Self-initiated versus externally triggered movements. I. An investigation using measurement of regional cerebral blood flow with PET and movement-related potentials in normal and Parkinson's disease subjects

We have read with interest the article, 'Self-initiated versus externally triggered movements I. An investigation using measurement of regional cerebral blood flow with PET and movement-related potentials in normal and Parkinson's disease subjects' by Jahanshahi et al. (1995). The authors argue that their results substantiate the assumption that the supplementary motor area (SMA) contributes to the 'early' Bereitschaftspotential. We wish to raise some methodological points and propose a different interpretation of the results. (i) The term 'Bereitschaftspotential' should be used in an unequivocal sense. It was originally used for a parietally and centrally negative, frontally positive scalp potential appearing before self-initiated movements (Kornhuber and Deecke, 1964; Deecke et al, 1969). Therefore it is confusing as to why the authors also applied it to the electrical potentials recorded in advance of externally triggered movements. These potentials are not movement-specific since they can also be recorded in advance of an anticipated stimulus and were named contingent negative variation (Walter et al., 1964). (ii) The argument of the authors is based upon the statistical evaluation of the amplitude of the 'early' Bereitschaftspotential. For several reasons this is a questionable procedure. First, the division of the Bereitschaftspotential into an 'early' and a Mate' phase seems to be self-evident considering the different steepness of this potential during these two epochs. However, there is no evidence that different physiological processes or different brain areas cause these two epochs of the Bereitschaftspotential. Microelectrode studies (Wiesendanger and Wise, 1992) and intracranial recordings in humans (Ikeda et al., 1992) showed no difference in timing of pre-movement activity in different motor areas. The steepness of the two postulated parts of the Bereitschaftspotential are significantly correlated at various scalp electrodes (Botzel et al., 1993) suggesting a common origin of these potentials. As an alternative to the arbitrary distinction of 'early' and 'late', the time course of the Bereitschaftspotential can be conceived as an exponential