Cortical-depth EEG-fMRI at 7T: experimental setup and analysis pipeline to elucidate generating mechanisms of alpha oscillations

Abstract

Alpha band (8-13Hz) electroencephalography (EEG) oscillations are central to cognition, but their generating mechanisms are poorly understood. Most laminar studies investigating origins of alpha oscillations rely on invasive animal intracranial recordings. To relate these findings to human alpha generation, non-invasive techniques need to be developed. Ultra-high field (UHF, 7T) laminar functional Magnetic Resonance Imaging (fMRI) enables cortical depths resolved brain responses, and combined with simultaneous EEG, offers new insight into human alpha generation mechanisms. This work presents a framework for studying the generating mechanisms of electrophysiological signals non-invasively in humans using simultaneous EEG laminar-fMRI. Data were acquired on 10 participants during an eyes closed/eyes open paradigm. In 9/10 participants the quality of EEG and Blood Oxygenation Level Dependent (BOLD) fMRI data were sufficient to observe a significant negative correlation between EEG alpha power and the BOLD signal in visual cortex grey matter. To improve depth specificity and mitigate pial-surface BOLD signal increases due to draining veins, we applied “deveining”. The effects of each of the deveining analysis steps on the cortical depth profiles of the negative alpha-BOLD correlations was studied. The largest effect arose from excluding voxels in the tissue immediately surrounding veins. After deveining, the negative alpha-BOLD correlations were significantly weaker in the middle cortical depths than in superficial and deep depths. This depth-dependence was absent when modelling the task with a boxcar regressor rather than EEG alpha power, suggesting specificity to spontaneous electrophysiological (alpha) fluctuations. In conclusion, we have established a method to non-invasively interrogate the origins of electrophysiological signals. Our alpha-BOLD depth profiles during the eyes open-closed task (strongest correlations in superficial and deep layers) are consistent with activity in cortical depths typically associated with feedback/top-down processing, but we cannot definitively demonstrate causality from these data.