Reading point-light walkers and amorphous—A TMS study

Abstract

Objective: (a) Evaluate if human actions with and without pictorial information result in comparable motor facilitation; and (b) verify if the isolated features of movement and human shape increase motor excitability. Method: Motor evoked potentials (MEPs) of M1 were recorded from 18 healthy subjects using transcranial magnetic stimulation (TMS) during presentation of full-body video clips of everyday human actions either with (real movement, RM) or without (biological point-light, PLbio) pictorial information, non-biological/scrambled moving point-lights (PLnb), and static point-lights arranged in a human shape (PLs). All videos were approximately 1,000 ms long. Peak-to-peak MEP amplitude was individually averaged for each condition. Results: repeated measures analysis of variance (rmANOVA) considering MEP as dependent variable and stimuli as within-subject factor revealed an effect for Stimuli (F1,17 = 6.53; p < .001; η2p = .227). Such effect was due to lower MEP amplitude in PLs when compared to RM ( p = .002), PLbio ( p = .002) and PLnb ( p = .005). Conclusion: The similar corticospinal excitability (CE) increase during PLbio and RM observation corroborates the use of human PL in motor resonance/action observation studies. Noteworthy, PLnb also engaged the motor network, which could be due to kinematic aspects of human velocity profile or anthropomorphism of non-biological agents. Observation of PLs resulted in significantly smaller MEPs. Therefore, M1 activation seems restrict to movement perception but might not be strictly dependent on human form. Thus, planning the control stimuli and task context is crucial when using PL displays in the study of human action perception and the action observation network (AON).