Driving performance during word generation--testing the function of human brain lateralization using fTCD in an ecologically relevant context.

It has been hypothesized that cerebral lateralization of function enhances cognitive performance. Evidence was found in birds and fish. However, recent research in humans did not support this hypothesis. We aimed to replicate and extend these findings for single- and dual-task performance in an ecologically relevant task. We combined a word generation task which is assumed to be primarily processed in the left hemisphere with a driving task which is assumed to be primarily processed in the right hemisphere. For each task the individual strength and direction of hemispheric lateralization was assessed by using functional transcranial Doppler sonography (fTCD). For each subject (36 right-handed, 35 nonright-handed) performance was measured in the two single-tasks and in the dual-task condition. On average, subjects showed a left hemisphere bias for the word generation task, a right hemisphere bias for the driving task and dual-task interference. Within subjects, lateralization of language and driving were statistically independent. In accordance with earlier studies, the results show no indication of a positive effect of strength of lateralization on performance in single-tasks or dual-task efficiency. We also found no advantage of a typical compared to an atypical or a contralateral compared to an ipsilateral lateralization pattern. In right-handers, but not in nonright-handers, we even found a negative relationship between strength of lateralization and dual-task efficiency for atypically lateralized subjects. This further supports the suggestion that lateralization does not enhance cognitive performance in humans.