Aislin Sheldon^*1 (), Hannah Moser^*1, Kamar Abdullahi¹, Karly Allison¹, Carter Mulder¹, Kyle Killebrew¹, Samantha Montoya¹, Scott Sponheim^1,3, Małgorzata Marjańska², Michael-Paul Schallmo¹; ¹Department of Psychiatry and Behavioral Sciences, University of Minnesota, Minneapolis, MN, ²Center for Magnetic Resonance Research, Department of Radiology, University of Minnesota, Minneapolis, MN, ³Veterans Affairs Medical Center, Minneapolis, MN ^*Equal Contribution

Oscillatory activity across the cortex is linked to multiple cognitive processes, including visual perception. However, recent studies which separate oscillatory and aperiodic components of electrophysiological activity have highlighted the functional role of aperiodic activity. Studies in animal models suggest the aperiodic slope of electrophysiological power spectra reflects the local ratio of excitatory:inhibitory synaptic transmission. We hypothesized that shallower aperiodic slopes from human EEG, measured over the occipital lobe, would correlate with higher MRS measures of occipital glutamate, reflecting cortical excitation. EEG was collected from 26 healthy adults during eyes-open rest. Power density spectra from occipital electrodes were analyzed using the fitting oscillations and one over f (FOOOF) toolbox, yielding a measure of the slope of the aperiodic (1/f-like) component for each participant. In the same participants, we separately acquired magnetic resonance spectroscopy (1H MRS) data at 7 tesla with a stimulated echo acquisition mode (STEAM) sequence (TE = 8 ms, TR = 5 s). Occipital MRS data were collected in a single voxel (30x30x18mm3) centered over the calcarine sulcus while participants completed a central fixation task on a gray screen. MRS data were processed using the matspec toolbox in MATLAB and metabolites were quantified using linear combination of model (LCModel) software. Across participants, we observed a significant negative correlation between aperiodic slopes and glutamate concentrations (r24 = -0.47, p = 0.016), whereas the correlation with GABA levels (a marker for neuronal inhibition) was not significant (r24 = 0.17, p = 0.4), nor NAA - a metabolite that is assumed to remain unchanged with visual stimulation, NAA (r24 = -0.18, p = 0.4). Our results show that occipital glutamate concentrations relate to aperiodic activity during fixation, and support the use of aperiodic electrophysiological activity as a marker for occipital excitation.

Acknowledgements: K01 MH120278