Rats respond where it counts

Abstract

Summary Taniuchi, Sugihara, Wakashima, and Kamijo (2016) report the surprising finding that rats can transfer numerical discrimination to novel objects. Further experiments show that rat numerical discrimination is flexible, as it can both count homogeneous and heterogeneous objects and omit an odd object. Keywords Numerical processing . Object recognitionA popular procedure for the study of numerical discrimination in animals uses a row of objects, one of which contains a food reward. The object at a fixed ordinal number from one side of the row is always the correct one, and the question of interest is whether an animal can learn to reliably choose that object. In an article in Learning & Behavior, Taniuchi, Sugihara, Wakashima, and Kamijo (2016) looked for the property of abstraction. Humans use numbers to count virtually any number of objects. Is the ability of a rat to numerically discriminate number limited to the objects it is trained with, or can it transfer a numerical discrimination to novel objects? Rats responded to a row of 10 boxes, each of which contained a door that could be entered. From one trial to the next, four to six objects were placed in front of different sets of doors. Different types of objects were used throughout the experiments (e.g., glass bottles, metallic cans, ceramic dolls). Training began with presentation of a row of the same type of objects. As training proceeded, sets of new objects were introduced, always with accessible food behind only the third object. Next, periodic test trials were introduced on which a new set of objects was used to see if the rat would choose the third of these novel objects. Importantly, because the number of boxes used varied among test trials, and the spacing between boxes changed among trials, rats could not locate the correct box by its absolute position or by counting from the right side.Two of the four rats used in Taniuchi et al.'s (2016) first experiment learned to choose the third box when three different objects were used on different trials. Of particular importance, these two rats scored above chance on probe test trials with a fourth set of novel objects. In order to test the possibility that the rats' success was based on similarity perceived between the training and test objects, Taniuchi et al. used new objects in a second experiment. All three rats learned to choose the third object. When tested on probe trials with a set of novel objects, two of the three rats chose the third box above chance. In both Experiments 1 and 2, rats chose the third box on 60 %-80 % of the test trials. These results convincingly showed that rats did show abstraction by transferring numerical discrimination to a novel set of objects.Having shown clear evidence for abstract numerical transfer, Taniuchi et al. (2016) report some truly remarkable findings from Experiments 3 and 4. The two successful rats from Experiment 2 continued to be tested in Experiment 3 with the four types of objects used in that experiment-call them A, B, C, and D. Although the rats ...