Just as subjects are samples from a hypothetical population pool, so trials in an FMRI experiment are instantiations from a condition category. If cross-subject variability should be properly accounted for, why would cross-trial variability not be on an equal footing? In addition to improving generalizability, capturing trial-level effects may also gain estimation accuracy and statistical sensitivity. The analytical tool at the whole-brain voxel level is already available: 3dLMEr.
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Gang