Good morning to all SPM community,
I need to know if the mathematical sign of betas should be taken account during implicit masking of interaction contrast.
Reminder of the standard procedure:
To disambiguate an interaction contrast with 2 factor and 2 level ((A2>A1) > (B2>B1) and (B1>B2) > (A1>A2) which have both the contrast vector -1 1 1 -1), we usually use an inclusive masking with (A2>A1) and (B2>B1) if we are interest by the comparison ((A2>A1) > (B2>B1).
Now come my question:
I was surprise to found in the article Sass, K. et al (2009) [Taxonomic and thematic categories: Neural correlates of categorization in an auditory-to-visual priming task using fmri] a different procedure:
They studied the interaction [thematical related>unrelated]>[taxonomical related>unrelated] masking this [thematical related>unrelated] and !!![taxonomical unrelated>related]!!!
In respect to previous notation, It correspond to performed an interaction ((A2>A1) > (B2>B1) with (A2>A1) and !!!(B1>B2)!!! instead of (B2>B1) as mask.
Why the second mask was inverted in respect to the related>unrelated (2>1) of interest?
In attachment, you will found the contrast estimate plot of this article. We can notice that the comparison [thematical related>unrelated] give a positive difference and that [taxonomical related>unrelated] gave a negative difference.
The inversion of the second mask [taxonomical unrelated>related] permit to consider a positive difference.
Is this procedure right ? Why ?
Do you need to take account the mathematical sign of betas instead of the related>unrelated ?
If the response is yes, how can you manage the pattern differences in the whole brain (this graphic represent only one voxel)?
Thank you in advanced and best regards.
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