I think it is important to realize that any contrast of 1 -1 and -1 1 produce exactly the same t-statistic, only scaled by -1. SPM results mainly functions to apply a threshold to the t-maps to show what passed the significance criteria you enter. 

So, if you load the t-maps as an overlay in some other program, such as MRIcron, the 1 -1 and -1 1 will have identical t-values in every voxel, with the sign reversed. You can therefore never see overlapping activity in these two contrasts. 

I don't think you can at all apply the term "deactivation" in either of these contrasts. It is possible both conditions show an task related neural activity. The contrast shows if one conditions shows "larger" activity (as measured by Beta values). Now, things get complicated in that a zero is larger than a negative value. In this case, you can have a "decrease" in neural activity for condition A relative to baseline/rest/etc, and no real change in the condition B, which then shows as "greater activity in A", although perhaps a more accurate explanation is "less deactivation in A and B". 

For this reason, I think it is always important to look at the Beta values for each condition for your significant clusters. But then then a negative Beta value must b e interpreted with care, as it is negative with respect to everything else going on in your experiment. So if you have a short ISI, a negative Beta value may not be deactivation at all. I realize that is complicated and not necessarily helpful, but if fMRI was easy it wouldn't be so interesting.

Best of luck,
Colin. 


On 8 January 2014 14:44, Kailyn Bradley <[log in to unmask]> wrote:
Hello SPM users,

I have a questions that I know has been discussed before from review of the archives, but I am still unclear about it. I am unsure about whether or not it is appropriate to use the term "deactivation" in the following context:

In this design we are conducting a paired ttest between two conditions that our subjects were exposed to, with the two conditions being "competitor" and "unrelated". One of my paired ttest contrasts is competitor > unrelated, which I designate as 1 -1 in my paired ttest model in SPM. My other contrast of interest is the reverse of this, unrelated > competitor, which I designate as -1 1 in SPM.

My questions is, if I see activation of a certain brain area (right parahippocampal gyrus for example) in the unrelated > competitor contrast (which was -1 1 in the model), but no activation of this area that survives correction in the competitor > unrelated contrasts (1 -1 in the model), is it appropriate to say that there is therefore deactivation of the parahippocampal gyrus in the competitor condition since that activity was not seen in the competitor > unrelated contrast? My understanding is that this is an incorrect usage of the term deactivation. I am under the impression that if one area is active in a certain t contrast but not active in the reverse contrast, that this does not mean that the specific area in question is deactivated in the reverse contrast. Isn't the only way to know if an area is truly deactivated to do an ROI analysis of that particular region in the contrast of interest and then extract the contrast values to see if there is deactivation in that particular region?

Previously it was suggested to me that the contrasts of competitor > unrelated and unrelated > competitor are two one tailed tests that simply look at different directions of the same contrast. In one contrast you screen for positive t-values but in the other you screen for negative t-values. Therefore, since parahippocampal gyrus is activated in the unrelated > competitor contrast, it must be deactivated in the competitor > unrelated contrast. This seemed like a wrong explanation to me, so I wanted to seek the further guidance from the SPM community who might have more experience with this.

Could someone clarify and help add to my understanding of whether it is ever appropriate to use the term deactivation when referring to results from opposite paired ttest contrasts?

Thank you for the help.

Best,
Kailyn