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Are you saying that there is a trial every 300-500ms? If so, then yes, the BOLD response will probably not be linear. However, it wouldn't necessarily kill all the signal, which may be why the results you got look reasonable. If there are, say 2 trial types (gamble vs. sure), then perhaps the distribution of trials creates enough of a jitter. Andrew Jahn wrote: > Hi SPMers, > > I ran a subject through an experiment combining eyetracking and fMRI > scanning simultaneously. I calculated where they were looking at, and > depending on where the eye fixation fell, I coded that as a trial (for > example, focusing on choosing a gamble or focusing on getting a sure > thing). I included the duration of the fixation in the regressor and > convolved each with the HRF. > > Most of these fixations are only a fraction of a second long, usually > lasting ~300-500ms. My question is, with so many trials so close > together, does this violate the assumption of linearity in the BOLD > response as outlined by Dale & Buckner (1997)? My design matrix looks > fine, and there are no instances of singularity. Furthermore, the > results that I have gotten appear to be reasonable. I would > appreciate any feedback. > > > Thanks! > > -Andrew