Motor Affordances in Object Perception

Motor Affordances in Object Perception Stephen J. Flusberg (sflus@stanford.edu) Alexia Toskos Dils (atoskos@stanford.edu) Lera Boroditsky (lera@stanford.edu) Stanford University, Department of Psychology Jordan Hall, 450 Serra Mall, Building 420, Stanford, CA 94305 USA Abstract Recently, researchers have suggested that when we see an object we automatically represent how that object affords action (Tucker & Ellis, 2001). However, the precise nature of this representation remains unclear: is it a specific motor plan or a more abstract response code? Furthermore, do action representations actually influence what we perceive? In Experiment 1, participants responded to an image of an object and then made a laterality judgment about an image of a hand. Hand identification was fastest when the hand corresponded to both the orientation and grasp type of the object, suggesting that affordances are represented as specific action plans. In Experiment 2, participants saw an image of a hand before interpreting an ambiguous object drawing. Responses were biased towards the interpretation that was congruent with the grasp type of the hand prime. Together, these results suggest that action representations play a critical role in object perception. Keywords: object perception; motor affordances Background Traditional approaches to visual perception have assumed that the primary goal of the visual system is to construct a detailed internal picture of the external world based on a noisy retinal image (e.g. Marr, 1980). Recently, however, there has been a growing appreciation for the possibility that visual perception may be equally concerned with how we move around and act in our environment (Milner & Goodale, 1995). The idea that vision and action are intimately linked can be traced to the ecological psychology of James Gibson (1979), who argued that organisms see the world in terms of how it affords action. While Gibson eschewed the notion of mental representation, contemporary scholars have suggested that visual perception may be at least partially characterized by a mental representation of the affordances in the environment (Tucker & Ellis, 1998). For example, seeing the coffee mug on the desk before me might involve mentally representing how I could reach out and grasp it in order to drink from it. However, the precise nature of these affordance representations and how they relate to object perception remains unclear. Several researchers have suggested that affordances are represented as action plans in the motor systems of the brain (e.g. Tucker & Ellis, 1998; Chao & Martin, 2000). Tucker and Ellis conducted a series of studies to test whether people automatically generate a motor representation in response to the visual presentation of an object, even when there is no intention to act on the object (e.g. Tucker & Ellis, 1998, 2001). In one experiment, participants had to make a left or right-handed button press to indicate whether an image of an object on the screen was upright or inverted. The objects were chosen to have clear right or left-handed affordance (e.g. a frying pan with a handle oriented to the left affords a left-handed grasp). Participants responded faster and made fewer errors when their responding hand was congruent with the (task-irrelevant) affordance of the object on the screen. Similar stimulus/response compatibility (SRC) effects have been obtained for other types of m i c r o - affordances, such as grasp type (Tucker & Ellis, 2001) and wrist orientation (Tucker & Ellis, 1998). Others have argued that the motor representations activated when we observe an object form an integral part of our perception of the object (e.g. Helbig, Graf, & Kiefer, 2006). For example, Helbig et al. (2006) presented participants with images of two objects in quick succession. Participants were more accurate in naming the second object when similar actions were required to make use of two objects (e.g. pliers and a nutcracker). These findings raise important questions regarding, (1) the level at and specificity with which affordance information is represented, and (2) the potential causal role this information plays in the process of object perception. For instance, several researchers have argued that the SRC effects obtained by Tucker and Ellis actually reflect abstract response coding rather than specific motor plans (e.g. Anderson, Yamagishi, & Karavia, 2002). Indeed, even Tucker and Ellis (2001) suggested that these effects could not be explained by appealing exclusively to the neural systems responsible for the on-line control of actions because they were obtained using both images of objects as well as real-world objects that were out of reach of participants. Rather, affordance representations might be more abstract, specifying, for example, the general class of hand shape required to interact with an object rather than precise motor parameters. Further, while the results of Helbig et al. (2006) are consistent with motor affordance representations contributing to object perception, the data could also be explained by the fact that objects that are used in similar ways are typically similar to one another in other ways as well (e.g. semantically). In this paper we describe two experiments designed to address these issues. First, we wanted to know whether motor affordances might be represented as specific action plans for interacting with an object rather than as abstract response codes. To this end, in Experiment 1 we made use of a dependent measure that is known to draw on very