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This study investigated characteristics of motor cortex activation and response generation in Parkinson's disease with measures of electrocortical activity (lateralized readiness potential [LRP]), electromyographic activity (EMG), and isometric force in a noise-compatibility task. When presented with stimuli consisting of incompatible target and distractor elements asking for responses of opposite hands, patients were less able than control subjects to suppress activation of the motor cortex controlling the wrong response hand. This was manifested in the pattern of reaction times and in an incorrect lateralization of the LRP. Onset latency and rise time of the LRP did not differ between patients and control subjects, but EMG and response force developed more slowly in patients. Moreover, in patients but not in control subjects, the rate of development of EMG and response force decreased as reaction time increased. We hypothesize that this dissociation between electrocortical activity and peripheral measures in Parkinson's disease is the result of changes in motor cortex function that alter the relation between signal-related and movement-related neural activity in the motor cortex. In the LRP, this altered balance may obscure an abnormal development of movement-related neural activity.

The aim of this study was to investigate the neurophysiological mechanisms underlying Parkinson's disease patients' increased reliance on external cues for the initiation of movement. Lateralized movement-related cortical potentials were recorded in a noise-compatibility task with seven patients and seven age-matched control subjects. In this two-choice task, visual stimuli containing incompatible target and distractor elements, which simultaneously instructed for responses from both hands, initially caused activation of the motor cortex controlling the wrong response hand. The incorrect response activation was of higher amplitude in patients than in control subjects, causing a longer response delay relative to response times when target and distractors instructed the same hand. In addition, hand-specific motor cortex activation started earlier in patients than in control subjects. These results indicate that visual stimuli exerted an earlier and stronger influence on movement initiation in patients than in control subjects. We hypothesize that information from sensory stimuli relevant for the generation of a response can have rapid access to motor structures in Parkinson's disease patients, thereby facilitating the initiation of movement. The findings may reflect a compensatory mechanism, but could also be related to excitability changes in the motor cortex intrinsic to the pathophysiology of Parkinson's disease.

In choice reaction tasks, subjects typically respond faster when the relative spatial positions of stimulus and response match than when they do not match. A prominent explanation attributes this 'Simon effect' to automatic response activation elicited by spatial correspondence, which facilitates or competes with the controlled selection of the response demanded by the stimulus. To test this account, we applied repetitive transcranial magnetic stimulation (rTMS) on the dorsal premotor cortex (PMd), as this area may subserve the inhibitory control of automatic response activation. Temporary interference with PMd was predicted to release the automatic activation from inhibition and thereby enhance the Simon effect. The results confirmed a modulation for trials following an incompatible trial, providing new evidence for competition between automatic and controlled response activation as a mechanism underlying the Simon effect.

To determine the onset of movement-related EEG activity accompanying stimulus-induced movements, it is commonly isolated from overlapping stimulus-related activity by a subtraction procedure, yielding the lateralized readiness potential (LRP). In order to elucidate the generation of the LRP and to explore whether magnetoencephalographic (MEG) measures have advantages over the LRP as a measure of response selection, MEG activity was recorded in four healthy adults during self-paced and stimulus-induced hand movements. Self-paced movements were preceded by readiness fields in all subjects, explained by sources in contralateral and (for 2/8 response sides) also ipsilateral hemispheres. Movement-related activity preceding stimulus-induced movements could only be modeled adequately when stimulus-related activity was removed by subtracting MEG signals for left and right hand movements. Thus identified source locations showed no systematic deviation from the sources for readiness fields, supporting a generation of the movement-related activity in primary motor cortex. The corresponding source waveforms allowed latency determinations of motor cortex activity as markers for response-choice timing. MEG thus provides information on the time course of hand-specific motor cortex activation for each hemisphere separately, where the electro-encephalographic LRP provides a composite measure for both hemispheres.

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