Hippocampal Pallium Lesion Impairs Transitive Inference in Goldfish

Transitive inference, a process that involves drawing logical conclusions based on preliminary information, is considered a cornerstone of human deductive reasoning. Furthermore, transitive inference is a clear instance of representational flexibility as it implies the novel expression of learned information. In mammals and birds, both episodic memory and transitive inference critically depend on the integrity of the hippocampus. Comparative neurobiological evidence indicates that a hippocampus homologue can also be found in the telencephalic pallium of teleost fish. Here, we investigated whether goldfish demonstrate inferential behavior in a standard transitive inference task, and whether the hippocampal pallium of goldfish, akin to the hippocampus in mammals and birds, plays a role in transitive responding. We trained goldfish with hippocampal pallium lesions and sham-operated controls on a series of overlapping two-item visual premise pairs: A+B-, B+C-, C+D-, D+E-. The sham-operated animals readily learned the premise pair discriminations and responded transitively during the crucial test involving a novel pair of nonadjacent elements (B vs. D). However, hippocampal pallium-lesioned goldfish were impaired in the critical transitive inference test, although they successfully learned to discriminate the premise pairs. These findings suggest that a relational memory function, which supports the novel expression of learned information, could be a primitive feature of the vertebrate hippocampus. Such outcome contributes significantly to the ongoing debate regarding the evolutionary origins of episodic memory in vertebrates.