An Exchange on Franz, Rowse, and Ballantine (2002)

Handedness has been characterized as the clearest example of huinan behavioral lateralization (Dassonville, Zhu, Ugurbil, Kim, & Ashe, 1997). In the study of Dassonville and colleagueh, the volume of activation in the contralateral motor cortex was consistently greater during movements of the domirrant hand than during those of the nondominant hand (in both rightand left-handers). In another study, White et al. ( 1997’) performed a detailed light-microscopic morphological evuluation and found “that the preferred use of the right hand In humans occurs without a gross lateral asymmetry in the primary sensory motor system” (p. 1047). Furthermore, in a pertinent review of the issue, Beltramello et al. (1998) correcqtly suggested that the bilateral cortical activation that occur’r when individuals move the nondominant side, which has been demonstrated in emission imaging studies, “has something to do with the use of the nondominant left hand” (pp 261-283), although they offered no explanation for that statenlent. The question is whether the three classes of observations just exemplified can be reconciled. With the anatomy of handedness now securely at hand, the awwer is affirmative (Derakhshan, 2003e). Thus, when the miiin structural characteristic of handedness, that is, the tempcwal precedence of the dominant hand in simultaneous birnatrual activity, is questioned (Franz, Rowse, & Ballantine, 2002), it is noteworthy. I therefore welcome the opportunity to explore the laterality aspects of the observations of FranL and her colleagues in light of the new discovery. Removing the doubts they have raised will provide further suppon for the validity of my claim that it is the vectorial nature of callosal traflic that underpins lateralities of movement control in humans (i.e., the one-way callosal traffic theory ) . After a brief overview of the novel scheme, and using the insights gained from it, I will provide an explanation for the negative findings of Franz et al. (2002) concerning the precedence of the dominant hand, reconciling their data (to the extent possible) with the earlier observations they had set out to examine. Furthermore, I present data here indicating that their use of self-declared or inventory-driven methtrds for determining laterality in their participants partly accounts for the results they obtained. At the same time, I examine the role of the callosum in the inferior performance of the nondominant hand documented in their study. Then, I briefly point out that the same one-way scheme seems to be operative in the sensory realm, although in the opposite direction. The directionality of callosal traffic underpins the unity of experience we all have in our seamless appreciation of the world and in our reacting to it as individuals who possess one brain with two hemispheres. According to the theory under scrutiny, handedness is a reflection of (a code for) the proximity of all effectors on the dominant side of the body to the command center in the major hemisphere, by a callosum length. The neuronal aggregate devoted to voluntary actions is divided between the two hemispheres. The nature of the controlling influence exerted by the moiety (of that devoted ensemble) housed in the major hemisphere on its counterpart housed in the minor hemisphere is excitatory. Any interruption of that callosally mediated influence (e.g., whether chemical as in the Wada test, iatrogenic, traumatic, electrical, or vascular; Loring, Meador, & Lee 1994) will result in diaschitic symptoms, crystallized in the following clinical syndromes: (a) the garden-variety syndrome of contralateral paresis and ipsilateral apraxia (with or without aphasia) in lesions of the motor cortex of the major hemisphere; (b) lateralityindexed weakness or apraxia of nondominant limbs in lesions affecting the callosum (e.g. in Marchiafava-Bignami disease); (c) ipsilateral paresis affecting the limbs located beneath the cortical lesion involving the major hemisphere; (d) callosally mediated temporal improvement of the functioning of the nondominant hand in supracapsular lesions of the major hemisphere as the lesion heals, as was quantified recently (Derakhshan, 2003f); and (e) the callosally mediated improvement of the effects of neglect of the left side in right parietal lesions upon moving any of the effectors on the left side-improvement brought about by the activation of the minor cortex that results in turn from the activating influences of the command center on the left hemisphere and that is transferred to the right hemisphere via an extant callosum. The last two syndromes have come to light only recently as a result of the theory under examination (Derakhshan, 2003a, 2003d, 20030. By now it should be quite clear that it is the anatomy just described, that is, the topology (the linking of left and right) of the major and minor moieties of the neuronal aggregate devoted to voluntary activity, that underpins the laterality of movement control in humans, with the laterality of the dominant hand being a code for the directionality just summarized. Some very able clinicians are apparently unable to grasp that fact, obviously because of the difficulty of overcoming the didactic and