BOLD matches neuronal activity at the mm scale: a combined 7T fMRI and ECoG study in human sensorimotor cortex.

High resolution BOLD fMRI has the potential to map activation patterns of small neuronal populations at the scale of cortical columns. However, BOLD fMRI does not measure neuronal activity, but only a correlate thereof, since it measures blood dynamics. To confirm that BOLD activation maps reflect neuronal population activity patterns, a direct comparison with neuro-electrophysiological data from the same cortical patch is necessary. Here, we compare BOLD activation patterns obtained with fMRI at 7 T to electrophysiological patterns obtained with implanted high density electrocorticography (ECoG) grids in the same patch of human sensorimotor cortex, and with similar resolution (1.5mm). We used high spatially sampled high-frequency broadband (HFB) power from ECoG, which reflects local neuronal population activity. The spatial distribution of 7 T BOLD activation matched the spatial distribution of ECoG HFB-power changes in the covered patch of sensorimotor cortex. BOLD fMRI activation foci were located within 1-3mm of the HFB-power ECoG foci. Both methods distinguished individual finger movement activation within a 1cm cortical patch, revealing a topographical medial to lateral layout for the little finger to index to thumb. These findings demonstrate that the BOLD signal at 7 T is strongly correlated with the underlying electrophysiology, and is capable of discriminating patterns of neuronal population activity at a millimeter scale. The results further indicate the utility of 7 T fMRI for investigation of intra-area organization of function and network dynamics.