Differential conditioning as a function of exposure time to discriminative and nondiscrimniative cues preceding response

Abstract

Differential conditioning has concerned itself with the effects on performance that are produced by concurrently exposing subjects to different rewards in the presence of discriminative stimuli, Performance to a smaller reward alternative is typically found to be depressed below that of a group receiving small reward in both alternatives, The magnitude of this depression or "negative contrast effect" appears to be positively related to the magnitude of the larger reward, Also the presentation of discriminative cues prior to making the instrumental response has been shown to have an effect on the depression. The present experiment was designed to obtain further information concerning the nature and importance of the temporal presentation of the discriminative cues preceding the response, Second, the effect of different start box confinement periods without discriminative cue presentation was investigated. Third, the effect of an abrupt shift in start box confinement and/or cue presentation was investigated. Five groups of rats were used in a straight runway. During the first phase of the experiment three groups were confined to the start box for 0, 2, or 8 sec, under nondiscriminative conditions, Two additional groups received discriminative cues signalling large or small reward for 2 and 8 sec., respectively, in the start box. At the end of the first phase each group was randomly divided into three subgroups which received no start box confinement, 8-sec. confinement without discriminative cues present, or 8-sec. confinement with discriminative cues present. The results of the first phase indicated that all subjects learned the discrimination, Start box confinement was found to depress performance to both alternatives with longer durations producing greater depressions. However, discriminative cue presentation in the start box did not effectively reduce the depression in small reward speeds, because, most likely negative contrast failed to occur for reasons unknown to the investigator. During the initial portion of the second phase, the imposition of different start box conditions resulted in a general disruption in performance. During the terminal portion, appropriate responding was seen in the start and run measures. These results provide a clear demonstration that start box confinement can have a pronounced effect upon subsequent behavior. The failure to observe negative contrast effects and, consequently, the failure to observe their attenuation by presenting discriminative cues in the start box, indicate that there remain unidentified factors vital for the occurrence of contrast.