Letter to the editor

To the Editor: The recent article by Diedrichsen et al. (2003) mentions the presence of ipsilateral corticospinal innervation in humans on several occasions (for example in p. 2416) despite the ubiquitous occurrence of onset asynchrony (of 150 ms) in their controls—leaving the functional status (relevance) of such fibers (if they exist) in limbo. Neither have they committed themselves as to the reason for the onset asynchrony so nicely depicted in Fig. 8 of the article. The reason, I believe, is paucity of clinical perspective in such studies as follows: the essential feature requiring an explanation, not forthcoming from the doctrine of contralateral innervation (followed by the authors), is the asymmetry of signs and symptoms in patients with comparable lesions affecting the major and minor hemispheres. It is only natural to wonder if such an asymmetry is related to the persistent asymmetry of the reaction time documented in the control subjects of their study. Elsewhere I have shown in detail that the two are related and that the link is coded in the (neural) handedness of humans (Derakhshan 2003a,b). Thus the reason for the inescapable onset asynchrony of their subjects is the callosum-length proximity of the command center to the dominant hand of their subjects as compared with the nondominant, which is connected to the same command via the corpus callosum. This nondominant delay is the interhemispheric transfer time, which varies with the “tempo” of the activity; a fact known to musicologists as the melody lead of the right hand (in right-handed pianists) (Vernon 1936). Liepmann depicted this callosally mediated relationship (delay) in two of his articles, carrying the “ movement formula” from the major to the minor hemisphere (Liepmann 1900, 1920) (Fig. 1). In summary, the anatomy I have described (as a clinician) seems to be the explanation of what Hazeltine colleagues have recently characterized as “a major limitation of human performance in the ability to initiatetwo goal directed behaviors at the same time” (Hazeltine et al. 2003). Others have referred to the same as“the impossibility of humans generating multiple independent, unsynchronized parallel action plans for the two hands” (Laeng and Park 1999) or have simply called it “synchronization error” (Kristeva and Deecke 1979).