Hello everyone,

I am working an fMRI data set which has been smoothed with a 3mm fwhm Gaussian kernel. I would like to increase the smoothness of the data by applying additional Gaussian smoothing on top of this already smoothed data. Ideally I would like to have an equivalent effect like if applying a single Gaussian smoothing of 5mm fwhm.

I know from Gaussian blur on 2D images that I would get the equivalent effect of a 5mm kernel smoothing if I apply additional smoothing of 4 mm kernel on top of images that were already smoothed with 3mm kernel, as sqrt(3^2 + 4^2) = 5. I wonder whether this also apply to 3D brain volume?

I have tested it on the standard mask and T1 images and found that the result of 3mm + 4mm smoothing is very similar, but not identical to the result of a single 5mm smoothing. Do you know whether there is a better way to calculate what kernel size I need in order to get a smoothing effect of 5 mm kernel?

Thanks so much,

Yuan-hao