Differential effects of unilateral magnetic cortical stimulation on reaction time

Data from patients with brain lesions suggested that the right hemisphere is involved in the intention of simple movements, while the left is involved in more complex tasks. The contributions of each hemisphere to a reaction time (RT) task were assessed with cortical magnetic stimulation in five healthy right-handed subjects. Subjects were asked to push buttons with both hands as fast as possible after a visual start stimulus. At three different delays (25, 50 and 75 ms) after the start signal, a magnetic stimulus of 20, 40 or 60% of maximum intensity was given to either the right or the left hemisphere. Delay, intensity and side of stimulation varied in random order. Repeated measures analysis of variance showed two main effects: firstly, RT was longer on the body side innervated by the stimulated than by the non-stimulated hemisphere. Thus, cortical stimulation delayed the execution of a motor task, as shown previously. Secondly, there was an interaction between side of stimulation and delay of the cortical stimulus. At a delay of 25 ms, right-sided stimulation resulted in longer RTs than left-sided stimulation. At delays of 50 and 75 ms, the reverse proved true. In both cases the effect held for both hands. According to these results, the right hemisphere is predominantly involved in the early phases of an RT task, while the left hemisphere is more involved in later phases of processing. The results show that cortical magnetic stimulation can be used to investigate differential contributions of the hemispheres to motor tasks in vivo.