Interhemispheric processing in left- and right-handers

Diffusion Tensor Imaging Revealing the Relation of Age-Related Differences in the Corpus Callosum With Cognitive Style

People may differ in their ways of processing tasks or situations, which may be explained by cognitive styles that define individual differences in information processing strategies. The cognitive style ranges between two extremes: analytic and holistic processing style. The concept of cognitive style has been widely investigated in the literature, but its age-related differences in the neural substrates have remained elusive. In this study, we focused on the white matter structure of the corpus callosum and its possible link to age-related differences in cognitive style, given its functional ability to connect and facilitate efficient communication between the left and right cerebral hemispheres. Seventy-two participants aged 20–75 years participated in this study. Participants’ cognitive styles were measured by the Analysis-Holism Scale (AHS), and their white matter microstructures were acquired using diffusion-weighted magnetic resonance imaging. The results revealed that older adults tend to have a more holistic processing style than younger adults. We then compared the white matter of tracts of interest between high and low AHS groups and found that the white matter microstructure in the genu of the corpus callosum can be used to distinguish between AHS subgroups. Interestingly, we found that age negatively correlated with the white matter tracts across the brain, indicating that aging is associated with reduced microstructure integrity. Together, our findings suggest that analytic-holistic cognitive styles of information processing possibly reflect that the microstructure development in the anterior part of the corpus callosum may influence the type of age-related information processing.

Local-global interference is modulated by age, sex and anterior corpus callosum size

To identify attentional and neural mechanisms affecting global and local feature extraction, we devised a global-local hierarchical letter paradigm to test the hypothesis that aging reduces functional cerebral lateralization through corpus callosum (CC) degradation. Participants (37 men and women, 26-79 years) performed a task requiring global, local, or global+local attention and underwent structural MRI for CC measurement. Although reaction time (RT) slowed with age, all participants had faster RTs to local than global targets. This local precedence effect together with greater interference from incongruent local information and greater response conflict from local targets each correlated with older age and smaller callosal genu (anterior) areas. These findings support the hypothesis that the CC mediates lateralized local-global processes by inhibition of task-irrelevant information under selective attention conditions. Further, with advancing age smaller genu size leads to less robust inhibition, thereby reducing cerebral lateralization and permitting interference to influence processing. Sex was an additional modifier of interference, in that callosum-interference relationships were evident in women but not in men. Regardless of age, smaller splenium (posterior) areas correlated with less response facilitation from repetition priming of global targets in men, but with greater response facilitation from repetition priming of local targets in women. Our data indicate the following dissociation: anterior callosal structure was associated with inhibitory processes (i.e., interference from incongruency and response conflict), which are vulnerable to the effects of age and sex, whereas posterior callosal structure was associated with facilitation processes from repetition priming dependent on sex and independent of age.

Hemispheric activation and interaction: Past activity affects future performance

Previous studies have shown that when hemispheric activation is modulated by a lateralised task performed concurrently with a second task, performance in the second task is affected by the side of the more active hemisphere. This effect is thought to be produced by competition for limited resources required to complete the two tasks and/or by a greater allocation of attention to the hemifield contralateral to the more active hemisphere. Little is known on how task performance is affected by the pattern of activation in the two cerebral hemispheres before a target task is conducted. The present study investigated how manipulation of hemispheric activity influenced performance of a non-lateralised task (letter matching). Greater left hemisphere activity interfered most with performance of the letter- matching task and was more pronounced in the early learning stage. Male participants were most affected by this effect. The results are discussed in relation to hemispheric interaction, functional lateralisation, and allocation of attention.

Practice makes two hemispheres almost perfect

Some tasks produce a performance advantage for conditions that require the processing of stimuli in two visual fields compared to conditions where single hemifield processing is sufficient. This advantage, however, disappears with practice. Although no definitive evidence yet exists, there are several possible mechanisms that might lead to improved performance of within- compared to across-hemisphere processing with practice. These include a shift from a more demanding, algorithmic strategy to a less demanding memory-retrieval strategy (e.g., [G. Logan, Toward an instance theory of automatisation. Psych. Rev. 95 (1988) 492-527]), as discussed by Weissman and Compton [D.H. Weissman, R.J. Compton, Practice makes a hemisphere perfect: the advantage of interhemispheric recruitment is eliminated with practice. Laterality, 8 (4) (2003) 361-375], and/or a more generalised practice effect [K. Kirsner, C. Speelman, Skill acquisition and repetition priming: one principle, many processes? J. Exp. Psychol., Learn. Mem. Cogn., 22 (1996) 563-575]. Contrary to Weissman and Compton findings, our results suggest that although single-hemisphere performance improves with practice, bi-hemispheric performance also improves substantially. Furthermore, these effects do not appear to be due to a shift in strategy but rather due to a general practice effect.

Individual differences in stroop and local-global processing: a possible role of interhemispheric interaction

Two experiments are reported examining individual differences in the processing of centrally presented stimuli containing two dimensions of information lateralized to opposite cerebral hemispheres. Left-handers, arising from (a) their lesser degree of functional lateralization and (b) their greater degree of callosal connectivity, were hypothesized to exhibit greater interdimensional (and presumably interhemispheric) interaction. Experiment 1 utilized local-global stimuli, and left-handers were found to be impaired at keeping the two dimensions independent and superior at integrating the two dimensions. Experiment 2 used Stroop stimuli, and left-handers again were impaired at keeping the two dimensions independent (i.e., showed greater Stroop interference). Correlational analyses indicated that the mechanisms of interdimensional integration versus independence are at least partially independent from one another. Results suggest that aspects of interhemispheric interaction can be addressed via the use of nonlateralized input.

