Apparent motion perception in lower limb amputees with phantom sensations: “obstacle shunning” and “obstacle tolerance”

Abstract

Phantom limbs are the phenomenal persistence of postural and sensorimotor features of an amputated limb. Although immaterial, their characteristics can be modulated by the presence of physical matter. For instance, the phantom may disappear when its phenomenal space is invaded by objects ("obstacle shunning"). Alternatively, "obstacle tolerance" occurs when the phantom is not limited by the law of impenetrability and co-exists with physical objects. Here we examined the link between this under-investigated aspect of phantom limbs and apparent motion perception. The illusion of apparent motion of human limbs involves the perception that a limb moves through or around an object, depending on the stimulus onset asynchrony (SOA) for the two images. Participants included 12 unilateral lower limb amputees matched for obstacle shunning (n = 6) and obstacle tolerance (n = 6) experiences, and 14 non-amputees. Using multilevel linear models, we replicated robust biases for short perceived trajectories for short SOA (moving through the object), and long trajectories (circumventing the object) for long SOAs in both groups. Importantly, however, amputees with obstacle shunning perceived leg stimuli to predominantly move through the object, whereas amputees with obstacle tolerance perceived leg stimuli to predominantly move around the object. That is, in people who experience obstacle shunning, apparent motion perception of lower limbs was not constrained to the laws of impenetrability (as the phantom disappears when invaded by objects), and legs can therefore move through physical objects. Amputees who experience obstacle tolerance, however, had stronger solidity constraints for lower limb apparent motion, perhaps because they must avoid co-location of the phantom with physical objects. Phantom limb experience does, therefore, appear to be modulated by intuitive physics, but not in the same way for everyone. This may have important implications for limb experience post-amputation (e.g., improving prosthesis embodiment when limb representation is constrained by the same limits as an intact limb).

Limb apparent motion perception: modification by tDCS, and clinically or experimentally altered bodily states

“…As in previous studies (Saetta et al, 2018;Shiffrar & Freyd, 1990;Stevens et al, 2000), we found that fast SOAs biased LAMP towards the visually guided perception of physically impossible movements. Slow SOAs instead induced the perception of physically possible movements, which are, based on previous findings (Orgs et al, 2016;Stevens et al, 2000), sensorimotorically guided.…”

“…According to the motor theory of LAMP, the perception of physically possible movements is grounded in the observer's sensorimotor representations acquired through motor experience (Funk et al, 2005;Orgs et al, 2016;Saetta et al, 2018;Stevens et al, 2000;Thornton, 1998).…”

“…Training to execute impossible movements with a phantom limb has shown to enhance the visually guided perception of physically impossible movements, regardless of the SOAs (Moseley & Brugger, 2009). Furthermore, individuals with an amputation who typically experience a phantom limb to fade away or bend back once it crosses hindering solid objects ("obstacle shunning") are more likely to perceive a limb to move straight through an object in the LAMP task (Saetta et al, 2018). Taken together, these findings highlight a tight interrelation between the motor capabilities of a (phantom) limb and the LAMP phenomenon.…”

“…Embodied cognition theory advocates an essential contribution of the human body’s structure, functionality, and sensorimotor state on perception, action, and cognition (Barsalou, 2010; Bechara & Damasio, 2005). In this framework, the repertoire of feasible movements and basic principles of physics, such as the implicit notion of mutual impenetrability of two solid entities (the law of impenetrability, Heinemann, 1945), may guide visual perception of body movements (Saetta et al, 2018). Since such movements are often partially occluded, accurate prediction about them is crucial (Kilner et al, 2007).…”

“…Fast SOAs typically induce the perception of a short movement trajectory that reflects physically impossible movements: In violation of the law of impenetrability, the limb is perceived as going trough a solid object, or, in violation of the biomechanical constraints, the limb is perceived to move along a short angle of rotation incompatible with the joint’s biomechanics. Therefore, this kind of perception has been supposed to be visually guided , i.e., relying on visual perception that is independent of the observer’s motor capabilities (Saetta et al, 2018; Vannuscorps & Caramazza, 2016; Shiffrar & Freyd, 1990).…”