Asymmetric interference between concurrent tasks: an evaluation of competing explanatory models.

According to Kinsbourne's functional cerebral distance model, asymmetric interference between concurrent cognitive and manual tasks reflects the functional specialization of the cerebral hemispheres. However, alternative explanations include initial-values artifact (statistical bias model), motor asymmetry (manual dominance model), and a combination of functional cerebral distance and manual dominance (two-factor model). We evaluated the competing models in four experiments with right- and left-handed university students for whom manual dominance was in effect reversed by requiring the dominant hand to perform the more difficult manual task. The cognitive load of the nonmanual task was varied within each experiment. The results did not support any of the models but, instead, reflected only tradeoffs between manual and nonmanual performance. The primary implications for future research are that performance on both tasks must be measured, and each task must be sensitive to interference from the other task, if observed asymmetries are to be interpretable.