> Is it common to use a cluster threshold with FWE-correction or is this too stringent? We found a cluster of voxel size 1 which is probably not meaningful. Can we apply a cluster threshold and if yes, how do we determine the extent?
I interpret your question as saying that you applied a voxelwise FWE
correction (say, at p < .05), which left you with some results. These
results included a "cluster" of a single voxel, and you're wondering
how to deal with that.
The short answer is, I don't think there's a short answer.
The longer answer is that, as you've applied a completely appropriate
voxelwise correction for multiple comparisons, I think you would be
justified in interpreting that single voxel as "significant". In all
likelihood, that voxel is not an isolated point, but merely the peak
of an underlying subthreshold cluster, which has some spatial extent.
You can verify this for your own sanity by looking at the results (t
statistic) at a lower threshold, or by looking at the con* image (i.e.
the parameter estimate), or both.
As far as a cluster threshold, SPM should give you cluster-level
correction values in the results table, and these are appropriate to
use. It's unlikely that a single voxel would be significant. But,
this is at a different level of inference, and given that you have
already passed what is usually a more stringent threshold for
voxelwise significance, I don't think it necessarily makes sense to
worry about an additional test.
That being said, lots of people will set arbitrary cluster-size
thresholds (e.g., 2 or 5 voxels) when using FWE correction, precisely
to avoid what seems like it may be a dodgy interpretation. As long as
these thresholds are quite small I don't generally have a problem with
it, but others may feel differently.
All of which is to say that, I think there are points either way, and
it's up to you to decide what the most helpful and honest
communication of the data are (and convince some reviewers your'e
Hope this helps!
Dr. Jonathan Peelle
Department of Neurology
University of Pennsylvania
3 West Gates
3400 Spruce Street
Philadelphia, PA 19104