefully matched groups of marijuana users and controls (NU = NC = 41). Multivariate analyses of multiple combinations of variables, followed by separate univariate tests on each measure, revealed essentially no significant differences between users and nonusers on any of the neuropsychologic, intelligence, or personality tests. Furthermore, no relationship was found between level of daily use (high vs low) and test performance. These findings were discussed in terms of previous chronic and acute studies of changes in adaptive brain functions and personality as a function of marijuana intake.

Adolescent↗
PubMedDate not supplied

Demonstration of the glycocalyces associated with three oral gram-negative bacterial species using a modern acrylic resin technique.

The complex highly hydrated chemical composition of the bacterial glycocalyx renders it difficult to preserve and visualize at the ultrastructural level. Polyanionic stains such as ruthenium red help to maintain some structural integrity, and other more modern approaches include antibody stabilization, lectins, and the addition of lysine to the primary fixative. It has been suggested that the glycocalyx of certain disease-associated organisms may play a role in the pathogenesis of some microbially based diseases such as periodontitis. New, more adequate, modern methodologies are therefore required for the further study of this structure. In the present study a cold dehydration process in conjunction with LR white acrylic resin has been employed to study the glycocalyces of three oral gram-negative bacterial species reported to be periodontopathogens. When compared with organisms processed conventionally and with ruthenium red, the organisms processed by the cold dehydration and LR white method demonstrated a fibrous matrix that was not seen in the other specimens. These results indicate that a combination of reduced dehydration temperature and cold acrylic resin embedding provides the best methodology for the visualization of the fine structure of the bacterial glycocalyx. This approach may be particularly useful in the study of organisms within specific disease-associated environments such as the periodontal pocket.

Acrylic Resins↗
PubMedDate not supplied

Estimation of CSF, white and gray matter volumes in hydrocephalic children using fuzzy clustering of MR images.

An algorithm and set of procedures for measuring volumes of cerebrospinal fluid (CSF), white matter, and gray matter from transaxial magnetic resonance images (MRI) of the brain are described. The algorithm is a variant of the fuzzy c-means clustering method for texture identification. This technique is used mainly to solve the problem of volume averaging of tissue compartments, but also has other advantages. It is fast, accurate, and relatively operator independent. Furthermore, it does not depend on statistical assumptions such as data normality, nor does it require any a priori heuristics. The procedure was tested successfully on imaged phantoms of known volume composition and compared with results achieved using a standard morphometric measurement approach. The procedure was also applied to brain MRIs of three clinically normal children and three age-matched children with hydrocephalus using both proton density and T2-weighted images. The algorithm was able to detect the expected increased amounts of CSF and decreased amounts of white matter characteristic of the hydrocephalic brain.

Algorithms↗
PubMedDate not supplied

Comparisons of cutoff and regression-based definitions of reading disabilities.

This study addressed the issue of specificity in reading disability by comparing two approaches to defining and selecting children with reading disabilities. One approach defined reading disability according to cutoff scores representing appropriate levels of intelligence and reading deficiency, whereas the other approach adjusted these scores for their intercorrelation through regression procedures. Results revealed clear differences in which children were identified as reading disabled according to the two definitions. However, differences in neuropsychological performance between children whose reading scores were discrepant or not discrepant with IQ were small and nonspecific for both definitions. The results of this study show that children identified as reading disabled vary according to the definition employed; at this point, there is little evidence suggesting any specificity of reading disability according to definition.

Achievement↗
PubMedDate not supplied

[Insights into brain function though magnetic source imaging: A review of research and clinical applications].

INTRODUCTION AND DEVELOPMENT: The paper presents a brief outline of the rationale behind the use of non invasive functional imaging and of the features that any imaging technique should display in order to make a substantial contribution to the search of the brain mechanisms responsible for cognitive functions. One such technique, magnetic source imaging (MSI), that meets these specifications, is described in more detail. Advantages of MSI include the capacity to provide direct measures of regional neurophysiological activity, a millisecond range temporal resolution, and the capacity to provide images of brain activity on an individual basis. We then describe applications of MSI to the study of brain mechanisms involved in various language functions such as oral comprehension and reading. Among these applications, the accuracy of MSI protocols in determining hemispheric dominance for language functions and in identifying the precise location and extent language specific cortex (Wernicke s area) has been verified through comparison with standard invasive techniques (Wada procedure and electrocortical stimulation mapping) in over 60 consecutive cases. In another series of studies we combined data from MSI and direct cortical stimulation to determine the role of temporoparietal areas in phonological analysis of spoken language and in phonological decoding of print. Finally, we have used MSI to gain unique insights into the brain mechanisms that support reading in developmental reading disability. CONCLUSION: Results from over 21 children diagnosed with this disorder suggest that impaired reading is associated with aberrant functional connections between temporal and temporoparietal areas of the left hemisphere that are normally engaged in reading.

Brain Mapping↗
PubMedDate not supplied

Prevalence of reading disability in boys and girls. Results of the Connecticut Longitudinal Study.

We hypothesized that results of previous investigations indicating an increased prevalence of reading disability in boys compared with girls reflected a bias in subject selection. In an epidemiologic sample of 215 girls and 199 boys, we identified two groups of reading-disabled children: research identified and school identified. Results indicated no significant differences in the prevalence of reading disability in research-identified boys compared with research-identified girls in either second (17[8.7%] of 196 boys; 15[6.9%] of 216 girls) or third grade (18[9.0%] of 199 boys; 13[6.0%] of 215 girls). In contrast, school identification resulted in the classification of 27 (13.6%) of 198 boys and seven (3.2%) of 216 girls in second grade and 20 (10.0%) of 199 boys and nine (4.2%) of 215 girls in third grade. Our data indicate that school-identified samples are almost unavoidably subject to a referral bias and that reports of an increased prevalence of reading disability in boys may reflect this bias in ascertainment. These findings caution against relying solely on schools for identification of reading-disabled children.

Analysis of Variance↗