Perceiving the direction of articulatory motion in point-light actions.

Alex Davila,Karl Verfaillie,Ben Schouten,Matthew Longo

doi:10.1371/journal.pone.0115117

Abstract

Human observers are able to perceive the motion direction of actions (either forward or backward) on the basis of the articulatory, relative motion of the limbs, even when the actions are shown under point-light conditions. However, most studies have focused on the action of walking. The primary purpose of the present study is to further investigate the perception of articulatory motion in different point-light actions (walking, crawling, hand walking, and rowing). On each trial, participants were presented with a forward or backward moving person and they had to decide on the direction of articulatory motion of the person. We analyzed sensitivity (d') as well as response bias (c). In addition to the type of action, the diagnosticity of the available information was manipulated by varying the visibility of the body parts (full body, only upper limbs, or only lower limbs) and the viewpoint from which the action was seen (from frontal view to sagittal view). We observe that, depending on the specific action, perception of direction of motion is driven by different body parts. Implications for the possible existence of a life detector, i.e., an evolutionarily old and innate visual filter that is tuned to quickly and automatically detect the presence of a moving living organism and direct attention to it, are discussed.

Highlights

The Leuven psychologist Albert Michotte, a pioneer in research on the perception of animate motion, pointed out already several decades ago that ‘‘a thorough study of behaviour must take into account the way in which people and animals ‘understand’ the actions of other people and animals, as well as those they perform themselves’’ [1], p.34
In Experiment 4, we presented participants with a action for which the direction of articulatory motion probably is mainly perceived on the basis of the movements of the arms and the legs carry no useful information at all, namely hand walking
The main purpose of the present study was to examine the perception of motion direction of point-light actions on the basis of the articulatory relative motions of the limbs

Summary

Introduction

The Leuven psychologist Albert Michotte, a pioneer in research on the perception of animate motion, pointed out already several decades ago that ‘‘a thorough study of behaviour must take into account the way in which people and animals ‘understand’ the actions of other people and animals, as well as those they perform themselves’’ [1], p.34. One aspect that has received attention in the literature is the perception of the direction of walking (forward vs backward) of a walker moving on a treadmill, in human observers (e.g., [8, 13,14,15,16]), as well as in nonhuman primates [17]. With this stimulus configuration the global translatory component of motion (common motion) is set to zero and discrimination of the direction of motion can only be based on the articulatory movements of the body parts in relation to each other (relative motion; e.g., [18, 19])

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