Syntactic Belief Update as the Driver of Garden Path Processing Difficulty

2026-06-25

AI Summary

Traditional lexical surprisal struggles to effectively predict processing difficulty in garden-path sentences. This work proposes a syntactic-structure-based measure of processing difficulty that does not rely on lexical probabilities. Specifically, it employs incremental syntactic parsing to dynamically maintain a probability distribution over syntactic trees—termed “syntactic belief”—and quantifies the magnitude of belief updating at critical ambiguity points using generalized Rényi divergence as a proxy for cognitive load. This approach significantly outperforms traditional lexical surprisal in fitting human reading time data, offering psycholinguistics a non-lexical, syntax-driven alternative metric grounded in belief updating. The findings open a new avenue for understanding online language processing through the lens of syntactic uncertainty resolution.

Abstract

Garden path sentences present a processing difficulty for humans -- the sentence prefix leads the listener towards one interpretation, until the listener hears a critical word that shows that the initial interpretation was wrong. Lexical surprisal, a measure that usually predicts sentence processing difficulty quite well, fails to provide good predictions for garden path sentences. We propose an alternative that actively predicts a probability distribution over syntactic trees (its syntactic belief) and updates that distribution after each new word. If a processor is led down a garden path, syntactic beliefs will be wrong and will require a large update at the critical word. The magnitude of the update is measured with a generalized Rényi divergence. Crucially, this metric is dependent on lexical items, but is fully independent of the probability of lexical items. This Syntactic Belief Update provides a better fit to the human reading time data on garden path sentences. This suggests a new research direction examining purely non-lexical alternatives to surprisal for psycholinguistics.

Problem

• garden path sentences
• sentence processing difficulty
• lexical surprisal
• syntactic belief
• psycholinguistics

Innovation

• Syntactic Belief Update
• Garden Path Sentences
• Rényi Divergence
• Sentence Processing Difficulty
• Non-lexical Surprisal