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The hippocampus of birds and mammals is critical for the learning of map-like memory representations of environmental space. It has been suggested that the hippocampus of rats also participates in non-spatial relational learning, including the learning of non-spatial transitive relationships among odor stimuli [Bunsey and Eichenbaum, Nature 1996]. Although transitive-like learning has been demonstrated in a variety of vertebrate species, from a comparative perspective the role of the hippocampus in this form of learning has not been tested in other amniote groups. We trained control and hippocampal-lesioned homing pigeons on a series of visual, non-spatial, go/no-go conditional discriminations and then tested them on novel transitivity probe trials. The hippocampal-lesioned pigeons were as successful as control pigeons in responding appropriately to correct and incorrect transitivity pairs. The finding that the homing pigeon hippocampal formation is not necessary for solving this serial, conditional discrimination task is important for further understanding hippocampal function across species, and represents one of the few studies that have attempted to localize a brain region responsible for the phenomenon of transitive behavior learning.

Stimulus control of food-site recognition and role of the hippocampal formation (HF) were investigated. Control and HF-lesioned pigeons were trained to find food located in a colored bowl, near a landmark beacon, in a constant room location. During later test trials, the sources of information were individually removed and/or disassociated. For all test trial types, HF-lesioned pigeons consistently chose bowls associated with one of the training stimuli. Controls were more sensitive to the changes introduced during the test trials; choosing like HF-lesioned pigeons on some test trials but choosing randomly on others. The data identify a critical role of the avian HF in learning the spatial relationship among environmental stimuli.

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