The relationship of LFPs, MUA, and SUA to the bold fMRI signal

The contribution of fMRI to our understanding of the functional anatomy of the brain is directly related to the degree to which the relationship between the MRI signal and the underlying neural activity is understood. It is established that activity in the brain is characterized by time-varying spatial distributions of action potentials superimposed on relatively slow-varying field potentials. To study how such potentials relate to the BOLD signal we scanned the visual cortex of anesthetized monkeys in a 4.7T scanner while recording local field potentials (LFPs), single (SUA), and multi (MUA) unit activity, by means of a novel, recently-developed recording technique. The geometry of novel electrodes and the active compensation built into the recording system minimized magnetic and electrostatic coupling respectively, permitting the elimination of any interference in subsequent off-line analysis. In each session, active sites were selected by first imaging the entire brain with a multi-shot, multi-slice gradient-recalled EPI MRI sequence (TE=20ms, TR=750ms), with FOV=128x128mm^2,128^2 matrix, 2mm thickness. Single slices, with areas of interest, were subsequently imaged with quadrature, transmit/receive volume or implanted surface coils with a voxel size of 250x250x660um^3 using time-resolved MR imaging (TE=20ms, TR=250ms). Our results provide insights into (a) the relationship of slow waves, single unit activity, and coherence functions of simultaneously-recorded single-units to the BOLD signal; (b) the temporal response function of the signals, and (c) the effects of anesthesia depth and stimulus strength on the modulation of each signal-type