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Hi Raphael,

Parametric modulation modulates the within-subject response by the parametric value. The canonical HRF value shouldn't change as the PM is orthogonal to the canonical HRF term.

The parametric regressor term represents the slope of the effect of the parametric modulator in each subject.

It seems that you interest is in the effect between subjects of the canonical HRF, but accounting for any subject differences in frequency. As such, you'd want to account for frequency in the group model as the PM term measures the within-subject effect of frequency.

Best Regards, 
Donald McLaren, PhD


On Wed, May 31, 2017 at 9:59 AM, Raphael F.C. <[log in to unmask]> wrote:
Dear experts,

I apologize for the basic question but I am not sure if I should use the parametric modulation.

I have finger tapping images of volunteers, whose tapping frequency was NOT paced by any external input. So tapping frequencies between subjects and even between blocks of the same run are different.

I am interested in the amplitude of activation discounted of the effect of different frequencies, since I assume activation will be greater for higher tapping frequencies.

Should I use parametric modulation for this? How?

Thank you very much,

Raphael