Pupil size not only responds to light changes, but also reflects an individual's internal cognitive processing. For instance, pupils dilate when people look at pictures of delicious foods or engage in mathematical calculations, as a result of heightened arousal level or increased mental effort.

A research team led by Prof. JIANG Yi found pupil size also can reflect high-level social cognitive process. They found that life motion perception unfolds in the pupils of the eyes.

The study was published online in Attention, Perception, & Psychophysics on May 31.

In our daily lives, we effortlessly perceive the movements of living organisms, such as pedestrians walking in a station or cats running in a garden. Previous studies have shown that the human visual system is highly sensitive to biological motion information. However, it is still unclear whether this visual sensitivity can be objectively measured by a no-report method. As one of the physiological indicators of the autonomic nervous system, changes in pupil size provide valuable insights into an individual's internal state, without relying on subjective reports from the observers. In the study, researchers recorded observers’ pupil sizes when they watched the biological motion, respectively. They found that the pupil size was significantly enlarged when observers viewed upright biological motion compared with the inverted counterparts (Experiment 1). In addition, neither the perception of static figure nor the perception of non-biological motion (with the acceleration of the movements removed) induced stronger pupil dilation, indicating that such an effect critically depended on the dynamic biological characteristics (Experiment 2 and Experiment 3). Interestingly, such effect could be extended to local feet motion signals (Experiment 4).

 Figure.1 The experimental stimuli and results. Image by CHENG Yuhui.

Taken together, these findings verify that the eye pupil can signal life motion perception, providing new empirical evidence that processing of bio-social information can be indexed by the pupil. As the upright and inverted biological motion differs in their motion cues aligned with the direction of gravity, these findings also highlight the importance of these gravity-compatible motion cues in the perception of biological motion. More importantly, with the convenience, objectivity, and noninvasiveness of pupillometry, the current study paves the way for the potential application of pupillary responses in detecting the deficiency of life motion perception in individuals with socio-cognitive disorders.

This research was supported by the Ministry of Science and Technology of China and the National Natural Science Foundation of China.

LIU Chen
Institute of Psychology Chinese Academy of Sciences
Beijing 100101, China.
E-mail: liuc@psych.ac.cn