Characterization of the effects of the human dura on macro- and micro-electrocorticographic recordings

Abstract

Objective Electrocorticography (ECoG) electrodes implanted on the surface of the brain have recently emerged as a potential signal platform for brain-computer interface (BCI) systems. While clinical ECoG electrodes are currently implanted beneath the dura, epidural electrodes could reduce the invasiveness and the potential impact of a surgical site infection. Subdural electrodes, on the other hand, while slightly more invasive, may have better signals for BCI application. Because of this balance between risk and benefit between the two electrode positions, the effect of the dura on signal quality must be determined in order to define the optimal implementation for an ECoG BCI system. Approach This study utilized simultaneously acquired baseline recordings from epidural and subdural ECoG electrodes while patients rested. Both macro-scale (2 mm diameter electrodes with 1 cm inter-electrode distance, 1 patient) and micro-scale (75 μm diameter electrodes with 1 mm inter-electrode distance, 4 patients) ECoG electrodes were tested. Signal characteristics were evaluated to determine differences in the spectral amplitude and noise floor. Furthermore, the experimental results were compared to theoretical effects produced by placing epidural and subdural ECoG contacts of different sizes within a finite element model. Main Results The analysis demonstrated that for micro-scale electrodes, subdural contacts have significantly higher spectral amplitudes and reach the noise floor at a higher frequency than epidural contacts. For macro-scale electrodes, while there are statistical differences, these differences are small in amplitude and likely do not represent differences relevant to the ability of the signals to be used in a BCI system. Conclusions Our findings demonstrate an important trade-off that should be considered in developing a chronic BCI system. While implanting electrodes under the dura is more invasive, it is associated with increased signal quality when recording from micro-scale electrodes with very small sizes and spacing. If recording from larger electrodes, such as traditionally used clinically, the signal quality of epidural recordings is similar to that of subdural recordings.

Continuous decoding of human grasp kinematics using epidural and subdural signals

“…Here, we decoded continuous whole hand grasping movements and linked to motor control literature (Santello et al 1998, Ingram et al 2008) showing that the many degrees of freedom inherent to finger movements can be represented by a few principal component representations. These results can also be seen as validating prior modeling studies, which have not necessarily predicted a worsening of decoding accuracy due to the dura’s presence (Slutzky et al 2010, Bundy et al 2014), even despite lower amplitude signals (Ramon et al 2014). …”

“…Previous studies that examined the effect of the dura and CSF on neural signals concentrated primarily on the relative power spectra or amplitude of the signals (Slutzky et al 2010, Bundy et al 2014, Olson et al 2015). Here, we evaluated how useful these signals would be for BMIs by using them to decode hand movements.…”

Visual epidural field potentials possess high functional specificity in single trials

“…Yet, probably because of the large number of neurons contributing to the EFP/ECoG signal (Miller et al, 2009), the additional attenuation of the signal by the dura makes epidural recordings a still rare case, despite its obvious clinical advantages. A recent study in monkey somatosensory cortex indeed indicated weaker decoding performance for epidural as compared to subdural signals (Bundy et al, 2014;Farrokhi and Erfanian, 2018). On the other hand, a rodent study on forelimb movements concluded that EFPs allow for the same decoding performance than intracortical LFPs (Slutzky et al, 2011).…”

Identifying the attended speaker using electrocorticographic (ECoG) signals

“…Penetration of the dura increases the risk of bacterial infection [38–42]. Epidural electrodes (i.e., electrodes placed on top of the dura) provide signals of approximately comparable fidelity [43, 44]. A single electrode placed epidurally could reduce risk, which should make this approach more clinically practical.…”