Investigation of the neural basis of natural action perception within the theoretical framework of perceptual decision making

Every day, we perceive many actions performed by people around us. Understanding the movements we perceive and the intentions behind these movements is critical to determining how to respond to the observed action. Neurophysiology studies and computational model theories suggest that this perceptual decision process consists of two stages: accumulation of encoded sensory evidence and selection of response. EEG studies in humans have identified a neural trace thought to represent the first of these two stages: Centro Parietal Positivity (CPP), which scales with the strength of the incoming sensory evidence and differs from the Lateralized Readiness Potential (LRP) that represents motor preparatory activity. However, these studies mainly utilized simple and artificial stimuli; therefore, whether perceptual decision-making processes will be similar when processing a more complex and ecologically meaningful stimulus, such as natural actions, is an open question. This study aimed to investigate this gap in the literature by examining the neural basis of perceptual decisions related to the perception of natural actions. To this end, in this study, which included three separate EEG sessions, twenty participants performed a discrimination task between action exemplars belonging to a different action class in each session (either one of the locomotion, skin-displacement actions, or manipulation actions). To control for task difficulty, the same coherence manipulation was applied to each action exemplar to create four levels of coherence, and behavioral data analysis showed that mean response times and miss rates decreased as the level of coherence increased. Furthermore, the results of event-related potential analysis showed that the CPP signal followed the level of coherence of the stimulus within all action classes, which is in line with the literature, and also that the manipulation of the action class had a significant effect on the CPP signal. In contrast to CPP, the LRP signal showed an independent build-up from both the strength of the sensory evidence and the identity of the action class. Taken together, the findings of the current study support the generalizability of perceptual decision-making stages and their neural basis, which are defined by utilizing much simpler stimuli, to decisions related to the perception of natural actions while revealing that the identity of natural movements significantly influences the decision process.