Abstract
A century after the first electroencephalography (EEG) recordings in human subjects, the relationship between EEG and the underlying neurophysiological signals, such as spiking activity and local field potentials (LFP), remains poorly understood. This limits inferences one can draw about intracranial signals from EEG, a critical step in bridging animal and human electrophysiological findings. We recorded LFPs and spiking activity from a 32-channel floating microarray permanently implanted in parafoveal V1 of a male macaque monkey and EEG using ten scalp electrodes. While the animal fixated, the screen flickered at different temporal frequencies (0, 5, 10, 20, and 40 Hz) to induce steady-state visual evoked potentials (SSVEP) in both LFP and EEG. The primary advantage of SSVEPs is that they generate high signal-to-noise ratios. We analyzed the relationship between the SSVEPs in LFP and EEG. We found robust SSVEPs in both LFP and EEG, particularly at 20 Hz. The LFP showed strong responses at the harmonics of the driving frequencies, which was not evident in EEG. Across-trial correlation of power at the driving frequency between EEG and LFP showed a stronger relationship at 20 Hz than at other frequencies. Coherence between EEG and LFP also depended on the driving frequency, with the highest coherence at 20 Hz. The trial-averaged time series of the LFP and EEG were strongly correlated, suggesting that EEG could be used to infer trial-averaged intracranial activity dynamics at sub-second resolution. Our study shows robust relationships between LFP and EEG signals under the high signal-to-noise ratio conditions of SSVEP stimuli. Even under those conditions, however, the coupling varies considerably across intracranial and scalp electrodes and, more surprisingly, across driving frequencies. Whether these are general rules, or idiosyncrasies attributable to electrode placement in a single animal requires further investigation.