The goal of this work is to dilucidate the mechanism that yields the associated EMG responses in patients with probable Alzheimer's disease. Seventeen subjects fulfilling this criteria and ten normal subjects as a control group were selected. They were asked to perform an oddball task in which they had to discriminate between two kinds of stimuli and produce a response to one of them. Results show that reaction times in controls and in patients when there is no associated EMG are equal in latency. When the associated EMG response occurs in the pathological group reaction time is significantly increased. Furthermore, the associated EMG response is delayed in 55 milliseconds when compared to the voluntary movement. Although these results may reflect some controversy, we interpret this finding as a contralateral inhibitory deficit associated to the corpus callosum abnormalities that occur in the evolving course of Alzheimer's disease.