TY - JOUR T1 - Decorrelated input dissociates narrow band gamma power and BOLD in human visual cortex JF - The Journal of Neuroscience JO - J. Neurosci. DO - 10.1523/JNEUROSCI.3938-16.2017 SP - 3938-16 AU - R. Butler AU - P. M. Bernier AU - J. Lefebvre AU - Guillaume Gilbert AU - K. Whittingstall Y1 - 2017/04/28 UR - http://www.jneurosci.org/content/early/2017/04/28/JNEUROSCI.3938-16.2017.abstract N2 - Although Functional Magnetic Resonance imaging (fMRI) using the blood-oxygen-level-dependent (BOLD) contrast is widely used for non-invasively mapping hemodynamic brain activity in humans, its exact link to underlying neural processing is poorly understood. While some studies have reported that BOLD signals measured in visual cortex are tightly linked to neural activity in the narrow band gamma (NBG) range, others have found a weak correlation between the two. To elucidate the mechanisms behind these conflicting findings, we hypothesized that BOLD reflects the strength of synaptic inputs to cortex whereas NBG is more dependent on how well these inputs are correlated. To test this, we measured NBG, BOLD and cerebral blood flow (CBF) responses to stimuli that either correlate or decorrelate neural activity in human visual cortex. Next, we simulated a recurrent network model of excitatory and inhibitory neurons that reproduced in detail the experimental NBG and BOLD data. Results show that the visually-evoked BOLD response was solely predicted by the sum of local inputs whereas NBG was critically dependent on how well these inputs were correlated. In summary, the NBG-BOLD relationship strongly depends on the nature of sensory input to cortex: stimuli that increase the number of correlated inputs to visual cortex will increase NBG and BOLD in a similar manner, while stimuli that increase the number of decorrelated inputs will dissociate the two. The NBG-BOLD relationship is therefore not fixed, but is rather highly dependent on input correlations that are both stimulus- and state-dependent.SIGNIFICANCE STATEMENTIt is widely believed that gamma oscillations in cortex are tightly linked to local hemodynamic activity. Here, we present experimental evidence showing how a stimulus can increase local blood flow to the brain despite suppressing gamma power. Moreover, using a sophisticated model of cortical neurons, it is proposed that this occurs when synaptic input to cortex is strong yet decorrelated. Since input correlations are largely determined by the state of the brain, our results demonstrate that the relationship between gamma and local hemodynamics is not fixed, but rather context dependent. This likely explains why certain neurodevelopmental disorders are characterized by weak gamma activity despite showing normal blood flow. ER -