Saccade related gamma-band activity in intracerebral EEG: dissociating neural from ocular muscle activity

Recent evidence suggests that transient increases in scalp electroencephalography (EEG) gamma band power (above 30 Hz) can be due to miniature eye movements. Although, these findings do not raise doubts about the widely established role of gamma range neural synchrony, it does call for caution when it comes to interpreting high frequency scalp EEG data. By contrast, gamma-band activity detected with intracerebral EEG (iEEG) is assumed to be immune to such miniature saccade artefacts. Here, we show for the first time, that while this is indeed largely the case, intracerebral recordings in the temporal pole of implanted patients can be contaminated by saccadic eye muscle artefacts resulting in typical high gamma-band power increases. By analyzing data from multiple depth electrodes, we show that this artefact is confined to the pole of the temporal lobe because of its immediate vicinity to extraocular muscles (rectus lateralis). For other brain structures, our analysis shows that the use of stereotactic EEG with a bipolar montage provides a robust and convenient tool to explore the functional role of gamma synchronization in humans with high anatomical accuracy during a wide range of cognitive processes, including oculomotor behaviour.