In order to study the influence of rate of attenuation of reinforcement on the occurrence and strength of behavior contrast effects in the rat, nine subjects were divided into three groups corresponding to 3 session, 1 session and immediate attenuation of reinforcement in phase 2 of a 3 phase (mult VIl'VIl'; mult VIl' Ext; mult VIl'VIl') paradigm using intensity of illumination.as the discriminative stimuli. A long baseline was employed in Phase 1 in order to ensure stability of responding in the constant component in Phase 2. None of the rats showed contrast effects and generalization testing confirmed this was the result of poor stimulus control.

A second experiment using a further 9 subjects and one pilot rat was performed after changes in procedure, including introduction of a 7 second change over delay, and extension of session length from 18 to 36 minutes. Again, no positive contrast effects were found but there was an increase in the degree of stimulus control,most noticeably the pilot rat which exhibited a negative contrast effect in phase 3. The implications of these findings in the light of other subsequent research employing long baselines was discussed with particular regard to features of long baselines which may retard stimulus control.