Influence of task complexity on manual asymmetries.

The degree of manual asymmetry is generally assumed to vary with task complexity. However, task complexity as a factor in manual asymmetries has rarely been examined directly. Further, the results of psychophysical studies indicate that manual asymmetry increases with task complexity, while physiological studies consistently report a reduction of manual asymmetries in more complex tasks. The use of different tasks (rather than different complexity levels within a given task) in many psychophysical studies might result in this inconsistency. This study investigated the influence of task complexity on manual asymmetries in 70 right-handed subjects. We used three complexity levels within a finger-tapping paradigm. A strong advantage of the preferred hand was particularly pronounced in the simple finger-tapping task. When the task was more complex, the advantage of the preferred hand, and thus, the manual asymmetry significantly decreased or disappeared. These results support previous suggestions that simple motor tasks involve localised neural networks confined to one cerebral hemisphere, while complex motor tasks are controlled by more widely distributed neuronal assemblies that involve both hemispheres. However, the influence of task complexity on manual asymmetry seems not to be monotonic.