On Wed, 21 Apr 1999, carol docherty wrote:
>
> Hi,
>
> Can someone please explain exactly what selecting the
> 'maxima' button does, when it comes to displaying a
> Zmap?? From the SPM96 notes, I was under the
> impression that, by selecting 'maxima', you were
> selecting the voxel with the greatest Z-score, so that
> you were then be able to plot out the time-series for
> that particular voxel.
>
> However, an example of an SPM output from a recent
> fMRI experiment is as follows:-
>
> ---------------------------------------------------------
> cluster voxel uncorrected x,y,z
> level{k,Z} level{Z} k & Z {mm}
> ---------------------------------------------------------
> 0.000(32,5.69) 0.000(5.69) 0.000 0.000 -8 -96 5
> 0.001(4.87) 0.000 4 -96 5
> 0.053(3.72) 0.000 8 -76 5
>
>
>
>
>
> ---------------------------------------------------------
>
> When I selected 'maxima', the voxel at {8,-76,5} was
> chosen and, when I tried to plot out the time-series,
> a z-score of 3.72 was given. Shouldn't the maximum
> z-score be 5.69??
>
> Thanks in advance,
>
> Carol.
Hi Carol,
The maxima output in Results is totally dependent on where the cursor is
on the glass brain - the algorithm goes on a limited "local" search from
the current cursor position. If you haven't moved it, it is likely that
the x,y,z starting co-ordinates are the centre of the image [0,0,0] so the
nearest local maxima to that point will be detected. You can pick up the
cross-point on the glass brain with your mouse (left-button) and move it
around to the desired position. Try this then press maxima and you may
find a differnt result.
Regards - Mike
--
_____________________________________________________________________
Mike Glabus PhD, Principal Physicist
MRC Brain Metabolism Unit Tel Intl: 44-(0)131 537 6818/6282
Royal Edinburgh Hospital FAX Intl: 44-(0)131 537 6110
Morningside Park Pager : 5236, via 0131 537 6000
Edinburgh, EH10 5HF, SCOTLAND
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