Inter-hemispheric transmission of visual information between the intact and the lesioned hemisphere

A unilateral lesion of the human central visual pathways results in a contralesional visual field loss known as homonymous hemianopia, a long-lasting and often permanent impairment. An important open question concerns the role of a commissural cross-talk between the intact and the lesioned hemisphere for recovery of homonymous hemianopia. One specific question is whether the presence of interhemispheric transfer of visual information depends on the location of the lesion causing hemianopia. Together with healthy control participants, we tested chronic hemianopic patients either with a lesion roughly restricted to V1 or with a more anterior lesion including visual extrastriate areas. The behavioural test used was the Poffenberger Paradigm that enables one to measure interhemispheric transfer time with a simple manual reaction time to lateralized visual stimuli. During behavioural performance we recorded visual event-related potentials (ERPs) to compare latency and amplitude of the ERPs components evoked by stimulation of the direct or the indirect commissural (callosal) pathway. In controls both pathways yielded a normal response. In hemianopics we found that visual stimulation of the intact hemifield evoked a normal P1-N1 component in the intact contralateral hemisphere (direct pathway). However, when recording the indirect callosal response in the ipsilateral lesioned hemisphere we found a different result as a function of the site of the lesion. In patients with a lesion restricted to V1 there was an indirect P1-N1 response similar to that of controls while this was not the case in patients with a more anterior and extended lesion. A likely explanation of this dissociation is that in the former group the widespread callosal connections between the extrastriate areas in the two hemispheres are partly spared while they are largely damaged in the latter group. In conclusion, a substantial visual commissural input from an intact to a lesioned hemisphere is present only following lesion restricted to V1 and this might suggest a favourable prognosis for recovery of vision following specific rehabilitation training.