LINE BISECTION IN A MAN WITH COMPLETE FOREBRAIN COMMISSUROTOMY

In judging whether horizontal lines flashed for 100 ms extended farthe r to the left or right of a central fixation mark, a man (L.B.) with c omplete forebrain commissurotomy showed a marked bias to judge the rig ht side longer, which is consistent with left hemineglect. Normal cont rols were considerably more accurate and were slightly biased toward t he left, which is consistent with right pseudoneglect. L.B. showed no rightward bias when viewing the lines freely or when asked in free vis ion to locate the center of a horizontal line, although his performanc e varied more than that of controls. Therefore, commissurotomy may ind uce strong but transient left hemineglect that is rapidly overcome in free vision. One explanation is that control is initially left-hemisph eric and subcortical transfer results in the left hemisphere receiving a spatially compressed representation of information in the left visu al field.