Inhibitory deficits in probable Alzheimer's disease. A study of movement related potentials and EMG response

We have recently reported contralateral associated EMG responses to voluntary hand movement in Alzheimer's disease. Several aspects of this process were not fully explained in our last paper. In the present one we present data on the register of movement-related potentials (Negative Shift, NS) and Lateralized Readiness Potential (LRP), which have shown a very fine capacity to reveal processes that occur in the motor cortex while movement execution is being prepared. The associated EMG responses (so called by us) have almost all the characteristics of the partial errors revealed by cognitive psychology. First, it is a covert response, so it can only be detected by EMG recording; second, the appearance of this partial error changes the reaction time in the same way as described by Coles: mainly, by increasing reaction time as compared with clear responses. Nevertheless, contrary to partial errors, the associated EMG response does not constantly appear before the correct response. Associated EMG responses always appear after correct responses, with a constant delay of 54 +/- 28 ms. Our results show also an incorrect response preparation related to associated EMG response. We interpreted this specific feature in relation to inhibitory deficits in motor cortex and associated callosal pathways that avoid a correct response performance in Alzheimer patients.