Several studies have shown that the strength of classically conditioned fear varies inversely with the length of the CS-UCS interval (McAllister & McAllister, 1971). If fear conditioning is important in the vicious-circle (VC) phenomenon (Brown, 1969; Melvin, 1971; Mowrer, 1947), then the interstimulus interval between brightness cues (CS) and shock (UCS) may similarly affect VC behavior. Experiment 1 examined effects of brightness cue change on VC responding. Forty male hooded rats were assigned to four groups in a 2 x 2 design, using the presence or absence of shock in the third segment during extinction and the presence or absence of cue change as independent variables. Experiment 2 examined the effects of pre-shock brightness cue changes on VC behavior, and varied the interval between the cue change location and shock. Eighty male hooded rats were assigned to eight groups in a 2 x 4 design, using the presence or absence of shock in the third segment and cue change placement as independent variables. The results indicated that brightness cue changes can reduce VC behavior. Experiment 2 also demonstrated that a cue change in the first alley segment reduced vc·responding as effectively as a total alley cue change. Changing the lower startbox cues was less effective and changing Segment 2 cues had little effect on VC behavior. The results were consistent with the ISI effects found in conditioned-fear research, since there was a relationship between the strength of VC responding and the spatio-temporal interval between changed cues and shock. / Master of Science