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 pa ired presentations of visual face stimuli using functional magnetic resonan ce imaging (fMRI). Photographs of faces were presented singly or in pairs w ith either a 1-s or 6-s intrapair interval UPI). 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 se nsitive to blood-oxygenation-level-dependent (BOLD) contrast. To examine th e refractory properties of the HDR, we subtracted the single-stimulus hemod ynamic response from the composite response evoked by face pairs for all vo xels significantly active on single face trials. The residual represents th e contribution of the second stimulus to the fMRI signal. Event-related pre sentation of faces evoked activity in medial calcarine cortex and the fusif orm gyrus bilaterally. In both calcarine and fusiform regions, the hemodyna mic response to the second face in a pair was of lower amplitude and of inc reased 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 wits 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. T here was a significant difference in mean latency to HDR peak between calca rine and fusiform cortex, with the HDR peaking about 400 ins earlier in cal carine cortex. We conclude that characteristics of the HDR, notably its amp litude, latency, and refractory properties, differ across visual cortical a reas. (C) 2001 Academic Press.