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ARTICLES: Alonso R. Riestra, Kyle B. Womack, Gregory P. Crucian, and Kenneth M. Heilman Is the middle between both halves?: Midpoint location and segment size estimation in neglect Neurology 2002; 59: 1580-1584 [Abstract] [Full text] [PDF]

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Is the middle between both halves?: Midpoint location and segment size estimation in neglect

I Derakhshan   (22 January 2003)

Reply to Letter to the Editor

Alonso Riestra   (22 January 2003)

Is the middle between both halves?: Midpoint location and segment size estimation in neglect 22 January 2003
I Derakhshan, Formerly, Associate Professor of Neurology Cincinnati and Case Western Reserve University Charleston WV Send Correspondence to journal: Re: Is the middle between both halves?: Midpoint location and segment size estimation in neglect idneuro{at}hotmail.com I Derakhshan	In answering the question of Riestra et al in “Is the middle between both halves?” [1] it is incorrect to resort to semantics or philosophical deliberations, as the title and the discussion of the paper implies. The issue is biological in nature, and there is enough data in the literature to provide (at least partial) answer to their enquiry, as follows: It is known that the (visual) neglect in such syndromes is ameliorated with activity involving nondominant effectors and may worsen by the activity of an effector on the dominant side. [2] The review just cited contains an interesting exception in a left-hander in whom it was moving the right hand that ameliorated the neglect. [3] I have given the account of the role of the callosum in mediating these remedies, and its connection to movement control in human (as codified in handedness) elsewhere. [4] I would now address what seems to be a new observation the novelty of which have escaped the attention of the authors, because they are following a conventional dogma regarding movement control in human (see below): The right handed patients did better when turning their eyes to the left of the midline than they did when turning them to the right (in performing according to instructions). According to one-way callosal traffic theory referred to above, it is this movement that ameliorated the patients neglect as reflected in their performance, as recorded. Such nondominant activities energizes the minor hemisphere (via the callosum) making it active and functional so the patient can see the things on the neglected side once again, [5] albeit temporarily. Direct experimental support for the above is plenty, but I will suffice with two. In a paradigm exploiting TMS, Semmler and Nordstrom [6] documented increased amplitude of MEP of the active nondominant hand when stimulating the minor hemisphere. This occurred only with (increasing) activity but not at rest. The MEP of the dominant hand did not show such increased excitability as the major hemisphere was stimulated. In another paradigm that used fMRI, Baker et al documented continuous engagement of the major motor cortex for the eye regardless of the laterality of gaze. The minor cortex for eye was active only when looking to the left. [7] These observations and at least part of that reported by Riestra et al are readily explained by the theory mentioned above, where the doctrine of contralaterality remains silent as to their occurrence. References: 1.Riestra AR, Womack KB, Crucian GP, Heilman KM. Is the middle between both halves? Midpoint location and segment size estimation in neglect. Neurology 2002;59:1580-1584. 2. Pierce SR, Buxbaum LJ. Treatment of unilateral neglect: A review. Arch Phys Med Rehabil 2002;83:256-268. 3. Humphreys GW, RIddoch MJ. Detection by action: Neuropsychological evidence for action-defined templates in search. Nat Neurosci 2001;4:84-88. 4.Derakhshan I. Conflict and integration of spatial attention between disconnected hemispheres. J Neurol Neurosurg Psychiatry 2003; (In press) 5.Derakhshan I. In defense of the sinistral: Anatomy of handedness and the safety of prenatal ultrasound. Ultrasound Obstet Gynecol 2003; (In press) 6.Semmler JG, Nordstrom MA. Hemispheric differences in motor cortex excitability during a simple index finger abduction task in humans. J Neurophysiol 1998;79:1246-1254. 7.Baker JT, Donoghue JP, Sanes JN. Gaze direction modulates finger movement activation patterns in human cerebral cortex. J Neurosci 1999;19:10044-10052.
Reply to Letter to the Editor 22 January 2003
Alonso Riestra Mexican Institute of Neurosciences Send Correspondence to journal: Re: Reply to Letter to the Editor alonso_riestra{at}intramedios.com.mx Alonso Riestra	We appreciate Dr. Derakhshan’s interest in our paper. We hope he finds the following remarks helpful. It is commonly believed that in order to calculate the middle of a line a subject must compare the size of both sides of the line thus; the spatial bias in line bisection in neglect must be the result of perceptual size underestimation of the neglected side. This approach overlooks the fact that before finding the middle of a line there are not well-defined line segments to be compared and that the subject must find the point that defines this segments before comparing their size. In our paper we proposed that during line bisection, finding the midpoint of a line must be, at least initially in the process, independent from calculating the size of two equal segments or halves. Far from philosophical deliberations, the rhetorical suggestion of a paradox in the title only intends to make it interesting to the reader. Our experimental approach in subjects with brain lesions obeys to the biological nature of this question. Dr. Derakhshan comments on interhemispheric activation for motor control are consistent with but tangential to the content of our paper because our experiments explored perceptual not motor functions. The only motor demands were those related to visual search and, as we wrote, our findings cannot be explained by deficits in visual exploration or hemianopia. Dr. Derakhshan’s proposal that our subjects did better locating the mark when it was on the left because there was a stronger activation of the right hemisphere cannot be sustained in the context that both our patients had left hemianopia. Arguing that this right hemispheric activation came from the left hemisphere through the callosum does not explain why this same mechanism did not take place when the mark was on the right. Dr. Derakhshan’s comments do not provide additional information or explain better the main findings of our experiments, namely the spatial bias only in the where task and the dissociation of where determinations and line size comparisons in our subjects with neglect.