So my first question is....if my conjunction analysis (I'm using the Global null conjunction hypothesis) returns activation patterns with negative t values, does that mean the activation patterns are actually deactivation? And maybe more importantly are negative t values even statistically relevant (i.e. can I report them as actual results?). I find that the probability of getting negative t values increases with more subjects I include in the conjunction analysis (i.e. all positive t values with a conjunction of 10 subjects, but a majority of negative t values with a conjunction of 20 subjects). I also get negative t values when I'm looking a deactivation patterns (i.e. areas less active during the stimulus over baseline). So I guess what I'm really asking is how do I interpret the negative t value results.

My second question revolves around how to display results with negative t values. I'm able overlay the blobs of negative t values on the three slices using the overlay slices button on the Results GUI, but rendering the results on the 3D brain only displays positive t values and same with using the display_slices command. If I use the save button to save the results as an .img file, SPM does not save any of the negative t values....I think SPM somehow writes any t value of zero or less as an NaN. And so I've been advised that in order for SPM to save negative t values, it would involve re-writing SPM code which is definately beyond my scope of expertise.

Anyways, I'm wondering if anyone else has had this sort of problem with displaying results with negative t values, and maybe how to overcome it...I'm hoping that I won't be limited to using the 3 slice overlay button, since every three slices elicits a different t value colour range.

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Nicholas Masse, M.Sc.
Research Technician
Neurovascular Research Lab
School of Kinesiology
University of Western Ontario
Rm. 3110 Thames Hall
519.661.2111 x88526
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