Objects that Work Together May be Perceptually Grouped

Objects That Work Together May Be Perceptually Grouped Collin Green (cgreen@arc.nasa.gov) NASA Ames Research Center, Moffett Field, CA 94035 John E. Hummel (jhummel@psych.ucla.edu) Department of Psychology, University of California, Los Angeles CA 90095 were more easily noticed than metric changes (e.g., when a chair was turned toward a table in the target scene, subjects successfully rejected lures in which the chair was turned away from the table but not in which the chair was farther from [but still facing] the table). One interpretation of these findings is that scene represen- tations include general information about the semantics of objects (but not much visual detail) and information about meaningful relations between objects (but little about the specific metrics of those relations). In other words, scene- level representations may emphasize the information in the perceptual stimulus which is useful for understanding and interacting with an environment, omitting specific visual details: information that is specifically functional. Building on this interpretation, we recently proposed an account of scene processing based on the representation of functional groupings of objects within larger visual scenes (Green & Hummel, 2004a). In short, groups of interacting objects within a visual scene may serve as a basis for recog- nizing scenes and for connecting visual information to goal- relevant actions. We hypothesized that functional groups are explicitly represented mental entities, and that these rep- resentations affect the allocation of visual attention; pre- liminary data support this hypothesis (see Green & Hummel, 2004b). The current work further investigated whether functional groups affect object identification in simple vis- ual scenes. The hypothesis that functional interactions between ob- jects influence visual processing has prior empirical support. Evidence suggests that functional relations affect the identi- fication of visual objects in neuropsychological patients with parietal damage (Riddoch, Humphreys, Edwards, Baker, & Willson, 2003; Humphreys, Riddoch, Forti, & Ackroyd, 2004). Riddoch et al., (2003) studied patients who showed extinction when trying to report the names of two simultaneously-presented objects. When objects were presented together but were not positioned to interact (i.e. were not working together to accomplish some larger goal), patients could report the name of one object, but not both. When the objects were positioned so that they interacted, accuracy in reporting the names of both objects increased markedly. Control conditions indicated that semantic asso- ciations between objects were not sufficient to explain the improved performance; rather, it was functional information that facilitated the simultaneous selection of the interacting objects. The work by Riddoch and colleagues is evidence in favor of our hypothesis. If functional groupings of objects are Abstract Perceiving functional relationships between objects may be fundamental to understanding visual environments. Even the identification of objects in a scene may be influenced by the functional relations that exist among those objects. In two experiments, normal observers identified a briefly-exposed target object presented with a semantically associated or unas- sociated distractor object. The target object and distractor were either arranged to work together, or not to do so. Identi- fication was more accurate when target objects were arranged to work with an associated distractor than when they were ar- ranged so as not to work with the distractor. In contrast, iden- tification was worse when target objects worked with unasso- ciated distractors than when they did not. A sensitivity to change in stimulus onset asynchrony suggested that the for- mer (facilitatory) effect was perceptual in nature. We propose this as evidence that sets of objects engaged in familiar func- tional relationships are perceptually or attentionally grouped. Keywords: Scene perception; object recognition; function; perceptual grouping. Introduction Real world visual scenes are full of information. At the lowest level, the retina encodes a scene as a set of brightness and color values. Later, systems specialized for object and space representation encode the identities and locations of objects in the environment. The result of processing is a representation that permits the observer to recognize the type of place being viewed, and to understand what activi- ties can and should be performed. Clearly, a scene is repre- sented very differently at the retinal level than at this higher level. While the properties of retinal representations are relatively well understood, the properties of the later, more abstract representations (those connected to behavior) are less clear. Work by Mandler and colleagues (Mandler & Parker, 1976; Mandler & Ritchey, 1977) demonstrated that certain types of information are preferentially encoded while view- ing organized scenes. Participants were asked to discrimi- nate between studied (target) scenes and unstudied (lure) scenes. Lure scenes were constructed by making subtle alterations to target scenes and were used to test participant sensitivities to a variety of changes in object and relational information. Results indicated that lures containing object type changes were more easily rejected than those contain- ing object token changes (for example, changing a mug to a plate was more noticeable than changing a mug to a differ- ent mug). Additionally, qualitative changes to relations