Magneto-encephalographic correlates of the lateralized readiness potential

To determine the onset of movement-related EEG activity accompanying stimul us-induced movements, it is commonly isolated from overlapping stimulus-rel ated activity by a subtraction procedure, yielding the lateralized readines s potential (LRP). In order to elucidate the generation of the LRP and to e xplore whether magnetoencephalographic (MEG) measures have advantages over the LRP as a measure of response selection, MEG activity was recorded in fo ur healthy adults during self-paced and stimulus-induced hand movements. Se lf-paced movements were preceded by readiness fields in all subjects, expla ined by sources in contralateral and (for 2/8 response sides) also ipsilate ral hemispheres. Movement-related activity preceding stimulus-induced movem ents could only be modeled adequately when stimulus-related activity was re moved by subtracting MEG signals for left and right hand movements. Thus id entified source locations showed no systematic deviation from the sources f or readiness fields, supporting a generation of the movement-related activi ty in primary motor cortex. The corresponding source waveforms allowed late ncy determinations of motor cortex activity as markers for response-choice timing. MEG thus provides information on the time course of hand-specific m otor cortex activation for each hemisphere separately, where the electro-en cephalographic LRP provides a composite measure for both hemispheres. (C) 1 999 Elsevier Science B.V. All rights reserved.