Abstracts

Rationale: To minimise surgical morbidity, knowledge of not only eloquent cortex but also its connectivity is imperative. This may be a particular challenge in patients with dyplastic lesions. A recent study used cortico-cortical evoked potentials (CCEPs) to demonstrate functional connectivity of the anterior and posterior language areas (1). Using the same technique we investigated the connectivity between the anterior language area and frontal motor areas, including primary motor cortex, premotor cortex and supplementary motor area (SMA), in patients with and without focal cortical dysplasia (FCD) within the left inferior frontal gyrus (IFG). Methods: 5 left hemisphere language dominant patients underwent invasive monitoring with a minimum of 64 contact subdural grid electrodes, for pre-surgical evaluation of medically intractable epilepsy. 3 patients had left FCD within the IFG, 1 had a previous pilocytic astrocytoma resected from the middle frontal gyrus and 1 was non-lesional. (See table 1 for patient characteristics). All had medial, lateral and posterior frontal coverage and 3 patients had additional SMA coverage. Electrodes associated with speech arrest were identified in the anterior language area by conventional extra-operative electrical stimulation. 15-30 bipolar single pulse electrical stimuli (SPES) were delivered to the language electrodes to investigate cortico-cortical connections, and CCEPs were obtained by averaging time locked to SPES. Time from onset of stimulus to peak depolarization (early N1, or late N2) or repolarization (early P1) was recorded. Results: The anterior language area had a variable position in the IFG and was displaced in some patients with FCD. Early and late CCEPS occurred in close proximity to the particular language electrode demonstrating strong connectivity with surrounding cortex, and mouth and tongue motor areas (N1 latency of ≈ 20 ms, and N2 latency of ≈ 60 ms). In patients with diffuse or middle IFG FCD, and in the non lesional patient, CCEPs were recorded from a large area over the medial and lateral frontal cortex including primary motor mouth area, and premotor area. In patients with SMA coverage early CCEPs (N1≈ 15 ms) were also seen within this area. In the patient with a large posterior IFG FCD, stimulation resulted in a more limited field of CCEPs. Conclusions: Our work suggests direct connections between the anterior language area, primary motor cortex, premotor areas and SMA in a widespread cortico-cortical network. Lesions within these areas may lead to a reorganisation of functional connectivity, and results need to be reproduced in a larger patient population with FCD to better understand plasticity. Anatomically the connections may be wide spread, however elucidating functionality will facilitate safe surgical planning. Reference 1)Matsumoto R. et al. (2004) Functional connectivity in the human language system: a cortico-cortical evoked potential study. Brain 127:2316-30.