Now They See It, Now They Don't: An Enquiry into Lateralities of the Seeing Hand and Neglect

摘要

The seeing hand is the hand that restores a person's peripheral vision; moving it eliminates the "hemianopia" seen in lesions affecting the minor hemisphere. This article documents the role of directionality of callosal traffic that underpin this well-documented but as yet unexplained phenomenon. By indexing cases in which macular vision was spared or damaged in lesions affecting the minor or major hemisphere, respectively, it is concluded that the role of the minor hemisphere in vision is to widen the expanse of visual experience rather than participating in seeing per se. As to somatosensory data arising from the nondominant side of the body, evidence indicates that awareness of dimensionality in space awaits the arrival of the signals from the minor to the major hemisphere via the callosum. This explains the fact that neglect is limited to lesions affecting the minor hemisphere. Evidence also indicates that the improvement of neglect upon moving the seeing hand requires an extant callosum (anteriorly), so as to allow the entry of excitatory commands from the major hemisphere for moving the nondominant side awaken the injured (dormant) minor hemisphere; thus representing the contralateral hemispace ignored prior to these movements. The seeing hand is therefore farther from the major hemisphere by a callosum-width (i.e. it is the hand with the longer reaction time). Using data from inspection time, lexical decision and reaction time studies, it is shown that the relationship of the seeing hand to behavioral laterality is statistical.