Evidence for a refractory period in the hemodynamic response to visual stimuli as measured by MRI.

We investigated the effects of paired presentations of visual stimuli upon the evoked hemodynamic response of visual cortex measured by magnetic resonance imaging (MRI). Stimuli were identical 500-ms high-contrast checkerboard patterns, presented singly or with an interpair interval (IPI) of 1, 2, 4, or 6 s (onset-to-onset), followed by an intertrial interval of 16-20 s. Images were acquired at 1.5 Tesla using a gradient-echo echoplanar imaging sequence sensitive to blood-oxygenation-level dependent (BOLD) contrast. Single checkerboards evoked a hemodynamic response from visual cortex characterized by a rise at 3 s, peak activation at 5 s, and return to baseline by 10 s. We subtracted subjects' single-stimulus hemodynamic response from their paired-stimulus responses to isolate the contribution of the second stimulus. If the hemodynamic responses were fully additive, the residual should be a time-shifted replica of the single stimulus response. However, the amplitude of the hemodynamic response to the second checkerboard was smaller, and the peak latency was longer, than for the first. Furthermore, the amplitude decrement was dependent upon IPI, such that the response to the second stimulus at 1 s IPI was only 55% of that to a single stimulus, with recovery to 90% at a 6 s IPI. Peak latency was similarly dependent upon IPI with longer latencies observed for shorter IPIs. These results demonstrate an extended refractory period in the hemodynamic response to visual stimuli consistent with that shown previously for neuronal activity measured electrophysiologically.