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 reson ance imaging (MRI). Stimuli were identical 500-ms high-contrast checkerboar d 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, I mages were acquired at 1.5 Tesla using a gradient-echo echoplanar imaging s equence sensitive to blood-oxygenation-level dependent (BOLD) contrast, Sin gle checkerboards evoked a hemodynamic response from visual cortex characte rized by a rise at 3 s, peak activation at 5 s, and return to baseline by 1 0 s. We subtracted subjects' single-stimulus hemodynamic response from thei r paired-stimulus responses to isolate the contribution of the second stimu lus. If the hemodynamic responses were fully additive, the residual should be a time-shifted replica of the single stimulus response. However, the amp litude of the hemodynamic response to the second checkerboard was smaller, and the peak latency was longer, than for the first. Furthermore, the ampli tude 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 recove ry 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 ex tended refractory period in the hemodynamic response to visual stimuli cons istent with that shown previously for neuronal activity measured electrophy siologically. (C) 2000 Academic Press.