Abstract
The present study examines stimulus generalization among inter-item associations and its decrement through overshadowing by an extra-series cue in rats' serial learning. Two groups of rats were trained with three two-item series, consisting of a varying number of 45 mg food pellets (10-10, 1-0, and 0-10), in a straight runway, presented in semi-random order in daily sessions. The Control group was held in either a black or white holding cage during the inter-run interval in all three series and not changed during the acquisition and test periods. However, the Experimental group was kept in one cage for the 10-10 and 1-0 series, but in a cage of the opposite color for the 0-10 series during the acquisition period. While the Experimental group showed better anticipation of the 0 pellet of 1-0 series, this was disrupted when the extra-series cage cue was removed during the test period. These results suggest that the extra-series cue overshadowed the 0'-10 association in which memory of 0 pellets signaled the following 10 pellets, and stimulus generalization from 0'-10 to 1'-O decreased as a result of this overshadowing. These findings support the basic assumption of a memory-discrimination learning hypothesis that explains phrasing effects in reward serial learning in terms of two processes: (1) extra-series phrasing cues overshadowing inter-item associations, and (2) stimulus generalization in reward signal strength among inter-item associations decreasing as the result of this over-shadowing.
