In addition to the method of exposing animals to a series of gradually strengthening excitors, concurrent presentation ofany stimuli-evoking antagonistic reactions is also effective in weakening responses to the target excitor without evoking an inhibitory process (Guthrie, 1952). These two methods are, respectively, referred to as toleration and interference (Poppen,1970). Their combination underlies many practical procedures, including systematic desensitization therapy (Wolpe, 1958,1961). Another example of such a combination can be seen in some stimulus fading studies. For example, by using aninterlocking device, Öst (1978) gradually brightened (or faded in) a phobic picture up to the full strength and dimmed (orfaded out) a pleasurable picture down to zero illumination for treatment of a patient with phobia.