A Computational PDP Model for Explaining Automatic Imitation

A Computational PDP Model for Explaining Automatic Imitation Matthias Scheutz (mscheutz@cs.tufts.edu) Department of Computer Science, 165 College Avenue Medford, MA 02155 USA Bennett I. Bertenthal (bbertent@indiana.edu) Department of Psychology, 10 th Street Bloomington, IN 47405 USA Abstract Recent evidence suggests that automatic imitation is mediated by an observation-execution matching system that cannot be reduced to the same processes responsible for other stimulus- response (S-R) compatibilities. A computational model is developed with different patterns of connectivity for imitative and spatial compatibilities, and it is successful in simulating the results from three different S-R tasks. Variations of the model with identical connections for mediating the two compatibilities reveal a significantly poorer fit. These results provide converging evidence that imitative and spatial compatibilities are mediated by different processes. Keywords: automatic imitation, S-R compatibility, connectionist modeling, direct matching hypothesis Introduction and Background The tendency to unintentionally and unconsciously mimic actions performed by others has long been noted. Charles Darwin, for example, commented that at leaping matches spectators would move their own feet as if imitating the athletes. More recently, Dijksterhuis and Bargh (2001) noted that we tend to whisper or speak louder when others do, scratch our heads upon seeing someone else scratch, or cycle faster after seeing a cycling race on TV. During social interactions, mimicry translates into a greater desire to want to cooperate and affiliate with those individuals imitating our gestures (Chartrand and Bargh, 1999). In spite of the frequency and significance of these behaviors, our understanding of the underlying mechanisms responsible for these automatic tendencies remains incomplete at best. The prevailing hypothesis for explaining spontaneous mimicry or automatic imitation is that the perception of some actions automatically activates corresponding motor programs. There are by now more than 75 experimental studies investigating automatic imitation, and most of the evidence is based on stimulus response compatibility paradigms, in which both stimuli and responses involve human movements (Heyes, 2011). In this paradigm, faster responding is observed when stimuli and responses correspond along some perceptual, structural or conceptual dimension than when they do not (referred to as a “compatibility effect”). When both the stimuli and responses involve human movements, it is often assumed that automatic imitation is involved. One problem with this interpretation is that the pattern of results for automatic imitation and all other S-R compatibility effects is exactly the same (i.e., faster response times for compatible than incompatible responses to the stimuli). As a consequence, these results beg the question as to whether the processes mediating automatic imitation are specialized or instead are the same processes involved in other S-R compatibility tasks. In order to resolve this issue, it is necessary to find a paradigm where the results for automatic imitation and other S-R compatibility tasks are predicted to be different. We recently provided such evidence by comparing imitative and spatial compatibilities in two experiments (Boyer, Longo, and Bertenthal, in press). The first tested for spatial compatibility with an imitative cue as the imperative stimulus, and the second tested for imitative compatibility with a spatial cue as the imperative stimulus. The stimulus consisted of a left or right hand with fingers spread apart and appeared on a computer screen from a third person perspective. Participants were instructed to respond to either the left-right spatial location or the anatomical identity of the index or middle finger tapping downward (Figure 1). Figure 1. Left panel depicts the left hand stimulus with the index finger tapping down. Right panel depicts the right hand stimulus with the middle finger tapping down. Responses consisted of pressing a key with the index or middle fingers on the right hand. In the standard S-R compatibility task (henceforth abbreviated “S-R task”), the responses were compatible with a task-irrelevant spatial stimulus when the left hand was presented (see Figure 1). For example, participants instructed to respond to the spatial cue would press a key with their index finger when responding to the left tapping finger. In this condition, both the stimulus and response are index fingers, and thus the response is facilitated via automatic imitation. Likewise, participants instructed to respond to the imitative cue would, for example, press a key with their middle finger when responding to the middle finger tapping. In this condition, both the stimulus and response correspond to the right side, and thus the response is facilitated via spatial compatibility. When the stimulus corresponded to a right hand, the