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 incr ease in the number of studies investigating lateralization of language func tions. One of the problems in relating findings of various studies to one a nother 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 anal yzed, in particular the threshold for significance of signal change. We pre sent a simple method to assess language lateralization that allows for some variation of tasks and statistical thresholding, but at the same time yiel ds 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) appro ach is geared toward identifying language regions that are involved in gene ric language functions rather than regions that are involved in functions t hat are specific to a single task. In two experiments CTA is compared to si ngle-task analysis in healthy right-handed males. In a third experiment lef t-handed males were examined. Results indicate that CTA: (1) improves detec tion of language-related brain activity in individual subjects and (2) yiel ds a high language laterality index (LI) in right-handed males with a small variance across subjects. The high LI matches the strong left-hemisphere d ominance for language that is typical for these subjects as reported in neu ropsychological and clinical tests in other studies. In the left-handed sub jects dominance was found either in the left (n = 4) or the right (n = 1) h emisphere or was absent (n = 3). The LI derived from CTA is more consistent across statistical thresholds for significance of signal change in fMRI an alysis than in individual-task analysis. Also, the CTA results are very sim ilar to those obtained with conjunction analysis of the same data. (C) 2001 Academic Press.