Abstract
Precise localization of electrodes is essential in the field of high-density (HD) electrocorticography (ECoG) brain signal analysis in order to accurately interpret the recorded activity in relation to functional anatomy. Current localization methods for subchronically implanted HD electrode grids involve post-operative imaging. However, for situations where post-operative imaging is not available, such as during acute measurements in awake surgery, electrode localization is complicated. Intra-operative photographs may be informative, but not for electrode grids positioned partially or fully under the skull. Here we present an automatic and unsupervised method to localize HD electrode grids that does not require post-operative imaging. The localization method, named GridLoc, is based on the hypothesis that the anatomical and vascular brain structures under the ECoG electrodes have an effect on the amplitude of the recorded ECoG signal. More specifically, we hypothesize that the spatial match between resting-state high-frequency band power (45–120 Hz) patterns over the grid and the anatomical features of the brain under the electrodes, such as the presence of sulci and larger blood vessels, can be used for adequate HD grid localization. We validate this hypothesis and compare the GridLoc results with electrode locations determined with post-operative imaging and/or photographs in 8 patients implanted with HD-ECoG grids. Locations agreed with an average difference of 1.94 ± 0.11 mm, which is comparable to differences reported earlier between post-operative imaging and photograph methods. The results suggest that resting-state high-frequency band activity can be used for accurate localization of HD grid electrodes on a pre-operative MRI scan and that GridLoc provides a convenient alternative to methods that rely on post-operative imaging or intra-operative photographs.
Highlights
Subchronic and acute electrocorticography (ECoG) are increasingly used in the clinical and scientific communities in order to investigate and map brain-functions and understand brain-disorders (Jacobs and Kahana, 2010)
For subjects S1 and S2 (Fig. 3A), for whom an angiogram was available, the measured high-frequency band (HFB) pattern significantly correlated with the estimated HFB pattern, with correlations values of 0.63 and 0.75 (p-value < 0.001), respectively
The results indicate that the model significantly predicts the measured HFB spatial patterns at the known grid position, and that the combination of both the angiogram and anatomy components can explain the HFB patterns better than any of these components alone
Summary
Subchronic and acute electrocorticography (ECoG) are increasingly used in the clinical and scientific communities in order to investigate and map brain-functions and understand brain-disorders (Jacobs and Kahana, 2010). Typical clinical grids used to localize seizure foci in patients with intractable epilepsy have an inter-electrode spacing of 1 cm (Salles et al, 1994; Diehl and Lüders, 2000; Lesser et al, 2010). These grids have been used in other applications, ranging from neurostimulation of the cortex of epilepsy patients to reduce seizure occurrence (Heck et al, 2014), to providing stroke patients with neurofeedback for rehabilitation (Gomez-Rodriguez et al, 2011) and Brain-Computer Interfaces (Leuthardt et al, 2004; Vansteensel et al, 2016). ECoG grids with increased spatial density, hereafter referred to as high-density (HD) grids, are useful for investigating fine-scale cortical dynamics (Kellis et al, 2016; Wang et al, 2016), given the fact that electrodes spaced as close as a few millimeters still provide discriminable information (Slutzky et al, 2010; Siero et al, 2013)
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