A critical characteristic of human language is our ability to understand multi-word sequences whose meaning is greater than the sum of their parts. Recent work from the PIs of this proposal (Toneva and Wehbe, 2019; Jain and Huth, 2018) and others (Schrimpf et al., 2020a; Caucheteux & King, 2020) has shown that cortical representations of multi-word sequences can be modeled much more accurately than before by using neural network language models, a machine learning technique that has revolutionized the natural language processing (NLP) field (Devlin et al., 2019; Radford et al., 2019). However, under the current paradigm these models must first be trained on separate NLP tasks and only then used to model the brain, creating a guess-and-check cycle that is not guaranteed to converge on the actual computations that humans perform. Here we propose to break this cycle by directly training neural network models to estimate the functions that the brain uses to combine words. To be able to optimally predict fMRI and MEG responses, these models will need to capture the composition principles governing which words the brain attends to, and how information is combined across words. These models will help uncover specific computations underlying language processing in the brain, enable computational testing of neurolinguistic theories, and inspire or directly improve models used in NLP. Accomplishing these goals, however, will require overcoming one major obstacle. Training neural net- work language models typically requires orders of magnitude more data than existing neuroimaging datasets. To address this issue, one central goal of this proposed project is to collect a very large fMRI and MEG dataset comprising roughly one million words of natural language stimuli. We plan to use the unique dataset and computational modeling framework to address three scientific aims. Aim 1: Create brain activity prediction benchmarks to foster interaction between neuroscience and NLP. Aim 2: Use data-driven models to test existing neurolinguistic theories & develop new accounts of the computations underlying word composition in the brain. Aim 3: Leverage information in different brain areas to help solve computationally defined language tasks. Successful completion of the proposed work will provide mechanistic insight into language processing, with a computational architecture tracing information flow among brain areas and describing the tasks they perform. Beyond its basic cognitive neuroscience implications, we expect this work will enable better understanding of language impairments and help identify targeted therapies. RELEVANCE (See instructions): Through collecting, analyzing, and disseminating large-scale neuroimaging datasets collected while participants listen to natural, narrative speech, this proposal aims to improve our understanding of the normal function of the language system. Specifically, this work seeks to improve and validate computational models of sp...