I personally wouldn't worry about using an implicit mask, but just
plot the data. When you use a mask, your imposing another constraint
on the data. Here are a few examples:

(1) Interaction, mask A2>A1 and mask B2>B1 --> Both must have the same
direction of change, lines are both positives; not a requirement for
an interaction

(2) Interaction, mask A2>A1 --> Only A must have a positive value, B
could be positive, 0 or negative. Less restrictive, but still not a
requirement

(3) Interaction, mask B2<B1 --> B must have a negative value AND A
must have a positive value (A could be negative, but then it wouldn't
show up in the interaction contrast unless the interaction was an F);
again not a requirement for an interaction.

(4) Interaction, mask A2>A1 and mask B2<B1 --> A and B must have
opposite direction of changes, one line is positive and one line is
negative; not a requirement for an interaction

(5) masks -- what is the threshold. you could have neither A or B
being different than 0 and still get an interaction.

(6) This leads to the following approach: Mask the interaction with
A2>A1, but at a very very liberal threshold of .5. This will give you
all the regions with an interaction where A is greater than B. You can
do the opposite to find the other direction with a mask B1>B2.

Best Regards, Donald McLaren
=================
D.G. McLaren, Ph.D.
Postdoctoral Research Fellow, GRECC, Bedford VA
Research Fellow, Department of Neurology, Massachusetts General
Hospital and Harvard Medical School
Office: (773) 406-2464
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On Sun, Jan 23, 2011 at 8:16 AM, Michael_Mouthon
<[log in to unmask]> wrote:
> 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.
>
>
>