Regional differences in the refractory period of the hemodynamic response: an event-related fMRI study

We investigated the characteristics of the hemodynamic response (HDR) to paired presentations of visual face stimuli using functional magnetic resonance imaging (fMRI). Photographs of faces were presented singly or in pairs with either a 1-s or 6-s intrapair interval (IPI). Each trial (single face or face pairs) was followed by an intertrial interval of 16-20 s. Faces were presented at fixation and passively viewed by the 10 subjects. Images were acquired at 1.5 Tesla using a gradient-echo echo-planar imaging sequence sensitive to blood-oxygenation-level-dependent (BOLD) contrast. To examine the refractory properties of the HDR, we subtracted the single-stimulus hemodynamic response from the composite response evoked by face pairs for all voxels significantly active on single face trials. The residual represents the contribution of the second stimulus to the fMRI signal. Event-related presentation of faces evoked activity in medial calcarine cortex and the fusiform gyrus bilaterally. In both calcarine and fusiform regions, the hemodynamic response to the second face in a pair was of lower amplitude and of increased latency at 1 s IPI, with significant recovery of both amplitude and latency toward single-stimulus values at 6 s IPI. At 1 s IPI, significantly greater recovery was found in posterior fusiform regions (50-60%) than in midfusiform regions (10-40%). These regional differences were not apparent at 6 s IPI. No differences were found across slices in calcarine cortex. There was a significant difference in mean latency to HDR peak between calcarine and fusiform cortex, with the HDR peaking about 400 ms earlier in calcarine cortex. We conclude that characteristics of the HDR, notably its amplitude, latency, and refractory properties, differ across visual cortical areas.