In their target article, Niedenthal et al. present the SIMS model, which provides a theoretical account of how embodied simulation is involved in interpreting smiles. We suggest that mechanisms like those proposed in the SIMS model are also involved in interpreting the spontaneous hand gestures that people produce when speaking. In past work, we have argued that gesture production is based on simulated actions and perceptions (Hostetter & Alibali Reference Hostetter and Alibali2008). In this commentary, we make the case that embodied simulation is also involved in gesture comprehension.

The Gesture as Simulated Action (GSA) framework (Hostetter & Alibali Reference Hostetter and Alibali2008; Reference Hostetter and Alibali2010) explains how representational gestures might arise from an embodied cognitive system. Aligned with theories of embodied cognition (e.g., Barsalou Reference Barsalou1999; Glenberg Reference Glenberg1997), the GSA framework holds that language processing activates perceptual and motor states. Speakers form mental images of information they are expressing; these mental images activate the same perceptual and motor systems that are involved in perceiving and interacting with physical objects in the world. This activation of the motor system is sometimes overtly expressed as representational gestures.

Here, we consider whether simulated actions could also be involved in gesture comprehension. Niedenthal et al. argue that when people perceive smiles, they engage their motor systems, either to mimic the perceived smiles or to simulate the experience of performing the perceived smiles. This mimicry or simulation contributes to perceivers' understanding of the perceived smiles.

We suggest that the same mechanisms – mimicry and simulation – might also be involved in comprehending representational gestures. Imagine a speaker describing how he makes pizza, who uses gestures to depict how he uses his fingertips and his knuckles to flatten the dough. Imagine further that the speaker's addressee has never made pizza crust before. In comprehending the speaker's description and gleaning information from his gestures, the addressee might overtly mimic the speaker's actions, or she might mentally simulate such actions.

Do speakers overtly mimic other speakers' gestures? A growing body of evidence indicates that they do (e.g., Kimbara Reference Kimbara2006; Reference Kimbara2008; McNeill, in press). Mimicry does not arise as a simple coincidence of people using similar gestures when they talk about similar things. Instead, similarity of gesture form is more likely when interlocutors can see one another than when they cannot, suggesting that mimicry is purposeful (Kimbara, Reference Kimbara2008). McNeill (in press) observes that researchers who code gestures often use mimicry as part of their effort to make sense of a speaker's gestures. He argues that addressees use mimicry to understand gestures that may not be readily interpretable otherwise.

Although overt mimicry of gestures does occur, it is not as widespread as might be expected if it were the primary mechanism of gesture comprehension. Recall that Niedenthal et al. emphasize that smile perceivers sometimes simulate the experience of the perceived smile, without overtly mimicking it. Along similar lines, it seems likely that gesture comprehenders sometimes simulate the gestures they perceive, without overtly producing them. These embodied simulations of gestural actions may contribute to comprehenders' interpretation of the perceived gestures. Support for this mechanism comes from studies that have shown activation of premotor areas when people observe actions (for reviews, see Jeannerod Reference Jeannerod2001; Rizzolatti et al. Reference Rizzolatti, Fogassi and Gallese2001). Premotor cortex is also activated when people observe gestures (e.g., Montgomery et al. Reference Montgomery, Isenberg and Haxby2007), and this activation is modulated by semantic information from the accompanying speech (Willems et al. Reference Willems, Özyürek and Hagoort2007). It has been argued that, when addressees perceive gestures, they interpret them, at least in part, by activating the cortical networks involved in producing gestures, via an “observation-execution matching system” (e.g., Holle et al. Reference Holle, Gunter, Rüschemeyer, Hennenlotter and Iacoboni2008; see Dick et al. Reference Dick, Goldin-Meadow, Hasson, Skipper and Small2009, for discussion). This idea is similar to the one we advance here.

The SIMS model holds that, in the case of smiles, eye contact triggers embodied simulation. It is possible that eye contact may also initiate embodied simulation of the co-occurring gestures; however, no studies to date have specifically examined the relation between eye contact and gesture comprehension. Instead, the little research that has explored the role of eye gaze in gesture comprehension suggests that addressees are most likely to attend to speakers' gestures when speakers direct their gaze to the gestures (e.g., Gullberg & Holmqvist Reference Gullberg and Holmqvist1999). Further, addressees are more likely to incorporate information from the gesture into their comprehension when speakers have fixated on the gestures (Gullberg & Kita Reference Gullberg and Kita2009).

One explanation for these findings is that speakers' gaze to their own gestures signals that the gestures are important; addressees then directly attend to those gestures and run “as-if” simulations of the gestures that facilitate comprehension. Speakers' shift in eye gaze may be particularly important for initiating addressees' simulations of speech-accompanying gestures, because addressees typically focus on speakers' faces, making it difficult to attend to detailed features of gestures produced in peripheral space.

Could mimicry and simulation account for individual and situational differences in addressees' reliance on gesture for comprehension? Research has shown that gestures make a greater contribution to comprehension when the material to be comprehended is complex, ambiguous, or challenging relative to the addressee's skills (Graham & Heywood Reference Graham and Heywood1976; McNeil et al. Reference McNeil, Alibali and Evans2000). It also seems likely that individuals differ in their reliance on gesture for comprehension, although there is little data directly addressing this point. We suggest that addressees vary in how likely they are to engage in overt mimicry and/or simulation. Across contexts, individuals may be more likely to engage in mimicry and/or simulation in situations where language comprehension is more difficult, as suggested by McNeill (in press).

In sum, building on our past work about simulated action as an explanation for why speakers produce gestures, we suggest that similar mechanisms may be involved in comprehension of gestures. Viewing a speaker's overt simulation of an action or event may elicit overt mimicry, or it may evoke a corresponding simulation in the addressee's mind. In the SIMS model, mimicry and simulation contribute to perceivers' understanding of smiles. We suggest that mimicry and simulation also contribute to addressees' comprehension of speakers' gestures.