Pergamon Neuropsychologia, Vol. 33, No. 10, pp. 1225-1242,1995 Copyright © 1995Elsevier Science Ltd Printedin GreatBritain.All rightsreserved 0028-3932/95 $9.50 + 0.00 0028-3932(95)00039-9 THE EMERGENCE OF THE CAPACITY TO NAME LEFT VISUAL FIELD STIMULI IN A CALLOSOTOMY PATIENT: IMPLICATIONS FOR FUNCTIONAL PLASTICITY KATHLEEN BAYNES,* C. MARK WESSINGER, ROBERT FENDRICH and MICHAEL S. GAZZANIGA Center for Neuroscience, University of California, Davis, CA 95616, U.S.A. (Received 16 December 1993; accepted 17 February 1995) Abstraet--Callosotomized patient J.W. has a well-documented history of right hemisphere language abilities, including an auditory and visual lexical-semantic system with limited phonology and syntax. However, J.W. has not previously exhibited the ability to name stimuli presented to the left visual field (LVF). We report the emergence of this ability. Experiments were conducted in which pictures and text were presented to the subject's LVF using retinal stabilization techniques to ensure lateralization. J.W. was able to correctly name approximately one-quarter of these stimuli under a variety of presentation conditions. The newly developed ability to respond verbally to complex LVF stimuli can be the result of (1) enhanced inter-hemispberic transfer of information via sub-cortical pathways, (2) sophisticated cross cueing strategies, or (3) control of motor speech in the right hemisphere. Although it appears that the first two mechanisms make a contribution to J.W.'s LVF naming performance, accuracy for unpredictable stimulus sets and the error patterns require acknowledgement that control of motor speech is now available to the right hemisphere. Key Words: right hemisphere language; hemispheric differences; laterality. INTRODUCTION The study of callosotomy and commissurotomyt patients has yielded striking documentation of the existence of independent, self-determined cognitive systems operating within a single being [13]. Of special interest is investigation of the differing language abilities in the disconnected left and right hemispheres. In general, the left hemisphere is language dominant and evidence of right hemisphere language capacity is absent in most patients. The studies that explore right hemisphere language in this population have concentrated on seven patients that have demonstrated this ability [4, 14, 31]. The insight that the disconnected right hemisphere was characteristically mute and required a non-verbal means of expression to respond was what first permitted observation of right hemisphere cognition in these patients [23]. However, subsequent *To whom all correspondence should be addressed. tCommissurotomy and callosotomy procedures are surgical interventions which have been employed to limit interhemispheric spread of seizure activity in patients who suffer from intractable epilepsy. A eommissurotomy includes resection of the anterior commissure as well as the corpus callosum; a callosotomy entails severing the corpus callosum only. 1225