Both PMs and regressors take away variance from the signal. If your regressor was identical to the PM term, then it wouldn't matter if you called it a PM or a regressor.

The key is what are you putting into the model. To put a regressor into the model, you need 1 value per timepoint. By entering 1 value per trial into the PM module, SPM will generate a regressor that has 1 value per timepoint by convolving the stimulus weighted by the PM values with the HRF.

If you had a behavioral regressor with 1 value per timepoint, you could put it in as a regressor and get a contrast value for it. However, I'm not sure how you'd interpret a behavioral covariate at each timepoint that isn't convolved with the HRF as you need something that would align to the BOLD signal (BOLD signal is the neural activity filtered with a hemodynamic response function (not necessarily the canonical HRF).

Hope this helps.

Best Regards,

Donald McLaren, PhD

On Thu, Aug 20, 2015 at 12:08 PM, Joelle Zimmermann <[log in to unmask]> wrote:

Hi Donald - thanks very much!

So by adding behavioural as a regressor, I'm taking any effects of behaviour (on the signal) away? I'm more interested in how behaviour across the trials actually modulates activation, which is what is done with a parametric modulator, right?

I wonder what it means if I do use behaviour as a regressor, and then create a contrast highlighting this regressor, and take that to the group level, where I do a one sample t-test with these .con images... would you be able to give me your thought about that?

On Thu, Aug 20, 2015 at 11:52 AM, MCLAREN, Donald <[log in to unmask]> wrote:
See below.

Best Regards,
Donald McLaren, PhD

On Thu, Aug 20, 2015 at 9:59 AM, Joelle Zimmermann <[log in to unmask]> wrote:
Hi SPMers,

I have a couple of questions about parametric modulators. I'd appreciate thoughts about any/all of them :-)

1) When entering a parametric modulator at the first-level, where I have one condition with 10 onsets, am I expected to have 10 parametric modulator values? (ie one per onset)? The PM values I want to enter are behavioural performance values, and indeed I have 1 performance value per onset, so logically this would make sense, I am just not sure what the program expects.

PMs are meant to modulate each trial, thus you need to have 1 value per trial.

2) Can I use raw behavioural performance scores as covariates? These performance scores are all numbers between 0 and 1. Or is some adjustment to the values advised?

As long as you think the BOLD response changes linear over the range of performance, no adjustment is necessary. The program should remove the mean from the values.

3) MOST IMPORTANTLY: What's the difference between adding behavioural performance as a parametric modulator, and adding it as a regressor?

Regressors aren't convolved with the HRF.

Thanks in advance,
Joelle