LET NOT THY LEFT HAND KNOW WHAT THY RIGHT-HAND KNOWETH - THE CASE OF A PATIENT WITH AN INFARCT INVOLVING THE CALLOSAL PATHWAYS

Following a cerebral vascular accident, a patient showed a classical d isconnection syndrome: left-hand tactile anemia, apraxia and dysgraphi a and right-hand constructional apraxia, What made the case unusual wa s the presence of hand asymmetry in the performance of some,matching-t o-sample tasks carried out in foveal vision. The left hand committed s ignificantly more errors than the right hand when it was not possible to identify on a perceptual basis the stimulus that was to be matched, because it was removed (memory condition) or was indicated verbally ( verbo-visual matching), or had the same name but not the same physical appearance as the match (capital and lower-case letter matching). No hand difference emerged when the stimulus remained in full view throug hout the matching task (perceptual condition). The hand effect, howeve r, was limited to colours and letters. Objects, geometrical shapes and unfamiliar faces were matched with equal proficiency by both hands un der every condition of presentation. Left-hand errors also significant ly outnumbered right-hand errors in sorting colours according to hue a nd colouring drawings. MRI showed an infarct in the left cingulate whi te matter that ran parallel to the trunk of the corpus callosum, and a n infarct of the splenium, However, the latter did not prevent the tra nsmission of colour and letter information between the two hemispheres , as shown by the performance on perceptual equivalence tasks and by t he correct right-hand responses to stimuli projected to the left visua l field. We propose that this pattern of deficit is contingent upon th e specific role that the left hemisphere plays in categorizing a given colour patch as belonging to a definite colour region (red, blue, etc .) and in grapheme recognition. Without the assistance of the left sid e the right hemisphere lacks the benefit provided by meaning identific ation, In our patient the left brain did receive information from the right brain and was able to process it, but was prevented by the parac allosal lesion from transmitting what it knew to the right motor area. No hand effect emerged for objects and geometrical shapes, because th eir semantic memory is bilaterally represented.