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Objects and Affordances: An Artificial Life Simulation

Objects and Affordances: An Artificial Life Simulation Giorgio Tsiotas (tsiotas@libero.it) Department of Computer Science, 7 Mura Anteo Zamboni Bologna, 40127 Italy Anna M. Borghi (annamaria.borghi@unibo.it) Department of Psychology, 5 Viale Berti Pichat Bologna, 40127 Italy Domenico Parisi (domenico.parisi@istc.cnr.it) Institute of Sciences and Technologies of Cognition (ISTC), 15 Viale C. Marx, Rome, 00137 Italy Abstract We simulated organisms with an arm terminating with a hand composed by two fingers, a thumb and an index, each composed by two segments, whose behavior was guided by a nervous system simulated through an artificial network. The organisms, which evolved through a genetic algorithm, lived in a bidimensional environment containing four objects, either large or small, either grey or black. In a baseline simulation the organisms had to learn to grasp small objects with a precision grip and large objects with a power grip. In Simulation 1 the organisms learned to perform two tasks: in Task 1 they continued to grasp objects according to their size, in Task 2 they had to decide the objects' color by using a precision or a power grip. Learning occured earlier when the grip required to respond to the object and to decide the color was the same than when it was not, even if object size was irrelevant to the task. The simulation replicates the result of an experiment by Tucker & Ellis (2001) suggesting that seeing objects automatically activates motor information on how to grasp them. Introduction In opposition to theories that posit perception and action as separate (Pylyshyn, 1999), it has recently been suggested that perception and action are strictly interwoven and that perception is guided and filtered by action (Berthoz, 1997; Ward, 2002). In a related way, recent theories emphasize the interconnections between sensorimotor and cognitive processes. In particular, it has been proposed that cognition is embodied, i.e., that it depends on the experiences that result from possessing a body with given physical characteristics and a particular sensorimotor system (Barsalou, 1999; Glenberg, 1997; Goldstone & Barsalou, 1998). This view of cognition is clearly in opposition to the classical cognitivist view according to which mind is a device for manipulating arbitrary symbols. More specifically, some theories suggest that object concepts re-enhance sensorimotor experiences with objects (Barsalou, 1999; Glenberg, 1997; Pecher & Zwaan, 2005). Studies on the relationships between the visual system and the motor system claim that seeing an object tends to evoke its affordances, re-activating previous experiences and interactions with the object. An affordance refers to a property on an object that influences how the object can be used (Gibson, 1979). For example, the properties of the handle of a door determine how one opens the door - by pulling, or pushing, or twisting, and so on. Accordingly, seeing an object such as a cup may re-activate the affordances linked to reaching and grasping the cup’s handle, even if the position of the handle is not relevant to the task at hand (Tucker & Ellis, 1998). Most evidence regarding the strict interrelations between perception and action concerns simple interactions with objects, rather than complex actions probably mediated by the actor's goals. So, for example, protruding object parts may activate reaching motor behaviors, whereas objects of a specific size may activate specific grasping behaviors. In order to refer to these “low-level” affordances, Ellis & Tucker (2000) created the expression of “micro-affordances”. Micro- affordances facilitate simple and specific kinds of interactions with objects but they do not pertain complex, goal-mediated actions. Most importantly, micro-affordances probably imply access to conceptual knowledge, as they are rather specific and suitable for a given object. So, for example, seeing an object does not elicit simply a grasping behavior, but a specific type of grasping behavior which is suitable for that particular object. Ellis & Tucker (2000) and Tucker & Ellis (2001) have demonstrated the role played by affordances in eliciting motor behavior by presenting participants with real objects of different sizes. Participants had to categorize the objects as natural or artefact using either a power grip or a precision grip. They found a compatibility effect between the kind of grasp and a task-irrelevant dimension, the object`s size. The effect was also observed when the object was located outside the reaching space, which suggests that seeing the object activates the simulation of a specific type of grasping. A similar compatibility effect was found between the direction of the wrist’s rotation and the kind of grasp required by the object. For example, objects such as bottles facilitated responses with a clockwise wrist rotation, while objects such as toothbrushes facilitated a counter-clockwise wrist rotation.