goodfeli/pylearn — comment on commit ff6995baefeebd891d248aa222b4ac193bab758a
File:  line 0
By goodfeli:

At the very least you need something to keep sigma from becoming negative on accident. I think this is the method that James used for the ssRBM. Beyond that, I'm not sure who else really learns sigma.

Another way of doing this would just be to parameterize sigma in a way that forces it to be positive (ie, make sigma be softplus of sigma_driver).