Categorization of visual stimuli by humans and monkeys

To come to grips with the complexity of the world, primates and other species have developed the ability to categorize objects. To study the mechanisms of categorization, we trained 3 monkeys (Macaca mulatta) in 2 tasks with 2 stimuli sets. The 1st set consisted of line drawings of faces and the 2nd of line drawings of fish, both differing along 4 dimensions. In the 1st task the monkeys rated the similarity of the stimuli presented in triads. This data was used to derive distances between the stimuli and to determine their psychological representation with Multidimensional Scaling (MDS). In the 2nd task the monkeys learned 2 classes of exemplars and then classified new ones in an unsupervised manner. We compared the data of the monkeys with data of 8 humans tested with the same paradigms. Both species showed similar performance, suggesting that comparable neural mechanisms may underlie perceptual classification. For both species the stimuli dimensions were not represented uniformly in the subjects' psychological space. The reconstruction of exemplar loci based on MDS solutions using Procrustes transformation yielded arrangement of points close to the configuration determined by the original exemplar coordinates. It is possible that the neural representation of multidimensional, parametrically defined patterns is closely related to their parametric configuration. The classifications of both species were best explained on the basis of the stimulus similarity to familiar exemplars rather than to class prototypes. In current neurophysiological experiments we are investigating the representation of stimulus-related information in the anterior inferiot temporal cortex