Can commissurotomized subjects compare digits between the visual fields?

Sergent (Brain 113, 537-568, 1990) flashed pairs of digits to the opposite visual fields of three commissurotomized subjects (L.B., N.G. and A.A.), and found that they were highly accurate in deciding which digit was the larger, but at little or no better than chance in deciding whether the digits were the same or different. Experiment 1 confirmed that these subjects were better at relative than at same-different judgements. However their performance on relative judgements was considerably lower than in Sergent's study and could be explained largely in terms of the subjects' use of information available to a single hemisphere. A fourth subject, D.K., with section of the posterior corpus callosum only, had little difficulty with either task despite previous evidence of visual disconnection, and appeared able to transfer the information verbally. In Experiment 2, L.B. was better able to make both relative and same-different judgements when the digits were presented within either the left or right visual fields than when they were presented in opposite fields. These results suggest little, if any, interhemispheric transfer of either shape or numerical information following complete forebrain commissurotomy.