Numerical and sensory categorization of serial information by animals

Rats were trained in a straight runway on a series consisting of 4 elements, each element being defined in terms of the number of .045-gm. Noyes pellets used as reward on 4 runway trials separated by about 20 sec. The series elements were 2, 6, 12, and 0 (2-6-12-0). The animals were trained each day on 3 such series, the series themselves being separated within a day by about 12 min. for 28 days before introducing a new transfer series to assess the relevance of the order of the series elements. In previous experiments with the simpler series, 2-12-0, alterations in order of elements eliminated the animals' tendencies to run much slower to the terminal nonreward element of the series, but transfer was clearly shown here when we shifted from 2-6-12-0 to 6-12-2-0. That is, the animals continued on the transfer test to approach the terminal element slowly, a result which we interpret as supporting the view that the rats either counted the rewarded trials and used that count to predict terminal nonreward, or they behaved on the basis of cues associated with the ordinal position of the trial.