THE EFFECT OF PACING OF EXPERIMENTAL STIMULI ON OBSERVED FUNCTIONAL MRI ACTIVITY

Neuroimaging activation studies typically observe signals during two o r more periods of differing cognitive activity which are then analyzed by a subtraction to test for localized neuroanatomical dissociations between cognitive tasks. Significant activity found between task condi tions is frequently assumed to reflect a novel cognitive process prese nt in one task and not the other. We present a conceptual framework th at considers the neural mechanisms underlying such observed neuroimagi ng changes. We propose that neuroimaging experiments which present sti muli at a fixed pace (where each trial takes the same amount of time) will be sensitive to changes in both duration and intensity of neural processing. In contrast, the signal observed during a self-paced desig n is derived from neural processing averaged over the reaction time an d hence could be less sensitive to differences in duration of neural p rocessing. As an empirical demonstration of these ideas, we studied no rmal subjects using echoplanar functional MRI during two visuospatial tasks (matching of either ROTATED or NONROTATED stimuli) performed usi ng FIXED and SELF-PACED designs. In both pacing designs, reaction time s were greater in the ROTATED than NONROTATED task, interpreted as a g reater duration of neural processing during the ROTATED compared to th e NONROTATED task. In the FIXED-PACED design, significantly greater si gnal was present within a parieto-occipital cortical region during the ROTATED task compared to the NONROTATED task. This difference was not observed during the SELB-PACED design. This result illustrates the im portance of considering trial pacing in the interpretation of function al neuroimaging activation studies. (C) 1997 Academic Press.