Gradiency and Visual Context in Syntactic Garden-Paths.

Through recording the streaming x, y coordinates of computer-mouse movements, we report evidence that visual context provides an immediate constraint on the resolution of syntactic ambiguity in the visual-world paradigm. This finding converges with previous eye-tracking results that support a constraint-based account of sentence processing, in which multiple partially-active syntactic alternatives compete against one another with the help of not only syntactic, semantic, and statistical factors, but also nonlinguistic factors such as visual context. Eye-tracking results in the visual-world paradigm are consistent with theories that posit limited interaction between context and syntax, but they are still consistent with related theories that allow immediate interaction but require serial pursuit of syntactic structures, such as the unrestricted race model. To tease apart the constraint-based and unrestricted-race accounts of sentence processing, the distribution of computer-mouse trajectories was analyzed for evidence of two populations of trials: those where only the correct parse was pursued and those where only the incorrect parse was pursued. We found no evidence of bimodality in the distribution of trajectory curvatures. Simulations with a constraint-based model produced trajectories that matched the human data. A nonlinguistic control study demonstrated the mouse-tracking paradigm's ability to elicit bimodal distributions of trajectory curvatures in certain experimental conditions. With effects of context posing a challenge for syntax-first models, and the absence of bimodality in the distribution of garden-path magnitude posing a challenge for unrestricted-race models, these converging methods support the constraint-based theory's account that the reason diverse contextual factors are able to bias one or another parse at the point of ambiguity is because those syntactic alternatives are continually partially-active in parallel, not discretely winnowed.