Finger-tapping interference as produced by concurrent verbal and nonverbal tasks: an analysis of individual differences in left-handers.

Verbal cerebral asymmetry was examined in 96 right- and left-handed college students using dual-task methods. The manual task was finger-tapping; verbal tasks were anagram-solution in silent and aloud conditions; nonverbal tasks were solution of Raven's Progressive Matrices and forms memory. Both handedness groups were sensitive to the task manipulation, i.e., verbal tasks produced a significant change in tapping rate from a no-change rate of zero; nonverbal tasks produced no significant change in tapping rate. Right-handers experienced the expected asymmetric manual performance indicative of left-hemisphere language laterality. Left-handers as a group experienced no asymmetric pattern but rather a percentage decrease in both hands which suggested bilateralization of language functions. Only when examining manual performance patterns within specific left-handed subpopulations (composed of a complete crossing of handwriting posture, familial sinistrality, and sex) did laterality patterns emerge. Specifically, asymmetric manual performance was found only in familial sinistral inverted posture females. Results demonstrate the advantage of controlling for subject characteristics when assessing cerebral laterality in left-handers.