I don't know about that exactly, but my understanding was that this is similar in justification to compelling a person to be fingerprinted or give a DNA sample. To me there does seem to be a fairly major difference between forcing someone to disclose information held in their mind and forcing them to provide a biometric. The former seems equivalent to compelling testimony against oneself. I have a hard time seeing the latter as compelling testimony against oneself, especially if giving fingerprints or DNA isn't.