The Chameleon Paradigm: An effective method for masking biological motion stimuli.

Continuous flash suppression (CFS) is widely used in research on unconscious visual processing due to its long-lasting masking. While CFS effectively masks static stimuli, its application to motion stimuli remains challenging. To resolve this issue, our previous work developed the Chameleon-1 paradigm (Zhao & Bao, 2022), an enhanced CFS technique that enables robust masking of translational motion stimuli for up to 10 s through precise spatiotemporal matching of color dynamics between target and masking stimuli. The current study systematically evaluated and optimized this paradigm through three studies. We first assessed the masking efficacy of the Chameleon-1 paradigm across different motion parameters and patterns (Study 1). Because Chameleon-1 failed to effectively mask biological motion (BM) stimuli, we then upgraded the paradigm to accommodate BM stimulus characteristics (Study 2). The results demonstrated that this Chameleon-2 paradigm achieved superior masking efficacy for BM stimuli, with average breakthrough time extended by over two-fold compared to Chameleon-1 and breakthrough ratios approximately 75% for upright and 45% for inverted BM stimuli during 10-s of BM presentation. We further employed this paradigm to investigate the neural correlates of conscious and unconscious BM processing using functional near-infrared spectroscopy in Study 3. Our work establishes a robust paradigm for sustained masking of BM stimuli and validates its utility in unconscious processing research. We also provide new insights into the neural mechanisms underlying unconscious BM perception.