It was brought to my attention (thank you, Neil Ranjan Chatterjee) that data values above 1.0 were truncated after being processed with the vbm5 toolbox. Investigating the source of this truncation, I discovered that it only occurs after installation of the spm5 update 1048 or later. Update 1048 was released on Jan 8, 2008, and in it, the function spm_write_vol.m was modified. Before this update, a scale factor was estimated such that the maximum range of integer values was used. After the update, a truncation of integer data may occur if the scaling factor is pre-defined. Unfortunately, I have used a fixed scaling factor for modulated data and uint8 as the data type in the vbm5 toolbox. This worked fine before the update, but might lead to a truncation after the update was installed.
Regrettably, the truncation might also affect the statistical analysis of these data. If you have updated spm5 to version 1048 or higher (or after Jan 8, 2008), please check your modulated data. This tool will check your data for a truncation above 1.0.
If your data was truncated, you do not have to estimate the segmentation again, but you need to rewrite the data with the new vbm5 version 1.18 (use option “Write already estimated segmentations”).
