VIEW-DEPENDENT OBJECT RECOGNITION BY MONKEYS

Background: How do we recognize visually perceived three-dimensional o bjects, particularly when they are seen from novel view-points? Recent psychophysical studies have suggested that the human visual system ma y store a relatively small number of two-dimensional views of a three- dimensional object, recognizing novel views of the object by interpola tion between the stored sample views. In order to investigate the neur al mechanisms underlying this process, physiological experiments are r equired and, as a prelude to such experiments, we have been interested to know whether the observations made with human observers extend to monkeys. Results: We trained monkeys to recognize computer-generated i mages of objects presented from an arbitrarily chosen training view an d containing sufficient three-dimensional information to specify the o bject's structure. We subsequently tested the trained monkeys' ability to generalize recognition of the object to views generated by rotatio n of the target object around any arbitrary axis. The monkeys recogniz ed as the target only those two-dimensional views that were close to t he familiar, training view. Recognition became increasingly difficult for the monkeys as the stimulus was rotated away from the experienced viewpoint, and failed for views farther than about 40 degrees from the training view. This suggests that, in the early stages of learning to recognize a previously unfamiliar object, the monkeys build two-dimen sional, viewer-centered object representations, rather than a three-di mensional model of the object. When the animals were trained with as f ew as three views of the object, 120 degrees apart, they could often r ecognize all the views of the object resulting from rotations around t he same axis. Conclusion: Our experiments show that recognition of thr ee-dimensional novel objects is a function of the object's retinal pro jection. This suggests that nonhuman primates, like humans, may accomp lish view-invariant recognition of familiar objects by a viewer-center ed system that interpolates between a small number of stored views. Th e measures of recognition performance can be simulated by a regulariza tion network that stores a few familiar views, and is endowed with the ability to interpolate between these views. Our results provide the b asis for physiological studies of object-recognition by monkeys and su ggest that the insights gained from such studies should apply also to humans.