Neurology -- Correspondence for Oliveri et al., 63 (11) 2139-2141

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Correspondence published:

Reply to Derakhshan: Massimiliano Oliveri (31 January 2005)

Overestimation of numerical distances in the left side of space: Iraj Derakhshan (31 January 2005)

Reply to Derakhshan 31 January 2005

Massimiliano Oliveri,
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Re: Reply to Derakhshan

maxoliveri{at}tiscali.it Massimiliano Oliveri

We thank Dr. Derakhshan for the intriguing comments about a putative influence of temporal factors in spatial biases observed in healthy subjects during a numerical distance judgement task. [1] Based on the hypothesis that motor and sensory activations of the non-dominant side involve a sequential activation from the dominant to the non-dominant hemisphere mediated by the corpus callosum [2], the suggestion is made that the longer interhemispheric transfer time (IHTT) in moving eyes to the left vs. right accounts for a leftward spatial bias (i.e. pseudoneglect).
We agree that the corpus callosum plays a critical role in the distribution of spatial attention. Related to this is the improvement of neglect syndrome following TMS of the left (“healthy”) hemisphere, which could reflect a transcallosal TMS-disinhibition of the right (“affected”) hemisphere. [3-5] However, we did not comment on any role of the corpus callosum on the leftward bias in spatial imagery observed in healthy subjects. In fact, we think that the main factor inducing this behavior is the right hemispheric dominance in performing spatial computations. This hemispheric asymmetry could explain a left-sided bias either for physical stimuli in the environment or for spatial images activated in the brain. [6] Other data confirm this interpretation.
First, the finding of non-significant effects of the hand used in the numerical distance judgement task does not support an exclusive role of sequential activation from the minor to the dominant hemisphere in making leftward spatial biases.
Second, Derakhshan’s hypothesis does not perfectly fit with cases of pseudoneglect reported with visual stimuli presented for intervals shorter than the saccadic onset time. [7] On the other hand, one could argue that a longer IHTT also contributes to covert orienting of attention towards the left side.
Finally, given the power of rTMS to interfere with transcallosal transmission [8], according to Derakhshan’s hypothesis left hemispheric rTMS should have even emphasized the IHTT from the left to the right hemisphere, leading to an increase of pseudoneglect. On the other hand, right parietal rTMS is likely to slow saccadic onset time and this would have led to a lengthening of leftward eye movements and so to a magnification rather than cancellation of pseudoneglect.
The lack of significant effects of left parietal together with the inhibitory effects of right parietal rTMS on pseudoneglect militate against an explanation of an exclusive role of corpus callosum in mediating this behavior.
References
1. Oliveri M, Rausei V, Koch G, Torriero S, Turriziani P, Caltagirone C. Overestimation of numerical distances in the left side of space. Neurology 2004; 63:2139-2141.
2. Derakhshan I, Ishiai S, Koyama Y, Furuya T. Conflict and integration of spatial attention between disconnected hemispheres. J Neurol Neurosurg Psychiatry 2003; 74:395.
3. Oliveri M, Rossini PM, Traversa R, Cicinelli P, Palmieri MG, Pasqualetti P, Tomaiuolo F and Caltagirone C. Left frontal transcranial magnetic stimulation reduces contralesional extinction in patients with unilateral right brain damage. Brain 1999; 122: 1731-1739.
4. Oliveri M, Rossini PM, Filippi MM, Traversa R, Cicinelli P, Palmieri MG, Pasqualetti P and Caltagirone C. Time-dependent activation of parieto- frontal networks for directing attention to tactile space. Brain 2000; 123: 1939-1947.
5. Oliveri M, Bisiach E, Brighina F, Piazza A, La Bua V, Buffa D, Fierro B. rTMS of the unaffected hemisphere transiently reduces contralesional visuospatial hemineglect. Neurology 2001; 57(7): 1338-40.
6. Heilman KM, Jeong Y and Finney GR. Spatial cognition. Neurology 2004; 63: 1994-1996.
7. Fierro B, Brighina F, Oliveri M, Piazza A, La Bua V, Buffa D and Bisiach E. Contralateral neglect induced by right posterior parietal rTMS in healthy subjects. Neuroreport 2000; 11(7): 1519-1521.
8. Marzi CA, Miniussi C, Maravita A, Bertolasi L, Zanette G, Rothwell JC, Sanes JN. Transcranial magnetic stimulation selectively impairs interhemispheric transfer of visuo-motor information in humans. Exp Brain Res 1998;118(3): 435-438.

