Pigeons discriminate shapes based on topological features.

The use of topological features in visual recognition has been demonstrated only in species with global-cue precedence. We investigated whether pigeons, with local-cue precedence, use topological features as cues for discriminating different shapes. The subjects in the topology group were required to discriminate stimuli based on whether the shapes contained one or no holes, whereas the subjects in the pseudocategory group were required to discriminate stimuli based on arbitrary categories. In contrast to the pseudocategory group, which showed little improvement in stimuli discrimination over the sessions, the topology group showed rapid improvement, indicating that the latter group performed better than what was expected from rote learning. Moreover, our data suggest that shape discrimination in pigeons is based on on-off feature information contained in the stimuli. We conclude that certain species with local-cue precedence are capable of perceiving topological features and that this type of perception may be a primitive component of the visual system.