Dear John & Richard,
Richard's intuition that small areas of fMRI 'activation' may represent
false positives seems sensible; but I wanted to raise a few possible
concerns for discussion.
First, is it really true that 'real' biological effects will be spread over
more than five voxels? In numerical terms, five voxels at typical
neuroimaging sizes (3x3x3) in gray matter will contain ~13 million neurons.
In cortical terms, the distance between cortical areas isn't always that
great; about 3cm (ten voxels) between foveal representation in posterior V1
and area V5 in humans. And single cortical areas aren't that large either.
V1 is about 4x8cm in humans so even without cortical magnification 5 voxels
aligned linearly might represent one fifth of the visual field horizontally.
But perhaps these concerns are rendered moot because functional imaging
data, as Richard points out, are typically very smooth anyway?
A second concern is that the (apparent) spatial extent of a blob will vary
with height threshold and so a threshold of (say) five voxels will have very
different implications at different Z thresholds. So I'm not sure how we can
justifiably claim that a spatial extent threshold of (say) five voxels is
grounded in biological plausibility, unless that biological effect is
determined at a particular height threshold.
So I guess my bottom-line question is: if we are interested in taking into
account the spatial extent of a cluster in deciding whether to report it,
shouldn't we be using the cluster level statistics that take into account
both peak height and spatial extent? Rather than applying an (inevitably
somewhat arbitrary) spatial threshold to voxel level statistics? Or is
everyone generally comfortable with the small-spatial-extent threshold
approach?
Best wishes,
Geraint
|