By the way, the reason why I thought about adding the betas of the parametric modulator comes from some studies of Weissman lab that seem to have employed this approach when dealing with RT differences between different conditions:
Carp et al., 2010, Front Hum Neurosci http://www.frontiersin.org/Human_Neuroscience/10.3389/fnhum.2010.00231/abstract
Carp et al., 2012, Neuroimage http://www.sciencedirect.com/science/article/pii/S1053811911008597
Kim et al., 2013, PLoS One http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0060710
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