Identity matching-to-sample with olfactory stimuli in rats.

Identity matching-to-sample has been difficult to demonstrate in rats, but most studies have used visual stimuli. There is evidence that rats can acquire complex forms of olfactory stimulus control, and the present study explored the possibility that identity matching might be facilitated in rats if olfactory stimuli were used. Four rats were trained on an identity match-to-sample procedure with odorants mixed in cups of sand as stimuli. Digging in the sample cup produced two comparison cups, and digging in the comparison cup that contained the same scent as the sample was reinforced. When criterion accuracy levels were reached, novel stimuli were added to the baseline training regimen. All 4 rats reached terminal performance of above 90% correct matching with more than 20 different baseline stimuli and matched novel stimulus combinations with above-chance accuracy; 3 of the 4 rats matched novel stimuli at levels significantly above chance. Accurate matching performance was demonstrated both with 2- and 3-comparison procedures. These results suggest that generalized matching-to-sample can be observed in rats when olfactory stimuli are used and, furthermore, that multiple-exemplar training may be important for its emergence.