Benefits of interhemispheric integration on the Japanese Kana script-matching tasks

We tested Banich's hypothesis that the benefits of bihemispheric processing were enhanced as task complexity increased, when some procedural shortcomings in the previous studies were overcome by using Japanese Kana script-matching tasks. In Exp. 1, the 20 right-handed subjects were given the Physical-Identity task (Katakana-Katakana scripts matching) and the Name-Identity task (Katakana-Hiragana scripts matching). On both tasks, a pair of Kana scripts was tachistoscopically presented in the left, right, and bilateral visual fields. Distractor stimuli were also presented with target Kana scripts on both tasks to equate the processing load between the hemispheres. Analysis showed that, while a bilateral visual-field advantage was found on the name-identity task, a unilateral visual-field advantage was found on the physical-identity task, suggesting that, as the computational complexity of the encoding stage was enhanced, the benefits of bilateral hemispheric processing increased. In Exp. 2, the 16 right-handed subjects were given the same physical-identity task as in Exp. 1, except Hiragana scripts were used as distractors instead of digits to enhance task difficulty. Analysis showed no differences in performance between the unilateral and bilateral visual fields. Taking into account these results of physical-identity tasks for both Exps. 1 and 2, enhancing task demand in the stage of ignoring distractors made the unilateral visual-field advantage obtained in Exp. 1 disappear in Exp. 2. These results supported Banich's hypothesis.

The missing link: the role of interhemispheric interaction in attentional processing

Although interhemispheric interaction via the callosum is most often conceived as a mechanism for transferring sensory information and coordinating processing between the hemispheres, it will be argued here that the callosum also plays an important role in attentional processing. Experiments will be presented that support this viewpoint, both when attention is conceptualized as a resource and when it is conceptualized as a selective mechanism for gating sensory information. Interhemispheric interaction is posited to aid attentional processing because it allows for a division of labor across the hemispheres, and allows for parallel processing so that operations performed in one hemisphere can be insulated from those executed in the other. Given this additional role for interhemispheric processing, it is suggested that the corpus callosum should be considered a component in the network of neural structures that underlie attentional control.

What does the haptic modality do during cognitive activities on letter shapes? A study with left- and right-handers

This study was undertaken to analyze intermanual (interhemispheric) transfer in left and right handed subject and to assess how information was extracted during finger scanning of letter shape at the different levels of letter processing: shape recognition during a physical matching task, letter recognition in a verbal "meaning" matching task and letter naming. The dichhaptic procedure was used to study interhemispheric relations. It was hypothesized that cognitive activities have a feed-forward effect on the exploration of shapes, and that the performance is related to the nature of the task and to handedness. The exploratory strategies of the two types of handedness were also analyzed. The results showed that response latencies were generally similar for left- and right-handed subjects, but accuracy was better for left than right handers in "verbal" matching with the same overall exploratory strategies. In physical matching, left- and right-handed subjects performed equally but used different exploratory strategies. The naming task was very difficult for both groups but failed to discriminate their on accuracy, response latency, and exploratory strategy. The results are discussed with reference to the different exploratory strategies used and the interhemispheric interaction at work in different cognitive processes.

Interhemispheric cooperation of left- and right-handers in mental calculation tasks

Relations between handedness and interhemispheric processing in cognitive tasks were examined. Thirty six right-handers and thirty left-handers (familial and nonfamilial left-handers) were asked to add two numbers which were presented tachistoscopically. Two numbers were displayed either to one visual field, or one to the left and one to the right visual field simultaneously. In Experiment 1, the numbers were displayed in Arabic numerals, and in Experiment 2 one of the numbers was displayed in Kanji and one in Arabic numerals. The results of Experiment 1 showed a bilateral advantage, however no subject group difference was shown in the performance of the three (left unilateral, right unilateral, and bilateral) presentation conditions. Though familial left-handers showed a weaker tendency to different patterns than right and non-familial left-handers, the results of Experiment 2 were largely similar to those of Experiment 1. These findings suggest that interhemispheric processing may not differ between left- and right-handers.

Handedness and progressive hydrocephalus in spina bifida patients

Hand preference and functional hand-dominance of 45 spina bifida patients (aged 13 to 25 years) were assessed by means of self-reports on an ordinal scale and a tapping task. Non-right-handedness was more frequent among patients with accompanying progressive hydrocephalus. Left-hand preference was significantly more frequent in this group than in patients without shunts. Non-right-handedness in hydrocephalic patients correlated positively with the estimated degree of pre-operative progress of the hydrocephalus. The apparent failure to establish right-handedness in these patients seemed to reflect a developmental disorder of lateralization, related to a more advanced degree of progressive hydrocephalus at an early age. It is suggested that the disorder may be attributable to dysfunction of the corpus callosum, caused by delay in the development of callosal white matter when the hydrocephalus is in a progressive state.