Say the following phrases at the title screen to change the Doctor's expression:

Say 'Cilantro' and he will make a unhappy face. In the European version say 'Coriander'.
Say 'Pickled plum' and he will make a sour face.
Say 'Brain Age Two' to make his head swell. In the European version say 'More brain training'.
Say 'Doctor' to make him laugh.