Higher Order Chunking in Serial Pattern Learning by Rats in the T-Maze

Rats were repeatedly exposed to two- and three-trial series consisting of different reinforced (R) and nonreinforced (N) trials in a fixed order on extended visually distinct runways in a T-maze. When initially presented with the same sequence, RR or RN in Experiment 1 and RRN or RNR in Experiment 2, on each series in a session (Separate Presentations), rats developed slower running speeds or bypassed the runway in returning to the start chamber on N more than on R trials. These serial response patterns occurred whether rats experienced each sequence on a specific runway (Associated group) or equally on different runways over sessions (Nonassociated group). When sequences were intermixed within each session, only the Associated group maintained its serial pattern responses to both sequences while the Nonassociated group only did so within the RRN sequence. When allowed to choose between runways over trials, the Associated group tended to select the runway associated with the RR sequence on the first two free-choice trials while rats in the Nonassociated group tended to alternate their choices of runways. Switching the relevancy of the runway context in the second experiment caused rats in each group to react like rats in the other group on forced-choice trials. In terms of Capaldi's (1992) model of levels of chunking in serial pattern learning, our findings indicate that rats learned to anticipate at least second trial outcomes by using higher order list or contextually cued chunking and also to anticipate third trial outcomes by using lower order series chunking.