diff --git a/spm_gmm_lib.m b/spm_gmm_lib.m
index 4a17e8e..b11de25 100644
--- a/spm_gmm_lib.m
+++ b/spm_gmm_lib.m
@@ -1502,12 +1502,12 @@
     alpha0 = b0_priors{1};
     beta0  = b0_priors{2};
     b0(k)  = 0;
-    alph   = N*S;
-    bet    = 1;
+    alph   = 0.5*N*S+1;
+    bet    = 0;
     for s=1:S
         [m,b,V,n] = get_posteriors(cluster,s);
         m1  = m(:,k) - m0(:,k);
-        bet = bet + m1.' * (n(k)*V(:,:,k)) * m1 + N/b(k);
+        bet = bet + 0.5*(m1.' * (n(k)*V(:,:,k)) * m1 + N/b(k));
     end
     b0(k) = (alph+alpha0-1)/(bet+beta0);
     % ---------------------------------------------------------------------