To follow-up all input files required are on the attachment to reproduce this issue

Using SDM 4.31 I did the following
Preprocessing - choose VBM gray matter, all other options are default
Mean - default options
Threshold - default options
Open MyMean_z_p_0.00500_1.000_10_neg.nii.gz to see the ring

I have tried increasing the t value and this can lead to a dot within the ring, if the coordinate is moved this can result in a normal single blob being produced. I'm wondering if it is therefore some interaction between the gray matter mask and the thesholding causing this

Thanks,

Matthew