Dear Editor:

As a clinical neurologist with interest in laterality of motor control, I read the recent article by Mutha and colleagues with interest [1]. To substantiate the existence of bi-hemispheric control for moving either arm as claimed, the authors must verifiably explain occurrences of well known laterality indexed syndromes seen at the bedside. Three of these are the syndromes of post-operative cognitive decline and epilepsy (occurring exclusively after damage to the dominant hemisphere) and the syndrome of conjugate eye deviation away from the paralysis and toward the damaged hemisphere (occurring exclusively in lesions affecting the nondominant hemisphere) [2-6]. The laterality indexed syndrome of ipsilateral paralysis in lesions affecting the major hemisphere in the absence of Kernohan-Woltman notch constitutes another diaschitic presentation with a similar message, i.e. motor communication between two hemispheres is one-way (from the dominant to the nondominant, via anterior commissure and the corpus callosum) and that this influence on the minor hemisphere is excitatory in nature [2, 5-7]. Bimanual simultaneous drawing is the most accurate, natural and readily available method to demonstrate the above understanding [7, 8]. Thus, when drawing two lines at the same time, the hand ipsilateral to the major hemisphere operates at a constant delay (equal to the interhemispheric transfer time) resulting in a shorter and less linear line (or smaller boxes with crooked angles) compared to the hand contralateral to the dominant hemisphere as the hands are moved simultaneously at any direction with a pencil held in each hand (documenting those movements). A lesion of anterior callosum affects the performances of the nondominant hand alone as shown by Preilowski [9] and Eliassen et al [10]. The imperfect linearity of the nondominant hand in this test reflects the loss of integrity (degrading) of the signal issued in the major hemisphere for moving the nondominant side of the body (including movements of diaphragms and saccades) while dual tasking affects the performance of the dominant hand more than it does the nondominant [6, 11].
On the sensory side, bilateral occurrence of somatosensory evoked potential upon stimulating the nondominant hand and the bilateral loss of the same signal upon stimulation of the median nerve on the nondominant side as seen in lesions affecting the minor hemisphere has been documented long ago [5, 12]. That the incidence of any of the above-mentioned syndromes does not exceed 50 % corroborates the binomial status of the human brain, with directionality in callosal traffic underpinning its presence.

References:

1. Mutha PK, Haaland KY, Sainburg RL. Rethinking motor lateralization: specialized but complementary mechanisms for motor control of each arm. PLoS One 2013; 8:e58582.

2. Derakhshan I. Letter by Derakhshan regarding article, "intraoperative magnesium administration does not improve neurocognitive function after cardiac surgery". Stroke 2014; 45.

3. Mathew JP, Laskowitz DT, Newman MF. Response to letter regarding article, "intraoperative magnesium administration does not improve neurocognitive function after cardiac surgery". Stroke 2014; 45: e45.


4. Derakhshan I. Anatomy of handedness and the laterality of seizure onset: surgical implications of new understandings in motor control. Neurol Res 2005; 27:773-779.

5. Derakhshan I. Laterality of seizure onset and the simple reaction time: revamping the Poffenberger's paradigm for seizure surgery. Neurol Res 2006; 28: 777-784.

6. Becker E, Karnath HO. Neuroimaging of eye position reveals spatial neglect. Brain 2010; 133: 909-914.

7. Derakhshan I. Right sided weakness with right subdural hematoma: motor deafferentation of left hemisphere resulted in paralysis of the right side. Brain Inj 2009; 23: 770-774.

8. Derakhshan I. Bimanual simultaneous movements and hemispheric dominance:
Timing of events reveals hard-wired circuitry for action, speech, and imagination. Psychol Res Behav Manag 2008; 1: 1-9.

9. Preilowski BF. Possible contribution of the anterior forebrain commissures to bilateral motor coordination. Neuropsychologia 1972; 10: 267-277.

10. Eliassen JC, Baynes K, Gazzaniga MS. Direction information coordinated via the posterior third of the corpus callosum during bimanual movements. Exp Brain Res 1999; 128: 573-577 (Figure 1B).

11. Hiscock M. Verbal-manual time sharing in children as a function of task priority. Brain Cogn 1982; 1:119-131.

12. Green JB, Hamilton WJ. Anosognosia for hemiplegia: somatosensory evoked potential studies. Neurology 1976; 26:1141-1144.