A) Re. “Follow up question: Should we be concerned that it states “Threshold volume  does not appear to be a stat!” - this is our t statistic volume?”:
No, this should not be concerning. I guess I should change the message to be “…does not appear to be an AFNI-produced stat”. It is looking for header info created by AFNI programs when creating stats. Since this statistic volume came from outside software, it doesn’t bear any of those markings, which is fine. Sorry for this confusion.
B) Re. “Follow up question:What does the  refer to in ‘fl.modperson.tva’? (The t value was specified to be sub-brick 1, assuming that’s the ‘’ before fl.mod…)”:
So, the  in the output is describing the “ithr” index—which is correctly listed here.
The second part of that line in single quotes—what appears as ‘fl.modperson.tva’—is the string “label” of the volume, which here does look a bit odd, but I don’t see how it has an affect. To verify this, what is the output of:
3dinfo -label modperson_concat.nii
? Normally, the label would be something descriptive—in an AFNI-produced dataset, for example, one might have something meant to codify what the item is (what kind of stat, or a coefficient) and where it came from (e.g., visual contrast, auditory contrast, visual - auditory contrast, etc.):
I am not sure where the displayed string label would be coming from—but let’s see what the output of that 3dinfo command above is.
C) Re. the 1sided/2sided aspects:
Some statistics (like the F-stat) only have values zero and greater; others (like the Pearson correlation or t-stat) can be negative or positive. For the former, which are >=0, I believe only (rightward) 1sided testing is ever performed. For the latter, 2sided testing can be performed.
This program is complaining about running a 2sided test on somethign that looks like it only has values >=0 (so, like an F-stat), for which 2sided testing would be inappropriate. I am not sure what statistic your volume holds—what is it?
Also, to check the range of values, what is the output of:
3dinfo -dmin -dmax modperson_concat.nii''
When you are displaying your data–the modperson_concat.nii file–are you specifying the th volume for both overlay and thresholding? Or is the th volume the overlay, and the th volume the thresholder? (In AFNI default file loading, I would expect the latter–we like to view the Beta weights or coefficients, and threshold based on the statistic, making use of both pieces of modeling information.)