Project Summary Our ability to understand other people in terms of the underlying mental states that drive their actions, termed theory of mind (ToM), is essential to human social behavior. Neuroimaging research has found that this sophisticated, abstract reasoning ability relies on a specific network of regions across association cortex, with one region, the temporo-parietal junction (TPJ), showing a particularly selective response and ToM-relevant information content. However, because the ToM response in TPJ has only been studied using noninvasive brain imaging techniques with limited precision, our understanding of the detailed functional properties of this region—what information is represented, how these representations evolve over time, what computations are performed—remains very limited. Here we propose to study the role of TPJ in mental state reasoning using intracranial electrocorticography in humans, providing a direct measure of neural activity with combined spatial and temporal resolution. We will study TPJ responses using narrative comprehension tasks, in which subjects listen to a story describing the actions and interactions of human characters, and answer questions that require reasoning about their mental states. In Aim 1, we will probe basic aspects of the time course of TPJ response, leveraging the high temporal resolution of the electrocorticographic signal. We will test the following hypotheses: 1) TPJ has rapid, transient increases in activity in response to novel mental state information in a narrative. 2) TPJ responses to mental state content increase over longer time scales in a paragraph-long narrative, as more contextual information is available. In Aim 2, we will use high-density recordings to probe the precise spatial and functional organization of responses to theory of mind, semantic comprehension, and episodic recall. We will test two hypotheses about the functional specificity of ToM responses: 1) responses to ToM content and to more generic semantic content will be spatially segregated within TPJ. 2) Areas responsive to ToM will also be engaged during the episodic recall of richly social events, but not during more generic recall processes without a social component. This research will provide the first electrophysiological characterization of a region involved in uniquely human social cognition, and lay the groundwork for a research program that will characterize the neural basis of mental state reasoning with increasingly precise recording devices.