Functional magnetic resonance imaging provides new constraints on theories of the psychological refractory period.

We used functional magnetic resonance imaging (fMRI) to investigate the psychological refractory period (PRP), the delay in the response to the second of two tasks occurring in immediate succession. Our results were consistent with prior work on the PRP in that when two visual-manual tasks were presented within 100 ms of each other, the second response was delayed on the order of 500 ms, compared with when the two tasks were separated by 1,500 ms. Surprisingly, in brain regions postulated to be important for executive functions, there was virtually no increase in brain activation in the short-interval compared with the long-interval condition. These data suggest that passive queuing, rather than active monitoring, occurs during the PRP.