The dataset you uploaded has negative values in it, so the smoothed dataset will have negative values; how did you create the original data?

% 3dinfo small+orig

Dataset File: small+orig
Identifier Code: XYZ_g0Jw5PHjIBqeGddU0Axy1w Creation Date: Thu Nov 17 11:53:49 2005
Dataset Type: Echo Planar (-epan)
Byte Order: LSB_FIRST [this CPU native = MSB_FIRST]
Storage Mode: BRIK file
Data Axes Orientation:
first (x) = Right-to-Left
second (y) = Anterior-to-Posterior
third (z) = Inferior-to-Superior [-orient RAI]
R-to-L extent: -118.125 [R] -to- 118.125 [L] -step- 3.750 mm [ 64 voxels]
A-to-P extent: -118.125 [A] -to- 118.125 [P] -step- 3.750 mm [ 64 voxels]
I-to-S extent: -58.100 -to- 81.900 [S] -step- 5.000 mm [ 29 voxels]
Number of time steps = 3 Number of values at each pixel = 1
Time step = 1.500s Origin = 0.000s
-- At sub-brick #0 'ts.reg.despi[0]' datum type is short: -16 to 5632
keywords = ts.reg.despike.oral.blur.fwhm.6.011.test+orig[0]
-- At sub-brick #1 'ts.reg.despi[1]' datum type is short: -19 to 5865
keywords = ts.reg.despike.oral.blur.fwhm.6.011.test+orig[1]
-- At sub-brick #2 'ts.reg.despi[2]' datum type is short: -19 to 5860
keywords = ts.reg.despike.oral.blur.fwhm.6.011.test+orig[2]

----- HISTORY -----
[nlobo@marie: Thu Nov 17 11:53:49 2005] 3dTcat -prefix small 'ts.reg.despike.oral.blur.fwhm.6.011.test+orig.[0..2]'