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The hippocampus and the amygdala are involved in avoidance learning in mammals. The medial and lateral pallia of actinopterygian fish have been proposed as homologous to the mammalian pallial amygdala and hippocampus, respectively, on the basis of neuroanatomical findings. This work was aimed at studying the effects of ablation of the medial telencephalic pallia (MP) and lateral telencephalic pallia (LP) in goldfish on the retention of a conditioned avoidance response previously acquired in two experimental conditions. In the first experiment, fish were trained in nontrace avoidance conditioning. In the second experiment, fish were trained in trace avoidance conditioning in which temporal cues were crucial for the learning process. An MP lesion affected the retention of the avoidance response in both procedures; in contrast, an LP lesion impaired the retention only in the trace-conditioning procedure. These data support the presence of two different systems of memory in fish, based on discrete telencephalic areas: the MP, involved in an emotional memory system; and the LP, involved in a spatial, relational, or temporal memory system. Moreover, these differential effects were similar to those produced by amygdalar and hippocampal lesions in mammals. We conclude that these specialized systems of memory could have appeared early during phylogenesis and could have been conserved throughout vertebrate evolution.

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In mammals, the amygdala and the hippocampus are involved in different aspects of learning. Whereas the amygdala complex is involved in emotional learning, the hippocampus plays a critical role in spatial and contextual learning. In fish, it has been suggested that the medial and lateral region of the telencephalic pallia might be the homologous neural structure to the mammalian amygdala and hippocampus, respectively. Although there is evidence of the implication of medial and lateral pallium in several learning processes, it remains unclear whether both pallial areas are involved distinctively in different learning processes. To address this issue, we examined the effect of selective ablation of the medial and lateral pallium on both two-way avoidance and reversal spatial learning in goldfish. The results showed that medial pallium lesions selectively impaired the two-way avoidance task. In contrast, lateral pallium ablations impaired the spatial task without affecting the avoidance performance. These results indicate that the medial and lateral pallia in fish are functionally different and necessary for emotional and spatial learning, respectively. Present data could support the hypothesis that a sketch of these regions of the limbic system, and their associated functions, were present in the common ancestor of fish and terrestrial vertebrates 400 million years ago.