Passive functional mapping of receptive language areas using electrocorticographic signals

Abstract

Objective: To validate the use of passive functional mapping using electrocorticographic (ECoG) broadband gamma signals for identifying receptive language cortex.

Methods: We mapped language function in 23 patients using ECoG and using electrical cortical stimulation (ECS) in a subset of 15 subjects.

Results: The qualitative comparison between cortical sites identified by ECoG and ECS show a high concordance. A quantitative comparison indicates a high level of sensitivity (95%) and a lower level of specificity (59%). Detailed analysis reveals that 82% of all cortical sites identified by ECoG were within one contact of a site identified by ECS.

Conclusions: These results show that passive functional mapping reliably localizes receptive language areas, and that there is a substantial concordance between the ECoG- and ECS-based methods. They also point to a more refined understanding of the differences between ECoG- and ECS-based mappings. This refined understanding helps to clarify the instances in which the two methods disagree and can explain why neurosurgical practice has established the concept of a "safety margin."

Significance: Passive functional mapping using ECoG signals provides a fast, robust, and reliable method for identifying receptive language areas without many of the risks and limitations associated with ECS.

Keywords: ECoG; Electrocorticography; Functional mapping; Intracranial; Receptive language.

Figure 1:. ECoG-based mapping of receptive language activity

Electrode locations for each of the 23 subjects are shown as black or red circles. Electrodes affected by significant signal artifacts or those that did not contain clear ECoG signals are indicated by small white circles. Electrodes whose broadband gamma activity significantly increased during the listening task are shown as large red circles. The diameter of each red electrode is related to the magnitude of task-related ECoG broadband gamma modulation (see Methods).

Figure 2:. Comparison between ECS- and ECoG-based mapping methods

Electrode locations for each of the 10 subjects with ECS-induced language inhibition are shown as black, blue, or red circles. Electrodes affected by significant signal artifacts or those that did not contain clear ECoG signals are indicated by small white circles. Electrodes whose broadband gamma activity significantly increased during the listening task are shown as large red circles (ECoG+). The diameter of each red electrode is related to the magnitude of task-related ECoG broadband gamma modulation (see Methods). Blue circles indicate electrodes for which ECS-induced language inhibition was reliably observed (ECS+). Large black circles indicate electrodes without ECS-induced inhibition of language function (i.e., “No Response”), while small black circles indicate electrodes that were not stimulated.

Figure 3:. Percentage of ECoG+ sites within a certain distance of ECS+ sites

The thick red line represents the fraction of ECoG+ electrodes within a certain distance of an ECS+ electrode, averaged across the subset of 10 subjects for whom ECS resulted in reliable inhibition of language function. The shaded region represents the standard error of the mean. The dashed vertical line indicates the 1.5 cm distance mark. 82% (± 7.1%) of ECoG+ electrodes are within that distance of an ECS+ electrode.