Comparative Perception and Attention Laboratory
Recognition of occluded objects
Casual observation suggests that animals, including pigeons, can recognize partially occluded objects. Pigeons are constantly interacting with partially occluded objects when they are foraging for food, returning to their nest after an outing, or finding their fledglings among branches and leaves. Surprisingly, most of researchers studying amodal completion in nohuman animals using two-dimensional objects found no evidence of it in pigeons. Our recent data suggest, however, that pigeons can recognize partially occluded objects in a two-dimensional environment if they are given sufficient training to separate the prevailing foreground and background cues. Moreover, we found that after sufficient experience with two-dimensional objects, pigeons begin to perceive the occluded stimulus as being more similar to the complete stimulus than to incomplete stimulus, suggesting the presence of amodal completion. Together, these results imply that pigeons placed in the laboratory world of two-dimensional images may require additional experience before they can effectively cope with object occlusion.
Nagasaka, Y., Lazareva, O. F., & Wasserman, E. A. (2007). Prior experience affects amodal completion in pigeons. Perception & Psychophysics, 69, 596-605.
Lazareva, O. F., Wasserman, E. A., & Biederman, I. (2007). Pigeons’ recognition of partially occluded objects depends on specific training experience. Perception, 36, 33-48.
Nonaccidental properties of objects
A single three-dimensional object can produce an infinite number of two-dimensional patterns on the retina. For example, a straight edge on a three-dimensional object produces a straight line in a two-dimensional projection to the retina; but, so too does a curved edge when the curve is precisely perpendicular to the line of sight. How, then, does the visual system decide whether a three-dimensional object has a straight edge or a curved edge using two-dimensional information?Theories of object recognition suggest that certain regularities of two-dimensional images, like curvilinearity, are highly likely to reflect the same regularities in three-dimensional objects. Although it is true that accidental alignment of the eye and a curved edge might result in the projection of a straight line, there are very few viewpoints from which this would be the case. Thus, the visual system simply ignores this possibility and assumes that straight edges in two-dimensional projections correspond to straight edges in three-dimensional objects.
I have worked on a project with Brett Gibson (University of New Hampshire), and in collaboration with Frederic Gosselin (University of Montreal), Philippe Schyns (University of Glasgow). We have asked which stimulus properties pigeons and people use to discriminate four three-dimensional, shaded objects. We found that both humans and pigeons used cotermination, the most diagnostic nonaccidental property of real-world objects, despite evidence from a model computer observer that cotermination was not the most diagnostic pictorial information in this particular task. This result implies that a nonmammalian visual system that is different anatomically from the human visual system is also biased to recognize objects from nonaccidental statistics.In a follow-up study, I trained pigeons and people to discriminate a target shape from a metrically-changed shape and a nonaccidentally-changed shape. Importantly, the stimuli had been controlled for physical similarity by using wavelet and pixel measures so that the dissimilarity in pixel energy between the metrically-changed shape and the target shape was at least as great (or greater) as the dissimilarity in pixel energy between the nonaccidentally-changed shape and the target shape. We found that both pigeons and people committed more confusion errors to the metrically-changed shape, suggesting that they perceived the metrically-changed shape to be more similar to the target shape.