Where is the default values for a session parameter stored

I need to write a stored procedure that will check the V$PARAMETER table to see if the default values of a parameter is used and if not, to change the values to the default values. I can determine if the parameter is not the default from the ISDEFAULT column, but what Oracle object stores the default values for a parameter. I've Goolged and found nothing. It has to be stored in a DBA/V$ or some other table just don't know which one.

Thanks in advance.