Theory of Mind brain regions are sensitive to the content, not the structural complexity, of belief attributions

Theory of Mind brain regions are sensitive to the content, not the structural complexity, of belief attributions Jorie Koster-Hale (jorie@mit.edu) and Rebecca R. Saxe (saxe@mit.edu) Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology Cambridge, MA 02139 Abstract A distinct group of brain regions, the ‘Theory of Mind (ToM) network’, is implicated in representing other people’s mental states, yet we currently know little about which aspects of mental state attribution are represented or processed in these regions. Using fMRI, we investigated whether ToM regions, compared to language-processing regions, are sensitive to two dimensions along which mental state attributions vary: (1) structural complexity and (2) social content of the attributed thought. In short vignettes describing a character's belief, the belief structure was either first-order or higher-order, and the content was mundane or socially-relevant. All ToM regions showed sensitivity to distinctions in content; no ToM region showed sensitivity to structural manipulation. By contrast, language regions were sensitive to both manipulations. We conclude that while increased structural complexity of belief attributions modulates language processing, this type of complexity is not part of the representational space of the ToM-network. Keywords: Theory of Mind; False Belief, Language, fMRI Introduction Mental state attribution exists in a very rich conceptual space – without much effort, we can ascribe a variety of mental states to other people, and make quick and subtle judgments about them. Moreover, we can easily characterize a mental state along a number of dimensions, such as who holds it, what kind of mental state it is (e.g. a belief, desire, or doubt), what the belief is about, how reasonable we find it, whether the content is relevant to our own lives, and how probable it is that it will be believed next week. Yet despite the range and flexibility of these inferences, mental state attribution gives rise to a surprisingly uniform neural response. A specific set of regions, often called the Theory of Mind network, consisting canonically of the bilateral temporo-parietal junction (TPJ), right superior temporal sulcus (rSTS), medial precunius (PC), and medial prefrontal cortex (MPFC), shows robust and systematic response to a variety of stimuli that invoke a mental state attribution, including stories and cartoons (Fletcher et al. 1995; Goel et al. 1995; Gallagher et al. 2000, 2002; Mitchell et al. 2002; Saxe and Kanwisher 2003; Perner, Aichhorn, Kronbichler, Wolfgang, & Laddurner, 2006; Gobbini, Koralek, Bryan, Montgomery, & Haxby, 2007; Van Overwalle 2009, Walter et al 2010). This combination of cognitive flexibility coupled with a robust and seemingly invariant neural response provides chance to examine the mapping between the neural response and final cognitive product: though we currently know very little about which aspects of mental state attribution are represented or processed in theory of mind regions, or what that representation looks like, we have the means to manipulate the cognitive representation at a fairly high level, and a precise place to look for changes in the neural representation. Thus, to begin answering these questions, we investigated the extent to which brain regions involved in theory of mind processing show sensitivity to features that vary within the space of mental state attribution. We asked whether ToM regions are sensitive to two broad dimensions along which mental state attributions can vary: (1) the structural (or syntactic) complexity and (2) the content of the attributed belief. We manipulated structural complexity by manipulating the first versus higher-order status of the belief – a manipulation that has often been employed to increase the difficulty of ToM tasks. We manipulated the content of the belief by varying its the social relevance. As well as varying features within the space of belief attribution, these manipulations vary along linguistic dimensions – saliency and syntactic complexity. Thus, to serve as a comparison, we asked whether high-level (sentence-level) language processing regions show sensitivity to these manipulations, and if so, whether the response profile in the ToM regions differed from the response profile in the language regions. We tested each of these possibilities in two steps. First, we used a functional localizer to identify language processing and Theory of Mind regions within the same set of individuals (Experiment 1). Second, we examined the effect that our manipulations of the structure and content of belief attributions had on the brain regions implicated in language and ToM (Experiment 2). Methods Participants: Twenty naive right-handed adults (aged 21-44, mean 27; 15 females) participated in the study for payment. All participants were native English speakers, had normal or corrected-to-normal vision, and gave written informed consent in accordance with the requirements of the internal review board at MIT. All 20 participants did both Experiment 1 and Experiment 2 in a single scan session. Two participants’ data were excluded due to excessive movement. Stimuli and Design: Experiment 1: ToM and Language Localizer The stimuli consisted of 24 short stories and 12 lists of non- words. Twelve of the stories were described a situation in which someone held a false belief, e.g.: After going to the gym, Kevin returned to his new apartment, which he had just recently moved into. He got upstairs and threw off his sweaty clothes, ready for a hot, steaming shower. Regrettably,