Separate processing mechanisms for encoding of geometric and landmark information in the avian hippocampus.

Domestic chicks bilaterally or unilaterally lesioned to the hippocampus were trained to search for food hidden beneath sawdust by ground-scratching in the centre of a large enclosure, the correct position of food being indicated by a local landmark in the absence of any extra-enclosure visual cues. At test, the landmark was removed or displaced at a distance from its original position. Results showed that sham-operated chicks and chicks with a lesion of the left hippocampus searched in the centre, relying on large-scale geometric information provided by the enclosure, whereas chicks with a lesion of either the right hippocampus or both hippocampi were completely disoriented (landmark removed) or searched close to the landmark shifted from the centre (landmark displaced). These results indicate that encoding of geometric features of an enclosure occurs in the right hippocampus even when local information provided by a landmark would suffice to localize the goal; encoding based on local information, in contrast, seems to occur outside the hippocampus. These findings provide evidence that the left and right avian hippocampi play different roles in spatial cognition, a phenomenon which had been documented previously only for the human hippocampus.