Biomedical subjects

N Y Weekes

Publications and source records attributed to N Y Weekes.

5 recordsLinked to original sources

PubMed2000

Parallel visuomotor processing in the split brain: cortico-subcortical interactions.

We tested nine patients with callosal pathology in a simple reaction time task with and without redundant targets in the same or opposite visual hemifield. Four patients showed large facilitation (redundancy gain) in the presence of a redundant target, exceeding probability summation models (neural summation). Five patients showed redundancy gain not exceeding probability models. Violation of probability models was not associated with a specific type of callosal lesion. Neural summation, which probably occurs at collicular level, may be modulated by cortical activity. To test this hypothesis, we used functional MRI. During detection of redundant simultaneous targets, activations in the extrastriate cortex were observed in a patient with callosal agenesis and redundancy gain violating probability models, but not in a patient with callosal agenesis and redundancy gain not exceeding probability models. We conclude that cortical activity in the extrastriate cortex may be a modulating factor in the magnitude of the redundancy gain during parallel visuomotor transforms.

Agenesis of Corpus Callosum↗

PubMed1999

Individual differences in the hemispheric specialization of dual route variables.

The dual route model suggests that reading of letter strings can occur through both a lexical and a nonlexical route. Hemispheric specialization of these routes has also been posited, suggesting that the left hemisphere has both lexical and nonlexical routes while the right hemisphere has only a lexical route. However, some recent data conflict with this hemispheric dual route model, suggesting that both hemispheres may have access to both routes. The purpose of the present study was to investigate individual differences in the hemispheric specialization of these routes and to determine whether these group differences in their specialization might explain conflicts in the literature. The effect of four individual difference factors was explored: handedness, biological sex, menstrual stage (i.e., fluctuations in estrogen), and self-rated degree of masculinity (i.e., sexual attribution). We looked at the interaction of these individual differences with the following dual route variables: (i) string length, (ii) word frequency, (iii) regularity of grapheme-phoneme correspondences of words, and (iv) the interaction of frequency and regularity using a bilateral lexical decision task. We observed that sex, menstrual stage, and masculinity each affected hemispheric specialization of the dual route variables, but did so in different ways. We posit that both hemispheres have orthographical (lexical) access as well as phonological (nonlexical) access to words. Further, we suggest that the presence of phonological processing in the right hemisphere depends on available resources and the strategies used, which are subject to individual differences.

Adult↗

PubMed1997

Interhemispheric relations in hierarchical perception: a second look.

This study reevaluates the role of interhemispheric interactions in the consistency effect (global interference with local decisions) in hierarchical perception. In an earlier study, Robertson et al. [22] (Neuropsychology, Vol. 7, pp. 325-342, 1993) tested three split-brain patients on a hierarchical perception task in which stimuli, consisting of large (global) letters made up of smaller (local) letters, were unilaterally or bilaterally presented for identification. They found that, in general, the consistency effect did not occur in split-brain patients and argued that the effect is interhemispheric and normally mediated by the corpus callosum. We repeated the experiment with new stimuli in two of the same split-brain patients. We found that both patients demonstrated evidence for global interference, implying that the neocortical commissures are not necessary for eliciting the consistency effect in hierarchical perception.

Corpus Callosum↗

PubMed1996

The effects of procedural variations on lateralized Stroop effects.

Several issues in the classic Stroop effect remain open, including (i) the stage of processing which gives rise to the effect, (ii) the effect of some procedural manipulations, (iii) the effect of hemispheric specialization and of interhemispheric interactions, and (iv) the existence of individual differences. In this paper, we investigate these issues using a series of experiments with central, lateral, and bilateral presentations of the Stroop stimuli. A total of 146 right-handed subjects took part in a multiexperiment study with relatively equal numbers of the two sexes participating in each experiment. We found that manual responses diluted but did not abolish the Stroop interference relative to vocal responses, arguing for a late processing stage account of the effect. However, separating the color patch from the color word either unilaterally or bilaterally did not significantly change the magnitude of the Stroop interference, arguing against a cost of interhemispheric transfer in the Stroop effect in normal subjects. We did, however, find evidence for hemispheric specialization in Stroop interference, greater in the left hemisphere than in the right hemisphere. Color words in the RVF produced the greatest Stroop effects regardless of the location of the color patch. In most cases, this laterality effect interacted with the sex of the subject, such that only males and a subgroup of females (i.e., those tested during a low estrogen phase in the menstrual cycle) showed evidence for greater Stroop effects when a color word projected to the left hemisphere than when it projected to the right hemisphere, regardless of the placement of the color patch.

Adult↗

PubMedDate not supplied

The association between stress, hemispheric specialization, and callosal interactions.

This study examines the effects of stress on hemispheric specialization and callosal transfer. Previous research suggests that the two cerebral hemispheres are asymmetric in their regulation of and response to stress. Further, studies have suggested that the activity of the corpus callosum may also be affected by stress. A group of 28 participants completed the Spielberger State-Trait Anxiety Inventory and performed a bilateral Stroop task. The bilateral Stroop task contains separate conditions which measure both hemispheric specialization and callosal transfer. A significant correlation was found between state anxiety and our measure of callosal transfer. Specifically, those individuals with greater state anxiety experience more efficient callosal transfer than did those with lower state anxiety. We conclude that state anxiety enhances callosal transfer without affecting hemispheric specialization.

Adolescent↗