Using MoTR to Probe Agreement Processing in Russian.

One important distinction in the syntax literature is between agreement that is external to the nominal phrase and agreement that is internal to it (sometimes called concord). How this type of agreement impacts sentence processing, however, is not well understood. In this paper, we ask whether agreement errors are processed differently based on their internal vs. external status. We investigate this question in Russian, a Slavic language that has a rich morphological agreement system and flexible word order, allowing us to control for several confounds. Our results are not fully conclusive but do provide moderate evidence that processing of agreement is modulated by internal vs. external status. We measure real-time language processing using Mouse Tracking for Reading (MoTR), a new web-deployable measurement tool that has been argued to improve over previous methods (e.g., self-paced reading) but has so far been tested only in English, and never for agreement processing phenomena. We find that MoTR can successfully pick up differences in our factorized psycholinguistic experiment in Russian, validating MoTR as a reliable tool for investigating agreement (error) processing. A direct comparison with existing data collected using in-lab eye-tracking-while-reading with similar experimental materials (Fuchs et al., 2025) suggests MoTR data yields larger effect sizes than does eye-tracking data.