The ability to readily distinguish animate entities from inanimate objects in the environment is essential to animal survival and social interaction. Biological motion (BM), represented by the movements of a few point lights attached to the major joints, can provide sufficient visual information for the detection and recognition of living entities. However, visual perception of BM in point-light displays (PLDs) is strongly impaired when the PLDs are presented upside-down. This phenomenon, known as the inversion effect in BM perception, is presumably a manifestation of an evolutionarily endowed perceptual filter (i.e., life motion detector) tuned to gravity-compatible BM.

The inversion effect in BM perception explained by the “life motion detector” has so far been observed in terrestrial animals. However, it remains largely unknown how it would be in lower aquatic vertebrates, such as teleosts, which live in a completely different environment from terrestrial animals. In a recent study, Professor JIANG Yi, Professor LIU Zuxiang, and their colleagues from Chinese Academy of Sciences used zebrafish as a model animal to systematically examine the ability of teleosts to discriminate between upright BM (gravity-compatible) and inverted BM(gravity-incompatible) signals through three experiments (Figure 1).

In Experiment 1, the upright BM and non-BM stimuli were respectively projected onto the two opposite sides of the test tank simultaneously. Results revealed that the fish preferred to approach and watch the upright BM than the non-BM stimuli (Figure 2a and 2b), indicating that the zebrafish are able to discriminate between the upright BM and the non-BM. Based on these results, in Experiment 2, the upright and the inverted BM stimuli were respectively projected onto the two sides of the test tank as in Experiment 1, while in Experiment 3, the upright or the inverted BM stimuli were presented on a single side of the tank, with each fish exposed to only one type of the stimuli. Results demonstrated that, relative to the inverted BM stimuli, the zebrafish were more willing to swim in proximity to the upright BM stimuli in Experiment 2 (Figure 2c) and spent more time orienting towards the upright BM stimuli in Experiment 3 (Figure 2d).

Figure 1. Experimental setup, stimuli and procedure. Upright BM, inverted BM and non-BM stimuli were represented as the animations of six white dots. Image by MA Xiaohan.

Figure 2. Results of the three experiments. Image by MA Xiaohan.

More intriguingly, when the recorded point-light video clips of fish were directly compared with those of human walkers and pigeons, a unique and consistent pattern of accelerating movements in the vertical (gravity) direction could be identified (see Table 1 in the published article).

Taken together, to our knowledge, the current study demonstrated for the first time that zebrafish own the ability to discriminate the upright BM of conspecifics from the inverted counterpart, exhibiting the inversion effect in BM perception. These findings suggest that the evolutionary origin of gravity-dependent BM processing may be traced back to ancient aquatic animals, thereby expand the applicability of the specialized “life motion detector” from terrestrial vertebrates to aquatic vertebrates. It is possible that this privileged ability of animacy detection is widely conserved in all vertebrates.

This study entitled “Gravity-dependent animacy perception in zebrafish” has been published online on August 31, 2022 in Research.

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