Behavioural evidence for distinct mechanisms related to global and biological motion perception

Abstract

The perception of human motion is a vital ability in our daily lives. Human movement recognition is often studied using point-light stimuli in which dots represent the joints of a moving person. Depending on task and stimulus, the local motion of the single dots, and the global form of the stimulus can be used to discriminate point-light stimuli. Previous studies often measured motion coherence for global motion perception and contrasted it with performance in biological motion perception to assess whether difficulties in biological motion processing are related to more general difficulties with motion processing. However, it is so far unknown as to how performance in global motion tasks relates to the ability to use local motion or global form to discriminate point-light stimuli. Here, we investigated this relationship in more detail. In Experiment 1, we measured participants' ability to discriminate the facing direction of point-light stimuli that contained primarily local motion, global form, or both. In Experiment 2, we embedded point-light stimuli in noise to assess whether previously found relationships in task performance are related to the ability to detect signal in noise. In both experiments, we also assessed motion coherence thresholds from random-dot kinematograms. We found relationships between performances for the different biological motion stimuli, but performance for global and biological motion perception was unrelated. These results are in accordance with previous neuroimaging studies that highlighted distinct areas for global and biological motion perception in the dorsal pathway, and indicate that results regarding the relationship between global motion perception and biological motion perception need to be interpreted with caution.

Keywords: Biological motion; Global motion; Motion coherence; Motion perception.

Fig. 1

Biological motion stimuli showing a right- and left-facing figure serving a tennis ball.

Fig. 2

Mean accuracy for each biological motion condition (error bars show SE and red line shows chance level). There was a significant difference in accuracy for normal compared to both random and scrambled stimuli (p < .05).

Fig. 3

Correlations between percentage accuracy for each biological motion condition and motion coherence (using RDK stimuli).

Fig. 4

Correlations between percentage accuracy for each biological motion condition. There is a significant correlation between normal and random (left) and normal and scrambled (centre). The correlation between random and scrambled (right) was not significant.

Fig. 5

Mean direction discrimination accuracy for biological motion stimuli with and without noise. Errors bars show standard errors of the mean and red line shows chance level. There is no significant difference in accuracy between the two conditions.

Fig. 6

Correlations between proportion accuracy for each biological motion condition and motion coherence (using RDK stimuli). There is a significant correlation between noise and no-noise biological motion (left), but no significant correlation of either biological motion condition with low-level motion coherence (centre and right). Four participants with relatively poor accuracy in the no-noise condition in the biological motion task are highlighted in red. It is clear that their drop in accuracy with the introduction of noise is specific to the biological stimuli. Note that excluding these four participants from analyses does not affect results. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)