LINE BISECTION IN A MAN WITH COMPLETE FOREBRAIN COMMISSUROTOMY

In judging whether horizontal lines flashed for 100 ms extended farther to the left or right of a central fixation mark, a man (L.B.) with complete forebrain commissurotomy showed a marked bias to judge the right side longer, which is consistent with left hemineglect. Normal controls were considerably more accurate and were slightly biased toward the left, which is consistent with right pseudoneglect. L.B. showed no rightward bias when viewing the lines freely or when asked in free vision to locate the center of a horizontal line, although his performance varied more than that of controls. Therefore, commissurotomy may induce strong but transient left hemineglect that is rapidly overcome in free vision. One explanation is that control is initially left-hemispheric and subcortical transfer results in the left hemisphere receiving a spatially compressed representation of information in the left visual field.