Abstracts

Rationale: The presumption of a deleterious impact of focal epileptic spike-and-wave activity is not new. Shewmon and Erwin demonstrated in 1989 that posteriorly localized spike-and-wave activity could transiently increase reaction time and error rate in a visual reaction time task. Studies combining electroencephalography and imaging have since suggested spike-related metabolic changes around the seizure focus and beyond. Use of a modality more directly measuring the dynamics of spike-induced deficits is warranted. We have amassed a sizable database of language-related electrocorticography (ECoG) data in focal epilepsy patients. This presents an opportunity to study the effects of interictal spikes upon on-going activities engaging higher order function, such as language. We determined whether interictal spikes from a frontal seizure focus transiently suppressed language-related augmentation of gamma activity at 80-100 Hz. Methods: A retrospective search of our patient cohort employed the following inclusion criteria: (i) an ECoG-defined left frontal seizure onset zone (SOZ) and (ii) mapping of naming-related gamma-augmentation via ECoG using an auditory naming task. Of the result, four patients survived the following exclusion criteria: (i) multiple seizure foci, (ii) massive brain malformations, or (iii) right language dominance by Wada testing or left-handedness when Wada test results are not available. An automated spike detection algorithm was utilized (Barkmeier et al. Clin Neurophysiol, 2011). Naming task trials were grouped into 'Spike' and 'Non-Spike' trials based on whether a spike occurred during stimulus presentation; any trials with spikes occurring between stimulus-offset and response-onset were excluded. Peak gamma activity between stimulus-offset and response-onset was determined for each trial and compared between Spike and Non-Spike trials. Results: Of all electrodes showing task-related gamma-augmentation across all patients, one site exhibited a reduction in peak gamma activity during Spike trials that survived Bonferroni correction; mean peak amplitude reduction was 58.4% of baseline (95% C.I.: 31.6% to 85.1%; p-value=0.000078). This site was located near to the SOZ in the dorsolateral frontal lobe and possessed a spike count similar to that of the SOZ. All other electrodes were located a greater distance away from or exhibited a spike count much lower than that of the respective SOZ; these sites showed no significant difference in peak gamma activity between Spike and Non-Spike trials. Neither within nor across patients was a change in response time found. Conclusions: We have demonstrated that a negative effect of interictal spikes originating from a single frontal lobe focus upon cortical language function: (i) can occur in the absence of a behavioral effect, (ii) may be more prominent at sites nearer to the SOZ, and (iii) may be more prominent at sites associated with epileptiform activity similar to that of the SOZ. The deleterious effects of interictal spikes were local, rather than global, in our small cohort; a preliminary result requiring further validation.