I ran parametric modulations on block design to investigate 2 regressors effects on my activations. But I was wondering several things about this analysis:

1) Does this approach allow us to reveal the effect of the regressor on the brain activity? or does it label this effect as a non-interest factor and suppress its effect on the brain activity?

Yes. It will show you which brain areas vary with your PM. The mean effect of the task will still be there.

2) If we have 2 parametric regressors in the same model, is it possible to investigate the effect of the first and the second regressors separatlety? even if they are integrated in the same 1st level analysis?

No. While you can look at the effect of PM1, the model still accounts for the effect of PM2, so they are not entirely separate. In the multiple PM model, the PM1 effect is the unique effect of PM1 after accounting for the effect of PM2 and vice versa.

If you want to control for the other PM, then putting a 1 over PM1 will test the effect of PM1 and a 1 over PM2 will test the effect of PM2.

Thanks a lot

Best regards

Anne-Lise