Priming biological motion changes extrapersonal space categorization

Highlights

• Priming with a point-light walker vs a scrambled motion a human RF leads to categorize the distance as compressed.

Abstract

Recent results have shown that the way we categorize space varies as a function of the frame of reference. If the reference frame (RF) is another person vs. an object, the distance is judged as reduced. It has been suggested that such an effect is due to the spontaneous processing of the other's motor potentialities. To investigate the impact of movement representation on space perception, we used biological motion displays as a prime for a spatial categorization task. In Exp. 1, participants were presented with a point-light walker or a scrambled motion, and then judged the location (“Near” or “Far”) of a target with a human body or an inanimate object as RF. In Exp. 2, participants were primed with point-light walkers of different speeds: a runner, a normal walker and a slow walker. In Exp. 3 they were primed with a point-light display depicting a human body sitting down on or standing up from a chair, with a human body RF either oriented or not oriented towards the target. Results showed a reduced judged distance when the human body RF was primed with a point-light walker (Exp. 1). Furthermore, we found an additional reduction of the judged distance when priming with a runner (Exp. 2). Finally, Exp. 3 showed that the human body RF has to be target oriented as a precondition for priming effects of the point-light walker.

Introduction

Space perception is not independent from the motor potential intrinsic to the body.

Previous research has shown that within reaching (or peripersonal) space the perceiver's ability to perform an intended action influences the perceived distance: we perceive an object as closer if we can interact with it (Witt et al., 2004, Witt et al., 2005). Also, when viewing a distance in the extrapersonal space (i.e. beyond the reaching space) we scale the distance according to the specific motor potential and action intention we have. When we plan to walk a distance, the perceived distance is scaled by how much walking effort would be required to traverse it, whereas viewing the same distance with the intention to throw a ball would evoke a scale based on throwing effort (Proffitt, 2006, Proffitt et al., 2003, Witt and Proffitt, 2008, Witt et al., 2004). In other words, specific anticipated action seem to shape our space perception following an optimal economic principle (see Proffitt, 2006).

Interestingly, it has been demonstrated that space is also socially scaled.

It seems that when observing another person around us, we automatically assume her visuo-spatial perspective (see Samson et al., 2010, Tversky and Hard, 2009) and we process the environment from literally “her body”.

In the reaching space, affordances are not only related to our own motor potentiality and intention but are also related to other people's motor potential. An object may afford a suitable motor act not only when it can be reached by our own hand but also when it is reachable by someone else (Cardellicchio et al., 2013, Costantini et al., 2010, Gallotti and Frith, 2013). In the same way, when we see a graspable object, the other's action opportunities modulate our perception: observing someone reaching an object with a tool, enlarges our own peripersonal space (Bloesch et al., 2012, Costantini et al., 2011). These evidences support that one's own actions and others' actions are represented in a functionally equivalent way (Sebanz, Knoblich, & Prinz, 2003).

While the research outlined above primarily relates to the peripersonal space, recent data suggest that similar effects are also present in the extrapersonal space. We recently exploited 3D virtual reproductions of a realistic environment and asked our participants to judge the location (“Near” or “Far”) of a target object located at progressively increasing or decreasing distances from an instructed reference frame (RF) (extrapersonal space categorization task through the psychophysical limit method) (see Fini et al., 2015a, Fini et al., 2015b, Fini et al., 2014). The data showed that the target object is judged as being closer to a human body than to another static object. Critically, this effect emerges only when the observer can attribute to the RF the intention or the potential to move (Fini, Brass, et al., 2015).

Further support for this comes from the observation that by manipulating the abstract belief that the wooden dummy is “living”, the effect is re-established. Specifically, a video extracted by the “Pinocchio” movie, in which Pinocchio behaves like a real human, has been used as a prime before the extrapersonal space categorization task. The more participants attributed to “Pinocchio” a biological nature, the more they perceived the distance as compressed with a wooden dummy adopted as RF (Fini et al., 2015a, Fini et al., 2015b). So, the perceptual or the abstract representation of the RF as a biological agent, able to walk the distance, appears to be crucial to determine the effect. Overall, these results suggest that, similar to what has been previously shown for the peripersonal space, the perception of a human body implicitly activates the representation of its motion potentiality or intention (i.e., the walking action to travel the distance) thus leading to a compression of space. In the present study, we aimed at testing this hypothesis more directly. We reasoned that if space compression in the presence of a human body is due to the spontaneous representation of motor intention and potentiality, the manipulation of the properties of this representation, should affect space perception accordingly.

With this idea in mind, we adopted a priming procedure in which biological motion sequences were presented before an extrapersonal categorization task. In three experiments, motion parameters of the priming stimulus (i.e., the motion type, the speed and the intention to walk towards the target) were systematically manipulated.

Point-light biological motion stimuli are animations composed solely of a dozen of points of light attached to the joints of a moving agent, allowing us to represent movement information in isolation (Johansson, 1973). It has been suggested that perceivers use their own implicit motor knowledge for the interpretation of biological motion stimuli (Casile and Giese, 2006, Grèzes and Decety, 2001). Saygin, Wilson, Hagler, Bates, and Sereno (2004) have shown the contribution of the motor system (inferior frontal and premotor areas) to biological motion perception and during action observation. These areas are crucial nodes, together with the posterior superior temporal sulcus (pSTS) and the inferior parietal lobe, of the mirror neuron system, or “action-perception system (APS)” (Buccino et al., 2004, Rizzolatti and Craighero, 2004, van Kemenade et al., 2012). This neural network is engaged during motor execution, simulation, verbalization and observation (Grèzes & Decety, 2001). The observation of a walking movement would induce a greater distance compression when a human vs. an object is the RF. We assume that, since a human is categorized as a biological agent, able to potentially perform a movement, this already present “biological” representation would be further strengthened by the observation of a walking movement.

In the first Experiment, we investigated whether the space compression is modulated by observing a walking movement. We first presented a point-light walker (Chang and Troje, 2008, Troje, 2002) or a scrambled motion, and then performed the same extrapersonal space categorization task described above, using as RFs a human body (Avatar RF) or an inanimate object (Object RF). In the second experiment, we tested whether the type of movement and the speed of the prime also affected the extrapersonal space judgment.

Thus, a runner point-light walker, a canonical point-light walker or a slow point-light walker were entered as primes in the task, adopting as RF a human body (Avatar RF).

Finally, in the last experiment we investigated whether the inferred intention to walk towards the target is necessary to obtain the space compression. We thus manipulated the potential intention of the prime (standing up or sitting down) and the orientation of the human body RF (towards the object or away from the object).