The homing pigeon hippocampus and space: in search of adaptive specialization.

The hippocampus (HF) of birds and mammals is essential for the map-like representation of environmental landmarks used for navigation. However, species with contrasting spatial behaviors and evolutionary histories are likely to display differences, or 'adaptive specializations', in HF organization reflective of those contrasts. In the search for HF specialization in homing pigeons, we are investigating the spatial response properties of isolated HF neurons and possible right-left HF differences in the representation of space. The most notable result from the recording work is that we have yet to find neurons in the homing pigeon HF that display spatial response properties similar to HF 'place cells' of rats. Of interest is the suggestion of neurons that show higher levels of activity when pigeons are near goal locations and neurons that show higher levels of activity when pigeons are in a holding area prior to be being placed in an experimental environment. In contrast to the rat, the homing pigeon HF appears to be functionally lateralized. Results from a current lesion study demonstrate that only the left HF is sensitive to landmarks that are located within the boundaries of an experimental environment, whereas the right HF is indifferent to such landmarks but sensitive to global environmental features (e.g., geometry) of the experimental space. The preliminary electrophysiological and lateralization results offer interesting departure points for better understanding possible HF specialization in homing pigeons. However, the pigeon and rat HF reside in different forebrain environments characterized by a wulst and neocortex, respectively. Differences in the forebrain organization of pigeons and rats, and birds and mammals in general, must be considered in making sense of possible species differences in how HF participates in the representation of space.