How can we understand each other during communicative interactions? An influential suggestion holds that communicators are primed by each other's behaviors, with associative mechanisms automatically coordinating the production of communicative signals and the comprehension of their meanings. An alternative suggestion posits that mutual understanding requires shared conceptualizations of a signal's use, i.e., "conceptual pacts" that are abstracted away from specific experiences. Both accounts predict coherent neural dynamics across communicators, aligned either to the occurrence of a signal or to the dynamics of conceptual pacts. Using coherence spectral-density analysis of cerebral activity simultaneously measured in pairs of communicators, this study shows that establishing mutual understanding of novel signals synchronizes cerebral dynamics across communicators' right temporal lobes. This interpersonal cerebral coherence occurred only within pairs with a shared communicative history, and at temporal scales independent from signals' occurrences. These findings favor the notion that meaning emerges from shared conceptualizations of a signal's use.social interaction | theory of mind | experimental semiotics | dual functional magnetic resonance imaging | conceptual knowledge H uman sociality is built on the capacity for mutual understanding, but its principles and mechanisms remain poorly understood (1). Given the pervasive ambiguity of communicative signals (2), how can we expect to understand each other? For instance, I might think of tacitly asking my friend Tom to enter a pub by virtue of a pointing gesture toward a nearby bike, believing that both of us recognized the bike of his girlfriend Emma, only to realize how my gesture would be interpreted differently as Tom tells me about his recent split from Emma (2, 3).An influential suggestion holds that communicators are mutually primed by each other's behaviors, with associative mechanisms automatically coordinating the production of communicative signals and the comprehension of their meanings (4-8). In this framework, mutual understanding arises by virtue of individual experiences with a signal's properties, as when linguistic features of a word are biased by recent experience of those features (9, 10). Alternatively, mutual understanding might require shared conceptualizations of a signal's use, abstracted away from specific experiences during a communicative interaction (11)(12)(13)(14). In this framework, mutual understanding arises from what communicators mutually know, "conceptual pacts" that are negotiated by communicators over the course of their interactions (11). Although both possibilities emphasize that communicative signals are context dependent (15), they put different emphasis on the relevance of the communicative signal. Both possibilities predict that mutual understanding is neurally implemented through temporally coherent and spatially overlapping activity across communicators (7,(16)(17)(18), but with different cerebral dynamics. If meaning ...