In any criminal action in which the mental competency of a person charged with a felony is in question, the circuit or county court or judge in vacation on motion duly made by the defendant or the district attorney, or on the motion of the court or judge, may order the person to submit to a mental examination by a competent psychiatrist or psychologist selected by the court to determine his ability to make a defense; any cost or expense in connection with such mental examination shall be paid by the county in which the criminal action is pending.