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October 2003

Callosal Neglect Reexamined

Arch Neurol. 2003;60(10):1493. doi:10.1001/archneur.60.10.1493-a

Heilman and Adams1 correctly advise caution in performing callosotomy on patients with multiple brain lesions. However, their comments concerning postcallosotomy findings in their patient ignore a plethora of evidence against their theoretical advocacy, as follows. The weakness and apraxia after callosotomy are always indexed to the subject's laterality of movement control, of which neural handedness is a code.2 Even voluntary saccades are planned and executed from the major hemisphere; their latency to the left is longer than to the right by an amount commensurate with the interhemispheric transfer time.3 The improvement of motor neglect is pegged to the patient's handedness,3,4 reflecting the specific excitatory effect of the major hemisphere on the minor hemisphere via the corpus callosum. The authors have completely discounted von Monakow's transcallosal diaschisis in the sensory realm. Evidence indicates that diaschisis causes contralateral neglect, which is commonly associated with ipsilesional findings. This is reflected not only in the time-resolved improvement in sensory neglect (as the authors pointed out) but also in the fact that the reaction time of the dominant hand substantially increases when the facilitatory influence of the minor hemisphere on the major hemisphere is abated in the presence of parietal lesions that cause neglect.5,6 Evidence shows that this increase in reaction time is topographically related to the lesion site, mainly affecting the foveal vision.6 Thus, the directionality of callosal traffic underpins both the laterality of movement control24 and conscious awareness. Neglect is an indicator of the directionality of callosal traffic in the sensory realm.7

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