Abstracts

Rationale: Expressive aphasia sites are defined as the brain region at which 50-Hz electrical stimulation elicits transient expressive but not receptive aphasia. Such sites can be identified in the left frontal or temporal lobe in epilepsy presurgical evaluation. The current study generated a four-dimensional map based on the intracranially measured cortico-cortical evoked potentials (CCEPs) and delineated the effective connectivity from the left temporal lobe expressive aphasia sites. Our hypothesis was that the left temporal lobe expressive aphasia sites would be intensely connected to the Broca's area in the left frontal lobe. Methods: We studied five right-handed patients who underwent cortical resection following extraoperative electrocorticography recording and revealed the expressive aphasia sites in the left temporal lobe. As a part of clinical management, we delivered trains of biphasic single-pulse electrical stimulation to adjacent electrode pairs within the expressive aphasia sites. We quantified the voltages of CCEPs at the remaining electrode sites, and animated the dynamics of CCEP voltages on the three-dimensional standard brain surface image. Results: The 50-Hz stimulation localized the expressive aphasia sites in the superior-, middle-, and inferior-temporal gyri of the left posterior temporal lobe. The single-pulse stimulation of the expressive aphasia sites elicited an initial negative CCEP component with a peak latency of 10-50 ms (referred to as N1) in both infra- and supra-sylvian structures of the left hemisphere. N1 involved the infra-sylvian regions mainly below the superior-temporal sulcus as well as the supra-sylvian areas, including the supramarginal, middle-frontal, superior-frontal, and precentral gyri. N1 minimally included the pars opercularis and triangularis of the left inferior-frontal gyrus. Conclusions: The spatial profile of N1 suggests that the expressive aphasia sites within the left temporal lobe are effectively connected to both infra- and supra-sylvian structures through a direct white-matter fiber. The effective connectivity between the left temporal lobe expressive aphasia sites and the Broca's area may be surprisingly small.  Funding: NIH grant NS064033 (to E. Asano).