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Methods

We present data from a number of event-related fMRI paradigms involving somatic and visual stimuli in which the aim was to model sigmoidal BOLD response profiles corresponding to activation below and above perceptual or cognitive thresholds. To model such a threshold-dependent activation, we used a three step procedure. First we estimated BOLD response magnitudes for each stimulus intensity using standard GLM single subject analysis. We then used a nonlinear fitting algorithm to fit a 3 parameter sigmoidal function to the curve of response magnitudes for each subject. In a final step, we applied group-level GLM to each of the 3 estimated sigmoid parameters, in particular the parameter that characterizes the steepness of the sigmoidal function. The group-level GLM statistical map for this parameter was then thresholded and corrected for multiple comparisons using a permutation procedure …