Background Activity in the living human brain can be studied using multiple methods, spanning a wide range of spatial and temporal resolutions. We investigated the relationship between electric field potentials measured with electrocorticography (ECoG) and the blood oxygen level-dependent (BOLD) response measured with functional magnetic resonance imaging (fMRI). We set out to explain the full set of measurements by modeling the underlying neural circuits. Results ECoG responses in visual cortex can be separated into two visually driven components. One component is a specific temporal response that follows each stimulus contrast reversal ("stimulus locked"); the other component is an increase in the response variance ("asynchronous"). For electrodes in visual cortex (V1, V2, V3), the two measures respond to stimuli in the same region of visual space, but they have different spatial summation properties. The stimulus-locked ECoG component sums contrast approximately linearly across space; spatial summation in the asynchronous ECoG component is subadditive. Spatial summation measured using BOLD closely matches the asynchronous component. We created a neural simulation that accurately captures the main features of the ECoG time series; in the simulation, the stimulus-locked and asynchronous components arise from different neural circuits. Conclusions These observations suggest that the two ECoG components arise from different neural sources within the same cortical region. The spatial summation measurements and simulations suggest that the BOLD response arises primarily from neural sources that generate the asynchronous broadband ECoG component.
Bibliographical noteFunding Information:
This work was supported by NEI grant RO1-EY03164 (B.A.W.), NEI grant K99-EY022116 (J.W.), NIH grant R01-NS0783961 (J.P.), and the Stanford NeuroVentures Program (J.P.). We thank Dora Hermes for helpful feedback on a draft of the manuscript and for providing advice and assistance on ECoG electrode localization on individual brain surfaces. We thank Vinitha Rangarajan for assistance with ECoG data collection and Hiroshi Horiguchi, Kai Miller, Anthony Norcia, and Justin Ales for helpful discussions.