Lateral but not medial telencephalic pallium ablation impairs the use of goldfish spatial allocentric strategies in a "hole-board" task.

Strong evidence suggests that the ventral region of the lateral telencephalic pallium of teleost fish, a structure involved in allocentric spatial cognition, is homologous to the hippocampus of tetrapods. This homology was first proposed on basis of anatomical data, and subsequently confirmed by developmental, functional and behavioural studies. Nonetheless, Saito and Watanabe [30,32] claim that not the lateral but, rather, the medial pallium participates in goldfish spatial navigation and should be considered the homologue of the hippocampus. Here, we further investigate the effects of selective pallial lesions on the spatial cognition abilities of goldfish, trained in a "hole-board" analogue task, to find the baited feeder within a 5 x 5 feeder matrix surrounded by visual cues. The task in the present experiment is similar to that used by Saito and Watanabe, but including thorough probe tests that enabled to define clearly the spatial strategies employed by the animals, and, therefore, the spatial deficits caused by the pallial lesions. The results showed that the lateral, but not the medial pallium lesions, produced a dramatic impairment in the implementation of allocentric spatial strategies. Thus, only lateral pallium lesioned goldfish, like hippocampus lesioned tetrapods, failed to reach the goal when the cues in its proximity were excluded, indicating that they used a guidance strategy. These results do not replicate Saito and Watanabe's, but are consistent to previous data indicating a close functional similarity between the lateral pallium of teleost fish and the hippocampus of amniotes.