Existence of working memory in teleosts: Establishment of the delayed matching-to-sample task in adult zebrafish.

Operant conditioning is a powerful tool to study animal perception and cognition. Compared to mammals and birds, there are very few behavioral studies using operant conditioning paradigm in teleosts. Here we aim to establish matching-to-sample task (MTS) in adult zebrafish, using visual cues (colors) as discriminative stimuli. Unlike simple one-to-one color-reward association learning, MTS requires ability for context integration. In this study, zebrafish learned to perform the simultaneous-matching-to-sample (SMTS) within 15 sessions. After the SMTS training, working memory was tested by inserting a delay period (delayed matching-to-sample; DMTS). Zebrafish could perform the DMTS with a delay of at least 3-4 seconds. They could also learn to perform the DMTS even with a delay period from the beginning of the training session. These results strongly suggest that adult zebrafish possess working memory. However, our study also indicates limitations of zebrafish in cognitive flexibility or attention: they could perform SMTS/DMTS only in a certain set-up. The presence of working memory without the mesencephalic dopamine neurons indicates the convergent evolution of this function in amniotes and teleosts.