Manifestation of ocular-muscle EMG contamination in human intracranial recordings

Abstract

It is widely assumed that intracranial recordings from the brain are only minimally affected by contamination due to ocular-muscle electromyogram (oEMG). Here we show that this is not always the case. In intracranial recordings from five surgical epilepsy patients we observed that eye movements caused a transient biphasic potential at the onset of a saccade, resembling the saccadic spike potential commonly seen in scalp EEG, accompanied by an increase in broadband power between 20 and 200 Hz. Using concurrently recorded eye movements and high-density intracranial EEG (iEEG) we developed a detailed overview of the spatial distribution and temporal characteristics of the saccade-related oculomotor signal within recordings from ventral, medial and lateral temporal cortex. The occurrence of the saccadic spike was not explained solely by reference contact location, and was observed near the temporal pole for small (< 2 deg) amplitude saccades and over a broad area for larger saccades. We further examined the influence of saccade-related oEMG contamination on measurements of spectral power and interchannel coherence. Contamination manifested in both spectral power and coherence measurements, in particular, over the anterior half of the ventral and medial temporal lobe. Next, we compared methods for removing the contaminating signal and found that nearest-neighbor bipolar re-referencing and ICA filtering were effective for suppressing oEMG at locations far from the orbits, but tended to leave some residual contamination at the temporal pole. Finally, we show that genuine cortical broadband gamma responses observed in averaged data from ventral temporal cortex can bear a striking similarity in time course and band-width to oEMG contamination recorded at more anterior locations. We conclude that eye movement-related contamination should be ruled out when reporting high gamma responses in human intracranial recordings, especially those obtained near anterior and medial temporal lobe.

Cortical representation of persistent visual stimuli

“…4) per second, and thus EM-driven activity would have taken the form of random sparse bursts in single trials, but these were not consistently observed. Second, although eye movements could not be tracked with fidelity in the clinical ICU setting, saccadic spike potentials elicited by the contraction of the ocular muscles at the onset of saccades (Yuval-Greenberg et al, 2008), can be detected intracranially in electrodes proximal to the eyes (Jerbi et al, 2009; Kovach et al, 2011). We were able to identify reliable saccadic events in one of our patients through the detection of intracranial saccadic spike potentials in two temporal pole electrodes, just posterior to the eye (Fig.…”

Exploring emotions using invasive methods: review of 60 years of human intracranial electrophysiology

“…Compared to functional MRI, HIE techniques offer comparable spatial resolution but with microsecond temporal resolution which is on the same order of magnitude as the speed of cognitive processing. Furthermore, HIE recordings are somewhat immune to muscle and eye movement artifact (Kovach et al, 2011). This allows for the study of phenomena involving motor actions which is difficult when using functional MRI, such as laughter.…”

Spanning the rich spectrum of the human brain: slow waves to gamma and beyond

“…While intracranial recordings may occasionally be contaminated by eye muscle activity (Ball et al 2009;Jerbi et al 2009a;Kovach et al 2011), these effects are predominantly restricted to recordings sites in the vicinity of the temporal pole and are efficiently reduced by using bipolar re-referencing strategies (Jerbi et al 2009a). Certainly, these recordings are performed in patients with brain pathologies, so individual results are best interpreted in the context of converging evidence from other techniques; nevertheless, each patient's pathology tends to be different; therefore, findings that remain consistent across patients can be regarded with reasonable confidence (Jerbi et al 2009b).…”