Dual-task interference and the cerebral hemispheres.

It has been argued that dual-task interference is reduced when the two cerebral hemispheres can carry out the two tasks independently. Evidence for this idea has arisen from studies involving manipulations such as lateralized stimulation or response, or requiring mental operations believed to depend primarily on a particular hemisphere. However, these studies have typically involved a very limited degree of response uncertainty, which appears critical in producing the most extreme dual-task interference (the psychological refractory effect). Pairs of tasks with independent response uncertainty were examined, and various manipulations were used to promote hemispheric task separation. Dual-task interference was not modulated by these manipulations. It seems likely that response selection represents a central bottleneck, in the sense that this process cannot operate simultaneously and independently in the two hemispheres.