Why nondominant hand movements cause bilateral cortical activation in emission imaging.

Abstract

To the Editor: The contribution of Kato et al1 contains important laterality related data. But the respected authors resort to undocumented and unwarranted assertions from the literature that one must address in order to arrive at a cogent interpretation of their data. They used the 1963 article of Nyberg-Hensen and Rinvik2 to support the existence of 10% to 15% uncrossed pyramidal fibers in humans. This article, which is often used for this very purpose, states that “the only safe conclusion to be drawn from the available data is that there may probably be considerable variation with regard to the proportion of crossed and uncrossed corticospinal fibers in man,” never offering or referring to such anatomical documentation in humans as asserted by Kato et al. On the other hand, the current techniques of cortical mapping with sufficient temporal resolution employing electroencephalography, magnetoencephalography, and transcranial magnetic stimulation (TMS) all have demonstrated sequential activation of the major followed by the minor hemisphere on moving the nondominant hand (see below). This temporal feature of bimanual coordination in humans translates into such daily life experiences as (1)) the double-click heard with snapping one’s fingers of both hands simultaneously (Derakhshan, unpublished data), (2) the melody lead of the right hand in piano playing3 (known to musicologists for 160 years), and (3) the precedence of the bowing hand to the fingering in violin playing, recently documented by Wiesendanger et al.4 This callosally mediated delay of 10 to 40 ms involving the nondominant hand requires an anatomical explanation not forthcoming from the (unmodified) …