BOLD fMRI vs electrophysiological

Google Image Result for http://www.bu.edu/dbin/anatneuro/assets/images/pics/lab/fmri.jpg: "While BOLD-based neuroimaging studies have provided unprecedented amount of insights into the workings of the human brain in vivo, the explanatory power of BOLD fMRI is currently limited since there is a fundamental gap in our understanding of the linkage between the observed BOLD contrast and the underlying neuronal physiology. In particular, the extent to which the magnitude and spatial scale of the BOLD signal correlates with neuronal physiology remains elusive. To this end, a small but increasing body of results suggests a predominantly linear coupling between BOLD and neuronal activity. For example, a recent study by Ogawa et al demonstrated a linear relationship between somatosensory evoked potentials and BOLD signals for brief stimulation durations. Rees et al. and Heeger et al. demonstrated a linear correlation between BOLD contrast in humans and suprathreshold spiking rate averaged over a cortical area in monkeys during the stimulation with nearly identical stimuli. A similarly linear relationship was observed also in anesthetized monkeys by Logothetis et al. in which single unit responses were acquired simultaneously with BOLD signals for the first time inside the MRI scanner. While the these results suggest that the fundamental coupling between BOLD and the underlying neuronal activity is approximately linear, important questions remain about the spatial scale over which the presumed linear coupling between BOLD and neuronal activity remains valid. Is the hypothesized linear coupling between BOLD and neuronal activity invariant across the different spatial scales of the cortical architecture? Can we assume a universal linearity from the spatial scale of entire cortical areas (several millimeters to centimeters) to individual cortical columns (sub-millimeter)?"