Jon K. Grossmann

Differential ambiguity reduces grouping of metastable objects

Two coaxial, ambiguously rotating objects tend to be perceived as corotating. Such grouping could be the consequence of bottom-up, cooperative interactions between the stimuli, or the top-down selection of object properties consistent with a model of the objects or scene. However, we find that the coupling between an ambiguous and unambiguous object is sharply reduced, presenting a challenge for both explanations of grouping. We describe experiments that support the idea that top-down feedback is necessary to select and stabilize a perceptual interpretation for ambiguous figures. Reduced coupling between objects of differing ambiguity can be explained if the feedback is global and proportional to ambiguity.

Competition in bistable vision is attribute-specific.

We employ ambiguous figures and rivalrous stimuli that have multiple ambiguous properties to show that the different attributes of an ambiguous stimulus can undergo independent switching dynamics. This suggests that competition is distributed and attribute-specific, consistent with the known functional segregation of visual processing. Conflicting evidence that binocular rivalry is an early or late visual process may be better understood as evidence for attribute-specific competition occurring at multiple stages of visual processing. Specifically, we show that whether perceptual selection during binocular rivalry is early and eye-based or late and percept-based depends on the particular ambiguous attributes of the rivalrous stimulus.

Asymmetries in Perception of 3D Orientation

Visual scene interpretation depends on assumptions based on the statistical regularities of the world. People have some preference for seeing ambiguously oriented objects (Necker cubes) as if tilted down or viewed from above. This bias is a near certainty in the first instant (∼1 s) of viewing and declines over the course of many seconds. In addition, we found that there is modulation of perceived orientation that varies with position—for example objects on the left are more likely to be interpreted as viewed from the right. Therefore there is both a viewed-from-above prior and a scene position-dependent modulation of perceived 3-D orientation. These results are consistent with the idea that ambiguously oriented objects are initially assigned an orientation consistent with our experience of an asymmetric world in which objects most probably sit on surfaces below eye level.

Recurrent global dynamics underlie perceptual grouping

Abstract Arrays of ambiguous objects often appear to group into a single interpretation. Examples include Attneaves triangles and the dynamic dot arrays of Ramachandran and Anstis. However, there are other displays in which perceptual grouping fails. We find that in a variety of such displays—which interpretations are seen, or whether grouping occurs at all—depends on whether the ambiguous property of the objects can be assigned values consistent with a single viewer position or motion. This implies that grouping depends on the visual system developing a 3-D scene model which constrains the relation of the viewer to objects in the scene.

Can Rotational Grouping Be Determined by the Initial Conditions

Objects rotating in depth with an ambiguous rotation direction frequently appear to rotate together. Corotation is especially strong when the objects are interpretable as having a shared axis. We manipulated the initial conditions of the experiment by having pairs of objects initially appear to be unambiguous, and then make either a sudden or gradual transition to ambiguous spin. We find that in neither case do coaxial counter-rotating objects persist in being perceived as counter-rotating. This implies that the perceptual constraint that favors coaxial corotation overrides the initial perceptual state of the objects.