Functional MRI and Wada studies in patients with interhemispheric dissociation of language functions

Abstract

Rare patients with chronic epilepsy show interhemispheric dissociation of language functions on intracarotid amobarbital (Wada) testing. We encountered four patients with interhemispheric dissociation in 490 consecutive Wada language tests. In all cases, performance on overt speech production tasks was supported by the hemisphere contralateral to the seizure focus, whereas performance on comprehension tasks was served by the hemisphere with the seizure focus. These data suggest that speech production capacity is more likely to shift hemispheres than is language comprehension. Wada and fMRI language lateralization scores were discordant in three of the four patients. However, the two methods aligned more closely when Wada measures loading on comprehension were used to calculate lateralization scores. Thus, interhemispheric dissociation of language functions could explain some cases of discordance on Wada/fMRI language comparisons, particularly when the fMRI measure used is not sensitive to speech production processes.

Figure 1. Concordance and discordance between Wada and fMRI LI indices

LI: Total Wada Laterality Index Ic: Wada Laterality Index – comprehension LIsp: Wada Laterality Index – speech production

Figure 2. FMRI activation map for each patient. Symmetric, sequential sagittal slices are shown

Activation maps were thresholded at uncorrected p < 0.001 and a minimum cluster size of 200 μl to give a whole-brain corrected p <.05. The images are presented as sequential sagittal slices from lateral to medial at regular 10-mm intervals. For each patient, the upper row shows left emisphere sections, and the bottom row shows mirror-symmetric right hemisphere sections.