Testing Video MLLM Sensitivity to Hand-Localized Visual Detail in Chinese Sign Recognition: A Matched Visual Intervention Study

Bowen Ye, Dengfeng Yao, Siyuan Jing, Peng Jin, Hongzhe Liu, Cheng Xu


Abstract
Recent benchmarks show that multimodal large language models (MLLMs) can answer questions about sign-language video, but accuracy alone does not reveal which visual cues support those answers. We test behavioral sensitivity to hand-localized visual detail using 150 front-view isolated signs from NationalCSL-DP. For each item, a hand-target condition (T) blurs tracked hand regions and a same-arm control (C) applies the same Gaussian operator, nominal σ, and translated mask geometry to nearby arm regions; the underlying regional content is not information-matched. The same four-choice question is used across conditions and four video MLLMs. All 1,800 final Primary responses are uniquely scorable, with 1,200 T/C responses entering the primary contrast. Mean accuracy is 38.17% on original videos (O), 33.50% on T, and 36.67% on C. The pre-specified C–T contrast is +3.17 percentage points (95% bootstrap CI [−0.17, 6.50]; item-cluster sign-flip p = 0.08172). The four model-level point estimates are descriptively positive, but two models are floor-limited. The evidence is therefore directional rather than confirmatory. The contribution is a matched-control diagnostic that complements aggregate benchmark accuracy while making its residual anatomical-content mismatch explicit.
Anthology ID:
2026.wslp-1.24
Volume:
Proceedings of the Second Workshop on Sign Language Processing
Month:
October
Year:
2026
Address:
Budapest, Hungary
Editors:
Sanjeet Singh, Abhinav Joshi, Pokala Dattatreya, Keren Artiaga, Khushi Jaydeep Pujara, Tomáš Železný, Ivan Gruber, Jakub Honzík, Sabyasachi Kamila, Facundo Manuel Quiroga, Marek Hrúz, Mohammed Hasanuzzaman, Ashutosh Modi
Venues:
WSLP | WS
SIG:
Publisher:
Association for Computational Linguistics
Note:
Pages:
243–248
Language:
URL:
https://aclanthology.org/2026.wslp-1.24/
DOI:
Bibkey:
Cite (ACL):
Bowen Ye, Dengfeng Yao, Siyuan Jing, Peng Jin, Hongzhe Liu, and Cheng Xu. 2026. Testing Video MLLM Sensitivity to Hand-Localized Visual Detail in Chinese Sign Recognition: A Matched Visual Intervention Study. In Proceedings of the Second Workshop on Sign Language Processing, pages 243–248, Budapest, Hungary. Association for Computational Linguistics.
Cite (Informal):
Testing Video MLLM Sensitivity to Hand-Localized Visual Detail in Chinese Sign Recognition: A Matched Visual Intervention Study (Ye et al., WSLP 2026)
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PDF:
https://aclanthology.org/2026.wslp-1.24.pdf