The response of signal intensity to transient (on-off) motor and sensory stimulation has been well studied; however, the dependence of signal response on the duration of stimulus requires further characterization. The objective of this study was to determine the time course of signal response in the human visual cortex to prolonged, sustained stimulation and to examine possible contributory physiologic mechanisms. Nine healthy volunteers underwent magnetic resonance (MR) imaging during sustained visual stimulation with light-proof binocular goggles. With photic stimulation, activation was observed in all subjects as an increase in signal intensity of the visual cortex. With sustained stimulation, a gradual decrease in signal intensity was subsequently observed, with progression toward an apparent steady state. Correlation with positron emission tomographic, MR spectroscopic, and visual evoked-potential data suggests that the initial uncoupling of cerebral blood flow and oxidative metabolism with a neuronal activation burst may represent a transient phenomenon. This quick-response phase may proceed to an equilibrium coupling of flow and oxidative metabolism, with a gradual normalization of venous deoxyhemoglobin levels and signal intensity.
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