Oops... There was a mistake in my first post: I get reasonable
activations with no thresholding when I contrast *each condition vs.
rest*, whereas the whole brain lights up when I do it for a *paired
comparisons* analysis, and not the other way around.
Sorry about that,
Stephane
Stephane Jacobs wrote:
> Hello everyone,
>
> In the process of trying to understand cluster thresholding, I played around
> with some different and sometimes non-sense values both for the Z score and
> the P value thresholds.
>
> My understanding of the cluster thresholding is that it comprises 2 steps:
> 1) a Z score threshold is apply to every voxel, keeping only the ones that
> pass the threshold.
> 2) the map obtained for the first step is thresholded again using the chosen
> P value, which determines a limit size for the clusters to be considered
> significant. This size is obtained from the gaussian field theory, but it's
> unclear to me how exactly the P value and the size correspond, if anybody
> wants to take a shot at explaining this... (this is actually my first
> question here I guess!)
>
>
> So, in one of my thresholding experimentations, I have thresholded my data
> to Z = 0 and P = 1.0, which means to me NO THRESHOLDING at all - all the
> voxels pass the Z score threshold (0), and the P value allows all the
> clusters as small as a single voxel to be considered significant. This is a
> stupid thresholding, but I just wanted to check that I had understood
> everything properly. Thus, I expected to get a really noisy map, with the
> whole brain lighting up (nice for Christmas...). To my wonder, I get totally
> reasonable patterns of activations!
> Of importance is that what I describe here happens when I do a
> paired-comparison analysis. When I just contrast each of my experimental
> conditions vs. rest, I do get a nice Christmas tree, as expected. Could
> anybody tell me the reason for that?
>
>
> Thanks a lot for any help,
>
> Stephane
>
>
>
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