Dear FreeSurfers,

I’m trying to replicate some functional analyses that were originally done in SPM2, and am using mri_volcluster. Here’s my command: mri_volcluster --in Old8/bold/taumax30_2_21/tal-ffx/RSF_v_NIF/sig --in_type bfloat --reg Old8/bold/taumax30_2_21/tal-ffx/register.dat --thmin 3 --minsizevox 8.

I’ve noticed that the size (in voxels) and number of clusters changes if I play with the –sign flag. I am confused by this.

Here’s the output not using the –sign flag:

Cluster   Size(n)   Size(mm^3)     TalX   TalY    TalZ              Max

  1          28       1792.0     -19.80  -96.70    8.53           5.28907

  2           8        512.0      39.60   26.79   -8.07           4.15380

  3           9        576.0      99.00   16.69  -54.67           4.09661

  4          10        640.0      87.12   63.11   18.95          -3.82155

  5          13        832.0      67.32  -42.44    5.81           3.79067

  6          11        704.0     -39.60  -16.43   59.78          -3.62503

  7          11        704.0     -87.12  -75.98  -43.30          -3.59305

  8          73       4672.0      43.56  -22.33   19.54          -3.43893

  9          18       1152.0      43.56  -44.81  -41.49          -3.16065

 10           8        512.0     -51.48  -32.30   53.20          -3.09981

 11          15        960.0     -63.36  -20.49   56.30          -3.00613

 12          48       3072.0     -71.28   -7.20   11.41          -3.00547

And here’s the output with sign set to positive:

Cluster   Size(n)   Size(mm^3)     TalX   TalY    TalZ              Max

  1          35       2240.0     -19.80  -96.70    8.53           5.28907

  2          21       1344.0      39.60   26.79   -8.07           4.15380

  3          13        832.0      99.00   16.69  -54.67           4.09661

  4          10        640.0     -71.28   33.20  -35.31           3.91335

  5          21       1344.0      67.32  -42.44    5.81           3.79067

  6           9        576.0      35.64  -88.58   15.49           3.63267

Why does the size of the first cluster change? Also, where is cluster 4 from the second output in the first output? Is there some re-calculation going on here?

Dan Dillon, Ph.D.

Post-doctoral Fellow

Affective Neuroscience Lab

Dept. of Psychology, Harvard University

Phone: (617) 496-5222

E-mail: dillon@wjh.harvard.edu