In the current study, we show that a reliable N400-effect (i.e. a neural correlate of comprehension difficulty) is elicited by sentence stimuli that are correct for a participant but incorrect for a jointly attending other (the confederate). Importantly, this Social N400-effect is elicited by the constraints of the social situation—if the confederate is removed from the room, exactly the same sentence stimuli fail to elicit an N400-effect. Additionally, we show that the canonical N400-effect in response to semantically implausible sentences is attenuated with only one sentence of disambiguating context.

The results of this study indicate that understanding someone else’s inability to grasp language content elicits a neural marker of semantic integration difficulty that is comparable to that elicited by failing to grasp something oneself. It is important to note that this study does not speak to how automatically we track the comprehension of others during communication—it provides information only about the cognitive process underlying those moments in which we do track knowledge about others (i.e. because of experimental task requirements). Further research is necessary to show how automatic the Social N400-effect is, as well as how one’s propensity to track the knowledge of other listeners is related to individual differences in personality (e.g. empathy) and related cognitive abilities (e.g. working memory, inhibition).

The current study shows that when participants do attend to the ability of a confederate to comprehend language stimuli, semantic mismatches for the confederate elicit a pattern of neural activation that is comparable to what is seen for semantic mismatches for the participant him/herself. The fact that detecting implausibility for another results in changes in the ERP signal that are temporally and spatially comparable to detecting implausibility for oneself lends weight to accounts suggesting that knowledge about others plays a primary role in language comprehension. In addition, this finding is in line with what has been reported in the joint action literature, namely that participants simulate the task demands of jointly acting others (e.g. Sebanz, Knoblich, & Prinz, 2003, 2005). This result opens questions about how two conflicting interpretations of sentence content are maintained and managed by the participant. Clearly, participants are capable of simultaneously entertaining multiple interpretations of the stimulus, as the confederate’s interpretation does not override the participant’s own understanding of the sentence. In many ways, this situation is akin to what happens when two meanings of an ambiguous word or homophone are simultaneously understood in a pun (e.g. The Cane and Able as the name of a health care centre)—the pun is only funny if both meanings are entertained (see, e.g., Coulson & Severens, 2007). We suggest that keeping track of one’s own versus another’s interpretation of a sentence will work in a similar fashion, and will certainly draw on additional cognitive processes such as working memory and inhibition (see also Lin, Keysar, & Epley, 2010; Brown-Schmidt, 2009). It is also worth noting that the mere presence of the confederate had an effect on accuracy judgments. Specifically, participants were significantly worse at detecting implausible sentences in the presence of a confederate than when seated alone. One possible explanation for this result is that participants in the Joint Group were attuned to the mismatch between their own and another’s interpretation and assumed that a mismatch in the opposite direction (i.e. participant finds sentence implausible, confederate finds it plausible) would also be included. In such cases, participants may have tried to second guess their interpretation of implausible sentences. While this explanation is speculative, the result itself highlights the fact that many cognitive tasks may be unwittingly affected by the mere presence of others.

The results of this study may differ from some previous reports highlighting egocentric processing biases (e.g. Keysar et al., 2000) because participants in our study were engaged in a true joint comprehension task. Previous experimental paradigms have investigated situations in which speaker and listener are united in dialogue (i.e. also a form of joint task), but share no immediate common action within the dialogue (i.e. speaker and listener tend to do different things at any given moment to achieve their common goal). Note that current theories suggest that language production and comprehension involve similar mechanisms (Pickering & Garrod, 2013), thus the distinction between speaker and listener may be less clear cut than previously envisioned. Nevertheless, it is clear that the overall process of producing an utterance versus understanding an utterance is different even if both draw on some common cognitive and neural processes. In contrast, in our study, participants jointly attended spoken stimuli alongside another. This means that the cognitive task of participant and confederate were better aligned than in previous studies, as both simultaneously perceived language input. When the cognitive tasks of participants are aligned, direct effects of one person’s knowledge are seen on the ability of the other to perform. When the cognitive tasks are misaligned (i.e. one person speaks while the other listens), the effects of knowledge about others may have a relatively later effect, as the primary cognitive tasks of speaker and listener are actually quite different.

In addition, this study provides converging evidence in support of the strong effect that discourse can have on lexical–semantic processing. In previous research, Nieuwland & van Berkum (2006) showed that the N400-effect is attenuated after participants are presented with three to five sentences of disambiguating context (i.e. attenuation of lexical-semantic expectations is slow; see also Warren, McConnell & Rayner, 2008). In the current study, the N400-effect is attenuated with only one disambiguating context sentence. We suggest that attenuation may have been so much stronger in the current paradigm, as the first sentence explicitly pointed out how the second sentence could be rendered plausible, i.e. in contrast to previous research, the first sentence of discourse was not implausible in itself (e.g. In the cartoon the dog worked in a bank), therefore it does not require the participant to come up with a situation model in which dogs wear suits, but rather it provides one explicitly. By explicitly describing the context in which the target sentence can be seen as plausible, we see a more or less immediate effect of discourse on the processing of target sentences. In support of this idea, Filik and colleagues have shown that, when an already well-known context provides disambiguating information (e.g. information about a cartoon character, like the Incredible Hulk), attenuation of the N400-effect is also very fast (Filik, 2008; Filik & Leuthold, 2008).

Our results have broad implications for understanding not only how we take others into account during conversation but also how we theorise about others’ mental states in general. While a great deal of research has been done on the behavioural consequences and neural correlates underlying theory of mind abilities, understanding the time course of mentalising has proven to be difficult (Apperly, 2011, 2012; Sabbagh & Taylor, 2000). The use of joint comprehension paradigms like the one introduced here has the potential to give excellent insight into the time course along which we take knowledge about others into consideration.