Although functional magnetic resonance imaging is an important tool for measuring brain activity, the hemodynamic blood oxygenation level dependent (BOLD) response is only an indirect measure of neuronal activity. Converging evidence obtained from simultaneous recording of hemodynamic and electrical measures suggest that the best correlate of the BOLD response in primary visual cortex is gamma-band oscillations ( approximately 40 Hz). Here, we examined the coupling between BOLD and gamma-band amplitudes measured with magntoencephalography (MEG) in human primary visual cortex in 10 participants. In Experiment A, participants were exposed to grating stimuli at two contrast levels and two spatial frequencies and in Experiment B square and sine wave stimuli at two spatial frequencies. The amplitudes of both gamma-band oscillations and BOLD showed tuning with stimulus contrast and stimulus type; however, gamma-band oscillations showed a 300% increase across two spatial frequencies, whereas BOLD exhibited no change. This functional decoupling demonstrates that increased amplitude of gamma-band oscillations as measured with MEG is not sufficient to drive the subsequent BOLD response.
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