Dear SPMers,
We are currently working with a model using parametric modulation to examine a linear increase in brain activation given two levels of difficulty (easy vs. hard). We assume that giving the program a contrast of 1, 0 will yield a con image that will show significant areas of activation as a linear increase from easy to hard compared to the global brain mean.
What does a negative contrast value mean in a parametric modulation? Would -1 0 yield a contrast in which a linear decrease in brain activation from easy to hard can be observed or will it yield a contrast in which a linear increase from hard to easy can be observed? Or, perhaps we are misinterpreting the meaning of -1 in a parametric modulation overall.
Any help will be greatly appreciated.
Best,
Siobhan Hoscheidt, M.A. & Elsa Baena, M.A.
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