Categorical learning in pigeons: the role of texture and shape in complex static stimuli

Pigeons are known to be able to categorize a wide variety of visual stimulu s classes. However, it remains unclear which are the characteristics of the perceptually relevant features employed to reach such good performance. He re, we investigate the relative contributions of texture and shape informat ion to categorization decisions about complex natural classes. We trained t hree groups of pigeons to discriminate between sets of photorealistic front al images of human faces according to sex and subsequently, tested them on different stimulus sets. Only the pigeons that were presented with texture information were successful at the discrimination task. Pigeons seem to pos sess a sophisticated texture processing system but are less capable in disc riminating shapes. The results are discussed in terms of the possible evolu tionary advantages of utilizing texture as a very general and potent percep tual dimension in the birds' visual environment. (C) 1998 Elsevier Science Ltd. All rights reserved.