Overestimation of numerical distances in the left side of space 31 January 2005

Iraj Derakhshan,
Mimicking Man Manually, Inc.
415 Morris St. Suite 401, Charleston, WV 25301
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Re: Overestimation of numerical distances in the left side of space

idneuro{at}hotmail.com Iraj Derakhshan

Oliveri et al reported that space is overestimated on the left side when right-handed healthy subjects assessed “the distance between the middle number and that positioned at its left side.” [1] This “pseudoneglect” was absent after the right parietal lobe was temporarily decommissioned by rTMS. The possible mechanism suggested is “a physiologic tendency to overestimate left side internal spatial representations...” Alternatively, they suggested “the specific involvement of right parietal cortex.” However, both of these explanations are restating the problem they set out to answer. They also did not mention of the role of corpus callosum in pathological neglect or in health.
Clinically, the role of corpus callosum in mediating neglect is manifested by a critical negative observation, i.e. absence of any reports concerning the well known temporary improvement of neglect upon moving the nondominant side without an extant anterior callosum.[2] Experimentally, the role of callosum in handedness and simultaneity judgments is revealed in the mandate that the stimulation of the nondominant side precede that of the dominant by an amount commensurate to interhemispheric transfer time (IHTT).[3] The impossibility of touching the same surface simultaneouly with both hands (as in playing the keyboard,underpinning the melody lead of the right hand)completes the newly discovered callosal loop in interhemispheric transmission. Thus, moving and sensing the nondominant side of the body are bi-hemispheric events mediated by the corpus callosum. [3, 4]
In right handers, this means an IHTT-length (or callosum-width) delay in moving eyes to the left compared to moving them to the right as the signals for moving the eyes to the left are transmitted to the right hemisphere for that purpose. [4] As in callosal delay in simultaneity, this callosally mediated delay in moving the eyes towards the left appears to the subject (that resides exclusively in the command center) in a spatial dimension; just as it occurs in Pulfrich’s illusion, [3] where the temporal disparity of signal transmission in one eye is interpreted as spatial discrepancy, creating the stereoscopic effect.
In the present case, the laterality indexed difference in the conduction pathway length for moving the eyes to the left produced the overestimation reported in Table 1. A mirrored arrangement (i.e. right hemispheric dominance associated with crossed nonaphasia, including attentional and motor processes) have been documented in neural left handers, corroborating the above interpretation. [5]
References
1. Oliveri M, Rausei V, Koch G, Torriero S, Turriziani P, Caltagirone C. Overestimation of numerical distances in the left side of space. Neurology. 2004; 63:2139-2141.
2. Derakhshan I, Ishiai S, Koyama Y, Furuya T. Conflict and integration of spatial attention between disconnected hemispheres. J Neurol Neurosurg Psychiatry. 2003; 74:395.
3. Efron R. The effect of handedness on the perception of simultaneity and temporal order. Brain, 1963; 186: 261-300. (pp 293, 299)
4. Derakhshan I. Handedness and macular vision: laterality of motor control underpins both. Neurol Res. 2004; 26:331-337.
5. Derakhshan I. Hund-Georgiadis M, Von Cramon YD. Crossed nonaphasia in a dextral with left hemispheric lesions: handedness technically defined. Stroke. 2002; 33:1749-1750. Erratum in: Stroke. 2002; 33:2524.