Reading point-light walkers and amorphous: a TMS study

Abstract

Objectives: (i) Evaluate if human actions with and without pictorial information result in comparable motor facilitation clarifying the adequacy of point-light (PL) human actions in motor observation and motor resonance research; (ii) Verify if the isolated aspects of human shape, i.e., without movement; and the isolated aspect of movement, i.e., PL in amorphous shape, are not enough for eliciting motor network response and thus that motor facilitation is specific to human action perception involving human motor and spatial configurations. Research question: Are PL human actions suitable stimuli for action observation experiments evaluating motor resonance response and what are the adequate parameters for preparing their control stimuli? Methods: 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, scrambled moving point-lights (PLscr) and static point-lights forming a human shape (PLs). All videos were approximately 1000ms long. Peak-to-peak MEP amplitude (maximal distance) was individually averaged for each condition (RM, PLbio, PLscr, PLs). Results: rmANOVA considering MEP as dependent variable and condition as within-subject factor revealed a main effect for Stimuli (F1,17= 6.91; p=.029; np2 =.16). Specifically, Fisher LSD post hoc revealed that such effect was due to inferior MEP amplitude in PLs condition when compared to RM (p=.016), PLbio (p= .006) and PLscr (p=.047). Conclusion: Our findings account for future studies elaboration by providing information on similar CE increase during PLbio and RM observation, thus corroborating the use of human PL in motor resonate/action observation studies. Noteworthy, PLscr 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 not to 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 activation.