Mismatch between electrocortical stimulation and electrocorticography frequency mapping of language

Background: Electrocortical Stimulation Mapping (ESM) is the gold standard for mapping eloquent cortex in neurosurgery. This technique, however, can cause seizures and requires good patient cooperation. Electrocorticography (ECoG) could replace ESM. Several studies have directly compared ESM and ECoG frequency mapping of language cortex, with mixed results. This may be due to ECoG mapping typically being limited to one or a few language tasks, potentially leading to underestimation of language representation. Objective: To evaluate the influence of the language task on the match between ECoG and ESM, we mapped language function based on episodes of spontaneous conversation with ECoG, and compared this with ESM data in eight epilepsy patients. A verb generation and picture naming task were used as reference standard. Methods: From the continuous ECoG and video registrations, periods of conversation were identified, distinguishing three conditions: speaking, listening and rest. A power-frequency analysis was done for each condition and the two language tasks. The match between ESM and ECoG frequency mapping was evaluated on the basis of sensitivity and specificity measures. Results: ECoG activation during spontaneous conversation and language tasks was seen in and around classic language areas. Comparing the electrodes activated in each condition with those indicated as language positive by ESM revealed that there was high specificity but relatively low sensitivity. Conclusion: The relatively poor match between ESM and ECoG mapping is not improved by using spontaneous language.