Interhemispheric Rivalry During Simultaneous Bilateral Task Presentation in Commissurotomized Patients

Abstract

Six commissurotomized patients were tested and the performances of their left and right hemisphere on a dot counting task were compared during unilateral and bilateral input to study aspects of interhemispheric interference. From 1 to 5 dots were flashed, either in the left or the right visual field alone, or in each of the two fields simultaneously, and the patients were asked to indicate the number of dots seen in each field by extending the same number of fingers with the ipsilateral hand. Comparison trials were also presented where the number of dots were replaced by a corresponding numeral to eliminate counting. Comparable incidences of responses extinction to the dots and to the numerals were frequently observed during bilateral input. Their occurrence bore no clear relationship to the performance levels during unilateral input. Rather, the predominant side of extinction varied among the patients and seems to be related to the presence of some contralateral brain damage. Despite frequent response extinctions, no concurrent increase in counting errors was observed during bilateral input, indicating that interhemispheric interference in the present case is not equally present in all stages of the input-output chain, but takes the form of an all-or-none rivalry in some gating mechanism. Ancillary findings support different modes of information processing by the two hemispheres, and a possible presence of weak ipsilateral projection of visual information.