The avian hippocampus, homing in pigeons and the memory representation of large-scale space

The extraordinary navigational ability of homing pigeons provides a unique spatial cognitive system to investigate how the brain is able to represent past experiences as memory. In this paper, we first summarize a large body of lesion data in an attempt to characterize the role of the avian hippocampal formation (HF) in homing. What emerges from this analysis is the critical importance of HF for the learning of map-like, spatial representations of environmental stimuli used for navigation. We then explore some interesting properties of the homing pigeon HF, using for discussion the notion that the homing pigeon HF likely displays some anatomical or physiological specialization(s), compared to the laboratory rat, that account for its participation in homing and the representation of large-scale, environmental space. Discussed are the internal connectivity among HF subdivisions, the occurrence of neurogenesis, the presence of rhythmic theta activity and the electrophysiological profile of HF neurons. Comparing the characteristics of the homing pigeon HF with the hippocampus of the laboratory rat, two opposing perspectives can be supported. On the one hand, one could emphasize the subtle differences in the properties of the homing pigeon HF as possible departure points for exploring how the homing pigeon HF may be adapted for homing and the representation of large-scale space. Alternatively, one could emphasize the similarities with the rat hippocampus and suggest that, if homing pigeons represent space in a way different from rats, then the neural specializations that would account for the difference must lie outside HF. Only future research will determine which of these two perspectives offers a better approximation of the truth.