Socially induced motor plasticity affects language comprehension

Socially Induced Motor Plasticity Affects Language Comprehension David Havas (dahavas@wisc.edu) & Julia Jenvey (julia.jenvey@gmail.com) Department of Psychology, 1202 W. Johnson Street Madison, WI 53706 USA Hayley Shilling (hshilling@wisc.edu) Department of Counseling Psychology, 244 Rust-Schreiner Halls Madison, WI 53715 USA Mitchell Nathan (mnathan@wisc.edu) Department of Educational Psychology, 1025 W. Johnson Street Madison, WI 53706 USA Abstract Language understanding is a socially coordinated activity, but the mechanisms of social coordination in language are poorly understood. Evidence from embodied cognition has shown that movement-induced fatigue of actions slows comprehension of language that refers to those actions. Research on the mirror neuron system suggests that action systems of the brain are also involved in social understanding of actions performed by another, empathy, and possibly language. Here, we show that simultaneous performance and observation of kinematically similar actions produced a fatigue-like effect in sentence judgment times relative to dissimilar control actions. The results suggest that the same action systems used in language processing are influenced by social actions. Keywords: language comprehension, embodied cognition, social cognition, joint action, motor plasticity, mirror neuron system. Introduction Language is fundamentally a social activity in which individuals coordinate their actions (Clark, 1996; Grice, 1975). Conversation involves intricately timed verbal and non-verbal signals for clarifying, initiating, guiding, and ending dialog (Clark, & Wilkes-Gibbs, 1986; Garrod & Anderson, 1987). The mechanisms of language coordination are of current interest. According to theories of dialog, conversation is successful to the extent that there is similarity of mental states between participants (Garrod & Anderson, 1987). For example, dialog requires that participants share a common ground, or similarity in mental states about referents (Clark, 1996; Clark & Wilkes-Gibbs, 1986). Interlocutors tend to show similarity across linguistic and non-linguistic levels, including word use (Garrod & Anderson, 1987), syntax (Branigan et al., 2000), semantics (Clark, & Wilkes-Gibbs, 1986) and movements (Chartrand & Bargh, 1999). It has been proposed that the same neural systems used for action imitation are also used in dialog (Garrod & Pickering, 2004; Pickering & Garrod, 2006b). Recent evidence from embodied cognition has shown a close link between action and language. Glenberg and Kaschak (2002), for example, implicated the action system’s influence on language comprehension. Participants in this study responded to a series of sentences depicting transfer, either away from (“ Close the drawer“) or toward the body (“ Open the drawer“). After reading each sentence, participants deemed it sensible or nonsense by pushing “yes” or “no” buttons and in so doing, were required to move their hands either toward their bodies or away. The results demonstrated an action-sentence compatibility effect (ACE). Sentence judgment times were shorter when the sentence depicted movement compatible with the movement required to make a sensible response. The ACE effect has been demonstrated for sentences describing both concrete and abstract (or metaphorical) transfer (Glenberg, Sato, Cattaneo, Riggio, Palumbo, & Buccino, 2008). More recent evidence has shown that language comprehension is influenced by prior fatugue of the motor system. Glenberg, Sato, & Cattaneo (2008) demonstrated that fatigue of specific actions influences comprehension of written language depicting those actions. To fatigue the neural motor systems involved in toward and away movement, participants were asked to engage in a repetitive action in a toward or away direction. Participants moved 600 beans individually from a container to a target. They were then asked to read a series of sentences describing transfer either toward (“Mark deals you the cards”) or away from the body (“You deal Mark the cards”), and judge them as sensible or nonsense by pressing a button. Response times (the time to read the sentence and push the button) were longer when the sentence depicted motion congruous to the movement the participant had previously fatigued. When the sentence depicted an incongruous movement, response times were shorter. This finding was taken as evidence that repeated movement induced plasticity in motor areas recruited in language processing. In particular, the repetition of movement induced muscular fatigue, forcing action-controlling neurons in Broca’s region to increase their output, but no longer target the specific action (Glenberg et al., 2008). Thus, participants’ comprehension of written depictions of the compatible action (toward or away) was slowed, compromised by shared involvement in both action and language in Broca’s region (Gallese, 2008).