Interhemispheric transfer of visual motion information after a posterior callosal lesion: a neuropsychological and fMRI study

Interhemispheric transfer of visual information was investigated behavioura lly and with functional magnetic resonance imaging (fMRI) 6 months after a lesion of the posterior two-thirds of the corpus callosum. On tachistoscopi cal left hemifield presentation, the patient was severely impaired in readi ng letters, words and geographical names and moderately impaired in naming pictures and colours. in contrast, interhemispheric transfer of visual moti on information, tested by verbal report of the direction of short sequences of coherent dot motion presented within the left hemifield, was preserved. The pattern of cerebral activation elicited by apparent motion stimuli was studied with fMRI and compared to that of normal subjects. In normal subje cts, apparent motion stimuli, as compared to darkness, activated strongly s triate and extrastriate cortex. When presented to one hemifield only, the c ontralateral calcarine region was activated while regions on the occipital convexity, including putative area V5, were activated bilaterally, A simila r activation pattern was found in the patient with a posterior callosal les ion; unilateral left or right hemifield stimulation was accompanied by acti vation in the contralateral and ipsilateral occipital convexity. Ipsilatera l hemifield representation in the extrastriate visual cortex is believed to depend on callosal input. Our observation suggests that this is not the ca se for visual motion representation and that other, probably parallel, path ways may mediate visual motion transfer after posterior callosotomy.