Hallmarks of a common forebrain vertebrate plan: Specialized pallial areas for spatial, temporal and emotional memory in actinopterygian fish

In mammals and birds different pallial forebrain areas participate in separate memory systems. In particular, the hippocampal pallium is implicated in spatial memory and temporal attribute processing, whereas the amygdalar pallium is involved in emotional memory. Here we analyze the involvement of teleost fish lateral and medial pallia, proposed as homologous to the hippocampus and amygdala, respectively, in a variety of learning and memory tasks, such as spatial memory; reversal learning; delay or trace motor classical conditioning; heart rate, emotional classical conditioning; and two way active avoidance conditioning. Results show that the damage to the lateral pallium produces a profound deficit in spatial learning and memory in teleost fish. In addition, lateral pallium lesions produce a significant deficit in trace classical conditioning, whereas they have no significant effects on delay conditioning, or in heart rate conditioning. In contrast, medial pallium lesions disrupt emotional, heart rate conditioning and avoidance conditioning, but spare spatial memory and temporal stimulus processing. These data demonstrate a striking functional similarity between the medial and lateral pallia of teleost fish and the pallial amygdala and hippocampal pallium of land vertebrates, respectively. The reviewed evidence suggest that these two separate memory systems, the hippocampus-dependent spatial, relational or temporal memory system, and the amygdala based emotional memory system, could have appeared early during evolution, having conserved their functional identity through vertebrate phylogenesis.