This study examined the development of interaction between the hemispheres as a function of computational complexity (Banich & Belger, 1990; Belger & Banich, 1992) in 24 children aged 6.5 to 14 years. Participants performed 2 tasks: a less complex physical-identity task and a more complex name-identity task. Children, like adults, exhibit an across-hemisphere advantage on the computationally more complex name-identity task, and neither a within-nor an across-hemisphere advantage for the computationally less complex physical-identity task. Correlations indicated that the younger the child, (a) the greater the size of the within-hemisphere advantage on the less complex task, (b) the greater the size of the across-hemisphere advantage on the more complex task, and (c) the poorer the ability to ignore attentionally distracting information in a selective attention paradigm. These results suggest that interhemispheric interaction in children, like that in adults, serves to deal with the heightened processing demands imposed by increased computational complexity.
"The second is to investigate reaction times for only the across hemisphere condition. The latter option is considered to be less accurate because processing delay is not controlling for left or right hemisphere deficits  . Therefore, the present study used the first option to estimate interhemispheric interaction. "
[Show abstract][Hide abstract] ABSTRACT: The present study applied the dimensional approach to test whether self-reported symptoms of Attention Deficit/Hyperactivity Disorder (ADHD) in adults are associated with the speed of interhemispheric interaction. A sample of first grade students (N = 112) completed the Conners' Adult ADHD Rating Scales and letter matching reaction time tasks. In the tasks, participants had to match a single target letter displayed below the fixation cross, either on left or right visual field, with one of two letters displayed above the fixation cross one letter on each visual field. For each task, identical letters were presented either within the same visual field (within hemisphere condition) or across visual fields (across hemisphere condition). Interhemispheric interaction was indexed as the difference in mean reaction time between within and across hemisphere conditions. Comorbid problems such as depression, anxiety and stress may affect task performance and are controlled for in this study. Findings indicated that self-reported ADHD symptomology, especially hyperactivity, in the presence of stress was weakly but significantly associated with fast interhemispheric interaction.
"A fundamental role of the CC, interhemispheric transfer of information, can be assessed in a number of different ways, from simple finger tapping exercises to complex visual hemifield stimulation. It has been suggested that there is both a clear increase in CC utilization throughout development and also increased utilization in children only, based on these behavioral measures (Banich et al., 2000; Marion et al., 2003). It has not been customary to examine the relationship between task performance and brain microstructure in normal development, however some studies have examined these associations in older individuals (Baird et al., 2005; Johansen-Berg et al., 2007; Roebuck et al., 2002; Sullivan et al., 2001). "
[Show abstract][Hide abstract] ABSTRACT: Cross-sectional and longitudinal volumetric studies suggest that the corpus callosum (CC) continues to mature structurally from infancy to adulthood. Diffusion tensor imaging (DTI) provides in vivo information about the directional organization of white matter microstructure and shows potential for elucidating even more subtle brain changes during adolescent development. We used DTI to examine CC microstructure in healthy right-handed adolescents (n=92, ages 9-24 years) and correlated the imaging data with motor task performance. The primary DTI variable was fractional anisotropy (FA), which reflects the degree of white matter's directional organization. Participants completed an alternating finger tapping test to assess interhemispheric transfer and motor speed. Task performance was significantly correlated with age. Analyses of variance indicated that 9- to 11-year-olds generally performed worse than each of the older groups. Males outperformed females. Significant positive correlations between age and FA were observed in the splenium of the CC, which interconnects posterior cortical regions. Analyses of variance indicated that individuals older than 18 years had significantly higher FA than 9- to 11-year-olds. FA levels in the genu and splenium correlated significantly with task performance. Regression analyses indicated that bimanual coordination was significantly predicted by age, gender, and splenium FA. Decreases in alternating finger tapping time and increases in FA likely reflect increased myelination in the CC and more efficient neuronal signal transmission. These findings expand upon existing neuroimaging reports of CC development by showing associations between bimanual coordination and white matter microstructural organization in an adolescent sample.
[Show abstract][Hide abstract] ABSTRACT: In the present study, we investigated whether a hemispheric division of labor is most advantageous to performance when lateralized inputs place unequal resource demands on the left and right cerebral hemispheres. In each trial, participants decided whether 2 rotated letters, presented either in the same visual field (within-field trials) or in opposite visual fields (across-field trials), were both of normal orientation, or whether one was normal and the other was mirror-reversed. To discriminate a letter's orientation, one must rotate the letter to the upright position. Therefore, we manipulated whether the two letters imposed similar or dissimilar demands on cognitive resources by varying the number of degrees that each letter needed to be rotated to reach the upright position. As predicted, in 2 experiments we found that the across-field advantage increased as the number of degrees each letter needed to be rotated became more dissimilar. These findings support a current model of hemispheric interactions, which posits that an unequal hemispheric distribution of cognitive load allows the cerebral hemispheres to take the lead for different aspects of cognitive processing.
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