Hi Nan,
3dsvm was originally developed for classification of temporal and minimally pre-processed FMRI data. Thus, temporal de-trending of the classifier output is performed by default to correct for temporal drift that we observed. Please take a look at
LaConte, 2007 (Fig. 4) for more details. However, performing de-trending of the classifier output is not the only and might not be the best way to correct for drift of the FMRI signal/classifier output, so you need to decide if it's necessary.
If you are NOT using temporal FMRI data (e.g. GLM beta maps, functional connectivity maps, structural data, etc.), please use the -nodetrend option to disable temporal de-trending.
Jonathan
