Contraction of body representation induced by proprioceptive conflict

Our body is not only an extended object in external space, but also the basis of our sense of self. Proprioceptive signals from muscle spindle organs, specifying body position, play a key role in this unique dual quality of body representation, as they define a ‘here’ or set of locations, where ‘I’ am located [1]. Position information from muscle spindles can be manipulated by vibrating the muscle tendon, generating illusions of position and movement [2]. For example, biceps vibration generates illusions of elbow extension, while triceps vibration generates illusions of flexion. Here we report that proprioceptive conflict induced by simultaneous vibration of antagonistic biceps and triceps muscle tendons alters representation of the body in a way qualitatively different from single vibrations. Rather than relocation or movement, this incoherent conflict of location produces perceived telescoping of the arm towards the elbow. Loss of coherent information about body position in space seems to produce contraction of the body representation itself. Our result suggests that basic sensory signals about body posture also play an essential role in representing the self as an extended object in space. In Experiment 1, we applied 100 Hz vibration to participants’ immobilised right forearm. In the ‘biceps alone’ condition, one vibrator stimulated the biceps tendon, proximal to the elbow. In the ‘dual tendon’ condition, identical vibration was applied to both biceps and triceps tendons simultaneously. In the ‘dual off Correspondence tendon’ condition, vibrators were placed on the dorsal and volar forearm, distal to the elbow. During vibration, participants made two types of judgment about the stimulated right arm. To ensure that tendon vibration induced proprioceptive illusions, we asked participants to match the position of their vibrated right arm with their left [2]. To investigate changes in perceived body extension in space, we asked participants to point with their left index finger to either their right index finger, or an elastic ring around the right forearm. A vertical Plexiglas surface prevented the arms touching (Figure 1). When participants matched elbow angles, biceps vibration alone produced expected illusions of elbow extension: the right arm was perceived as lower than it was (Figure 1). Crucially, we also found no matching bias in the dual tendon condition, consistent with previous findings showing no net illusion of movement following vibration of antagonistic muscle tendons [3]. Simultaneous vibration of antagonistic muscle tendons produced equal and opposite effects, which were not interpreted as overall proprioception biases in joint angle. Simultaneous vibration of both muscle tendons did, however, produce significant biases in pointing to body locations (Figure 1), with responses becoming more proximal. This telescoping effect was found both for pointing to the right fingertip, and to the elastic ring