Laterality of the Alien Hand: Directionality of Callosal Traffic Underpins Neural Handedness

Whereas the relationship of handedness to the ability to talk has been known to the ancients (Psalm 137, for example) the exact knowledge of anatomical underpinnings of this relationship at the public level is a new understanding. This article pursues another corollary of the anatomy sustaining laterality of motor control, i.e. the variability of the alien/unruly hand in patients who have suffered injury to the largest tract connecting the two hemispheres, i.e. the corpus callosum. It is shown that the lateralities of the command center (major hemisphere) and that of the alien hand always coincide, despite the apparent variability of the affected hand in such events (i.e. right hand of left hand in different right handed patients). According to the newly discovered circuitry, the hard-wired directionality in callosal traffic (neural handedness) determines the laterality of the alien/unruly hand after callosal injury, rather than the avowed handedness of the subject (behavioral handedness). The discrepancy between neural and behavioral handedness is explained clinically and physiologically. The latter provides for a noninvasive and inexpensive determination of neural handedness of the subject, i.e. the simple reaction time of two symmetrically located effectors in the body. The side with the longer reaction time is seated below the major hemisphere.