Reward magnitude, but not time of day, influences the trial-spacing effect in autoshaping with rats

The arousal hypothesis of the trial-spacing effect suggests that spaced-trial training increases emotional arousal and thus invigorates Pavlovian behavior, relative to massed-trial conditions. Emotional arousal was manipulated by varying reinforcer magnitude during training (either one or five food pellets/trial, across groups). In addition, autoshaping training was administered either in the morning (0900 h) or in the evening (1700 h). Rats were housed in an enclosed colony room and exposed to a regular light:dark cycle (light from 0700 to 1900 h). Available evidence indicates that reinforcer magnitude and time of day are related to arousal levels. As expected, a larger reinforcer magnitude led to a highly significant trial spacing effect. Evening training led to a higher response rate than morning training, but the trial-spacing effect was equally strong whether training was administered in the morning or in the evening. These results provide partial support for the arousal hypothesis and are discussed in the context of research on schedule-induced behavior.