Gesture height reflects common ground status even in patients with amnesia.

When we communicate, we alter our language and gesture based on the mutually shared knowledge - common ground - that we have with our listener. How memory supports these alterations remain unclear. We asked healthy adults and patients with hippocampal amnesia to engage in a referential communication task. Previous work suggests that common ground can be encoded by distinct memory systems; Amnesic patients show normal learning and referential label use as common ground increases, but inconsistently mark these labels with definite determiners (e.g., the vs. a windmill). Which memory systems support the ability to mark common ground via hand gesture? We found that gestures of both healthy participants and amnesic patients reflected common ground status. Both groups produced high gestures when common ground was lacking, and were less likely to do so as common ground increased. These findings suggest that gesture can reflect common ground status during conversation, potentially via non-declarative memory.