Preliminary evidence of abstract same/different discrimination learning in rats

Abstract

Two rats were trained to discriminate stimulus sets that consisted of identical or different objects. Same-set stimuli consisted of four identical objects (AAAA, BBBB, etc.), while different-set stimuli consisted of four different objects (ABCD, CDBA, etc.). Same and different stimuli sets were placed on opposite sides of two compartments within a discrimination box. Staying on the same-set side (Rat 1) or the different-set side (Rat 2) was rewarded 60 s after the start of a trial. Rat 1 learned the acquisition task with three same-sets (AAAA, BBBB, and CCCC) and different sets but could not accomplish the four same-set tasks and was dropped from the experiment. Rat 2 accomplished the acquisition task with four different objects (A, B, C, and D) and responded significantly better than chance to stimulus sets with novel objects (E, F, G, and H). Rat 2 also learned to discriminate same- and different-sets with 8 different objects (A, B, C, D, E, F, G, and H) and showed reliable performance on test trials with novel objects (I, J, K, and L). These results are discussed in terms of quantitative entropy discrimination as well as qualitative relational same/different learning.