Language is the primary means for human interaction, and communicative success requires an ability to reason about a conversation partner’s beliefs, desires, and goals. Many questions remain about the precise nature of the relationship between language processing and Theory of Mind (ToM), and their neural substrates. On the one hand, the two domains share an intimate connection given that a) language appears to have been shaped by communicative pressures; b) pragmatic inference is prevalent in language; c) aspects of ToM are important for language acquisition; and d) language is, in turn, critical for acquiring some aspects of ToM. But interestingly, prior neuroimaging and patient work have provided some evidence for a dissociation between basic linguistic processing and reasoning about mental states. In this thesis, I use functional MRI to elucidate the relationship between these two core human capacities.
First, I review some of the past research that informs the relationship between language and ToM. Then, I examine the topography of language processing and verbal vs. non-verbal ToM in two fMRI datasets (n=90 and n=47). I find that verbal, but not non-verbal, ToM elicits robust responses throughout the language network. Lack of response to non-verbal ToM argues against the core role of the language regions in social reasoning. Next, I investigate the relationship between the language and ToM networks using a functional correlation approach during several naturalistic conditions (n=55 participants across three studies). I find that although the two networks are dissociable, they also show reliable synchronization both at rest and during story comprehension. This synchronization may provide one mechanism for inter-network information sharing. And finally, I report a case study of an individual (EG) born without a left temporal lobe (likely, due to perinatal stroke damage). Given that in cases of early left hemisphere damage, language processing usually shifts to the right hemisphere, EG presents an interesting case study of how language and ToM – whose most mentalizing-selective component resides in the right temporo-parietal junction – can co-exist within a single temporal lobe. I show that language and ToM overlap to a greater extent in EG compared to a large normative dataset, and some areas that support ToM in neurotypical individuals instead support language processing in EG. However, both their basic language processing, and pragmatic processing appear fully intact. These results suggest that a single temporal lobe is sufficient to support both language processing and ToM, and that some cortical areas may assume either ToM or language functions, pointing to a deep relationship between the two systems, and an inter-changeable nature of their cortical substrates at least early in development.
This thesis can be read here: https://www.dropbox.com/sh/ex973nef6rxd05x/AAB4svCaGT60TMPF5HhvBeK3a?dl=0&preview=Alex+Paunov+-+PhD+Thesis+-+Language-ToM+-+20181211.docx