Your blush gives you away: detecting hidden mental states with remote photoplethysmography and thermal imaging

Author:

Liu Ivan12ORCID,Liu Fangyuan2,Zhong Qi1,Ma Fei3,Ni Shiguang4

Affiliation:

1. Faculty of Psychology, Beijing Normal University, Beijing, China

2. Department of Psychology, Faculty of Arts and Sciences, Beijing Normal University at Zhuhai, Zhuhai, Guangdong, China

3. Guangdong Laboratory of Artificial Intelligence and Digital Economy (SZ), Shenzhen, Guangdong, China

4. Shenzhen International Graduate School, Tsinghua University, Shenzhen, Guangdong, China

Abstract

Multimodal emotion recognition techniques are increasingly essential for assessing mental states. Image-based methods, however, tend to focus predominantly on overt visual cues and often overlook subtler mental state changes. Psychophysiological research has demonstrated that heart rate (HR) and skin temperature are effective in detecting autonomic nervous system (ANS) activities, thereby revealing these subtle changes. However, traditional HR tools are generally more costly and less portable, while skin temperature analysis usually necessitates extensive manual processing. Advances in remote photoplethysmography (r-PPG) and automatic thermal region of interest (ROI) detection algorithms have been developed to address these issues, yet their accuracy in practical applications remains limited. This study aims to bridge this gap by integrating r-PPG with thermal imaging to enhance prediction performance. Ninety participants completed a 20-min questionnaire to induce cognitive stress, followed by watching a film aimed at eliciting moral elevation. The results demonstrate that the combination of r-PPG and thermal imaging effectively detects emotional shifts. Using r-PPG alone, the prediction accuracy was 77% for cognitive stress and 61% for moral elevation, as determined by a support vector machine (SVM). Thermal imaging alone achieved 79% accuracy for cognitive stress and 78% for moral elevation, utilizing a random forest (RF) algorithm. An early fusion strategy of these modalities significantly improved accuracies, achieving 87% for cognitive stress and 83% for moral elevation using RF. Further analysis, which utilized statistical metrics and explainable machine learning methods including SHapley Additive exPlanations (SHAP), highlighted key features and clarified the relationship between cardiac responses and facial temperature variations. Notably, it was observed that cardiovascular features derived from r-PPG models had a more pronounced influence in data fusion, despite thermal imaging’s higher predictive accuracy in unimodal analysis.

Funder

Beijing Normal University at Zhuhai Researcher Activation Fund

Shenzhen Key Laboratory of Next Generation Interactive Media Innovative Technology

Shenzhen R & D Sustainable Development Funding

Shenzhen Key Research Base of Humanities, Social Sciences for People’s Well-being Benchmarking Study

Guangdong Digital Mental Health and Intelligent Generation Laboratory

Publisher

PeerJ

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