[VoxBo] Question about FIR analysis for event-related designs

[Charan Ranganath]

Lila, I'm not sure I understand your question because I don't know the 
duration of your TR and what you're exactly going for. But my initial 
hunch is that this isn't the way that you want to do this. In a 
traditional analysis, you have a model specifying the onset and duration 
of NEURAL activity that is convolved with a hemodynamic response. The 
FIR set is designed to model the HEMODYNAMIC response to an event, based 
on the onset of a neural event, but without strict assumptions about the 
duration of neural activity.

A typical HRF lasts 16-20s. So let's say that you have a 2s TR, an event 
that is short (~2s) could be modeled with an FIR set with 8-10 points. 
For a longer event (~8s), you'd want to use more points (say 15?).

Regarding whether you can include other covariates that are not modified 
w/an FIR set, that is fine. But if the other covariates are not modeled 
in a realistic way, they might either be useless or suck up variance 
that they shouldn't.

Re your last question, I believe voxbo sets up a 0 covariate to model 
the onset of the event (t=0), whereas the other covariates model 
subsequent TRs.

Hope this helps. CR

Lila Chrysikou wrote:
> Hello everyone,
> 
> I am trying to analyze data from an event-related design using an FIR 
> analysis. I am interested in examining differences between two 
> experimental conditions relative to a perceptual baseline task. My 
> experimental conditions have a duration of 4TRs, whereas my Baseline has 
> a duration of 2TRs.
> While setting up my GLMs, a couple of questions came up:
> 
> 1) After introducing the diagonal set as a condition of interest, I have 
> selected each experimental condition and the perceptual baseline 
> separately and subsequently modified each selection with the appropriate 
> TR for each covariate type (i.e., 4 TRs for each experimental condition, 
> 2TRs for the baseline). Is that correct?
> 
> 2) If my model includes other covariates for which I am not interested 
> regarding the FIR, can I leave them as is?
> 
> 3) I noticed that voxbo automatically generates an additional covariate 
> (0); what does this covariate correspond to? Should one instead enter 
> the FIR TRs as n-1 to account for this?
> 
> I haven't been able to find much information on FIR analysis on the 
> mailing list; if anyone has additional details/advice/recommended 
> readings that wouldn't mind sharing, it would be greatly appreciated.
> 
> Many thanks in advance,
> 
> ~Lila
> 
> -- 
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> Evangelia G. Chrysikou, Ph.D.
> Post Doctoral Research Fellow
> Thompson-Schill Lab
> 
> Center for Cognitive Neuroscience
> University of Pennsylvania
> Office address: 3810 Walnut St.,Room 307, Philadelphia, PA 19104
> 215. 573.6726 (phone)
> 215. 898.1982 (fax)
> E-mail: evangelg at psych.upenn.edu <mailto:evangelg at psych.upenn.edu>

-- 
Charan Ranganath, Ph.D.
Associate Professor
Center for Neuroscience and Dept. of Psychology
University of California at Davis
1544 Newton Ct.
Davis, CA 95618

phone: 530-757-8750
fax: 530-757-8640
http://entorhinal.ucdavis.edu/~evan/index.php

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