Abstracts

Assessment of eloquent functions during brain mapping usually relies on testing reading, speech, and comprehension to uncover transient deficits during electrical stimulation. These tests stem from findings predicted by the Geschwind-Wernicke hypothesis of receptive and expressive cortices connected by white matter tracts. Later work, however, has emphasized cortical mechanisms of language function. We report two cases that demonstrate that conduction aphasia is cortically-mediated and can be inadequately assessed if not specifically evaluated during brain mapping. To determine the distribution of language on dominant cortex, we performed electrical cortical stimulation using implanted subdural electrodes during brain mapping before epilepsy surgery. We report our findings in two patients. There was an interelectrode distance of 1cm. We stimulated adjacent pairs of contacts using a 50 Hz square wave signal starting at a duration of 2 seconds and an intensity of 4.5 mA. We increased duration to 5 seconds and increased current by 1 mA increments until (1) a response occurred, (2) an afterdischarge or clinical seizure occurred, or (3) no response was achieved at a stimulus of 12.5 mA/5 seconds. We elicited a transient, isolated deficit in repetition of language with stimulation of the posterior portion of the dominant superior temporal gyrus in one patient and of the supramarginal gyrus in the second patient. These cases demonstrate a localization of language repetition to the posterior perisylvian cortex. Brain mapping of this region should include assessment of verbal repetition to avoid potential deficits resembling conduction aphasia. This study provides further evidence that the white matter disconnection hypothesized by the Wernicke-Geschwind model is not an obligatory cause of conduction aphasia.