Combined analysis of language tasks in fMRI improves assessment of hemispheric dominance for language functions in individual subjects.

Recent advances in functional neuroimaging techniques have prompted an increase in the number of studies investigating lateralization of language functions. One of the problems in relating findings of various studies to one another is the diversity of reported results. This may be due to differences in the tasks that are used to stimulate language processing regions and in the control tasks, as well as differences in the way imaging data are analyzed,in particular the threshold for significance of signal change. We present a simple method to assess language lateralization that allows for some variation of tasks and statistical thresholding, but at the same time yields reliable and reproducible results. Images acquired during a set of word-comprehension and -production tasks are analyzed conjointly. As opposed to the use of any one particular task, this combined task analysis (CTA) approach is geared toward identifying language regions that are involved in generic language functions rather than regions that are involved in functions that are specific to a single task. In two experiments CTA is compared to single-task analysis in healthy right-handed males. In a third experiment left-handed males were examined. Results indicate that CTA: (1) improves detection of language-related brain activity in individual subjects and (2) yields a high language laterality index (LI) in right-handed males with a small variance across subjects. The high LI matches the strong left-hemisphere dominance for language that is typical for these subjects as reported in neuropsychological and clinical tests in other studies. In the left-handed subjects dominance was found either in the left (n = 4) or the right (n = 1) hemisphere or was absent (n = 3). The LI derived from CTA is more consistent across statistical thresholds for significance of signal change in fMRI analysis than in individual-task analysis. Also, the CTA results are very similar to those obtained with conjunction analysis of the same data.