Serial learning with wild card items by monkeys (Cebus apella): implications for knowledge of ordinal position

We investigated monkeys' knowledge of the ordinal positions of stimuli that formed a 5-item serial list, ABCDE, by means of wild card items (W) that could substitute for items in the original series. In Experiment 1, training with wild cards was given on 3-, 4-, and 5-item series. In the last of these series, the wild card substitutions created five wild card sequences, WBCDE through ABCDW. During the final 10 sessions of training with each of two different wild cards (Items x and Y), the 3 subjects were able to successfully complete almost 60% of the wild card sequences. In Experiment 2, the two wild cards were presented on the same trial in 10 different double wild card sequences (e.g., AXCDY). The 2 monkey subjects correctly completed about 59% of the double wild card sequences during the final two training sessions. The performance levels achieved on single and on double wild card sequences, although well below that observed on the baseline sequence ABCDE (90% or better), support the view that the monkeys possessed some knowledge regarding the ordinal position of each baseline item. Consequently, an associative chain interpretation, which does not provide for knowledge of ordinal position, falls short as a complete account of the monkey's capacity for serial learning.