Increases in alpha-band electrocorticographic oscillations and decreases in fMRI signals reflect surround suppression in V1 but not extra-striate cortex

Abstract

Introduction: Electrical brain signals are typically decomposed into different frequency bands thought to represent different functions. In particular, 8-12Hz (alpha) oscillations are debated to reflect functionally important processing, or simply idling. Electrocorticography (ECoG) measures electrical activity directly in humans using subdural electrodes. Here, we model population receptive field (pRF) properties using both fMRI and ECoG in the same subject. A recent extension of the pRF modeling approach reconstructs a center-surround profile, driven by increases and decreases in fMRI amplitudes in V1. We link different frequency bands to fMRI signals and the underlying neural population’s properties. Methods: We used stimuli consisting of contrast-defined bar-sweeps though the visual field intermixed with mean-luminance blocks (baseline). We determined the pRF properties and visual area layout using 7T fMRI (Dumoulin and Wandell, 2008). Subdural ECoG electrodes were then implanted in the same subject and the same stimuli were shown. ECoG data was filtered into different frequency bands, which were analyzed separately using the same methods. Results: Using ECoG, 30-120 Hz oscillations (gamma) allowed pRF modeling in V1 and IPS but without a center-surround configuration. Different response profiles were observed in V1 and IPS visual field maps in alpha oscillations. In contrast to surrounding frequencies, alpha increased when the contrast-defined bar was in the surround of the V1 pRFs. This increase in alpha-oscillations corresponds to the decrease in response below the baseline using fMRI. In IPS no center-surround configuration was found in either fMRI or ECoG data. Unlike V1, alpha oscillations increased during mean-luminance blocks. Conclusions: Both fMRI and ECoG recordings produce comparable estimates of the pRF. The fMRI signal cannot be explained by one ECoG frequency band alone. The same frequency band can reflect different functional processing depending on cortical location. Alpha oscillations reflect inhibitory signals in V1 and resting-state in extra-striate cortex.

Meeting abstract presented at VSS 2012