The sense of agency shapes body schema and peripersonal space

Abstract

Body schema, a sensorimotor representation of the body used for planning and executing movements, is plastic because it extends by using a tool to reach far objects. Modifications of peripersonal space, i.e., a functional representation of reach space, usually co-occur with body schema changes. Here, we hypothesized that such plastic changes depend on the experience of controlling the course of events in space trough one's own actions, i.e., the sense of agency. In two experiments, body schema and peripersonal space were assessed before and after the participants' sense of agency over a virtual hand was manipulated. Body schema and peripersonal space enlarged or contracted depending on whether the virtual hand was presented in far space, or closer to the participants' body than the real hand. These findings suggest that body schema and peripersonal space are affected by the dynamic mapping between intentional body movements and expected consequences in space.

Figure 1

The present linear actuator is based on a motorized spool that manoeuvres the carriage by means of a cable and by an elastic element opposite to the cable. When the motor rotates clockwise, the cable is pulled by the spool and this causes the carriage to move in the direction of the motor and the elastic cord is placed into tension. When the rotation of the motor is reversed, the cable is slowly released from the spool, and the tensioned elastic cord provides the force for the carriage to move away from the motor. This type of linear actuator was used mainly for safety reasons: it is a passively safe device, since the only force produced directly towards the participant is given by the elastic cord, and not by the motor itself. In (a) a side view of the apparatus is shown; in (b) a reduced length version is shown for clarity; in (c) the same short version is shown, this time without the rails, in order to show the complete carriage and part of the cable and elastic cord which drive it; in (d) the same version is shown from under the top plane: a cross-section (shown in green) was executed in the illustration to show the passive pulley and the complete path of the cable and elastic cord.

Figure 2

(A) Leap motion training: we placed a leap motion controller near the palm of the right hand and a screen at approximately 140 cm from the participants’ sternum. Both the leap motion controller and screen were connected to a PC, so that leap motion controller was used to track in real time the participant’s hand movements in order to control a 3D virtual hand projected on the screen. (B) Screenshots of two tasks performed by participants during leap motion training (V2 Playground app).

Figure 3

Effects of leap motion training with the virtual hand presented farther forward than actual hand location. (A) Effects of leap motion training on forearm bisections. The graph shows the statistical comparison of mean distances from the elbow to the indicated forearm midpoint (in mm) as a function of Timing condition (Synchronous and Asynchronous leap motion training) and Session (Pre and Post leap motion training). Asterisk indicates a significant difference in forearm bisections between pre and post leap motion synchronous training. Error bars indicate standard error of the mean. (B) Effects of leap motion training on the Reaching distance estimation task. The graph shows the average reaching distance (in mm) as a function of the Timing condition (Synchronous and Asynchronous leap motion training) and Session (Pre and Post leap motion training). Asterisk indicates a significant difference in the reaching judgments between pre and post synchronous leap motion training. Error bars indicate standard error of the mean.

Figure 4

Subjective ratings of agency and ownership over the virtual far hand. The graph shows the average ownership, ownership control, agency, agency control ratings on a 7-point Likert scale as a function of the Timing condition (Synchronous and Asynchronous). Sense of agency was present in the Synchronous condition (>1). In contrast, sense of ownership was not positively affirmed. Asterisk indicates that agency category score (the mean of the three statements related to agency) was significantly greater than its respective control category in the synchronous condition, and greater in the synchronous than asynchronous condition. Error bars indicate standard error of the mean.

Figure 5

Effects of leap motion training with the virtual hand presented closer to the participants than actual hand location. (A) Effects of leap motion training on Forearm bisections. The graph shows the statistical comparison of mean distances from the elbow to the indicated forearm midpoint (in mm) as a function of the Timing condition (Synchronous and Asynchronous leap motion training) and Session (Pre and Post Leap motion training). Asterisk indicates a significant difference in forearm bisections between pre and post leap motion synchronous training. Error bars indicate standard error of the mean. (B) Effects of leap motion training on the Reaching distance estimation task. The graph shows the average reaching distance (in mm) as a function of the of the Timing condition (Synchronous and Asynchronous leap motion training) and Session (Pre and Post Leap motion training). Asterisk indicates a significant difference in the reaching judgments between pre and post synchronous leap motion training. Error bars indicate standard error of the mean.

Figure 6

Subjective ratings of agency and ownership over the virtual near hand. The graph shows the average ownership, ownership control, agency, agency control ratings on a 7-point Likert scale as a function of the Timing condition (Synchronous and Asynchronous). Sense of agency was present in the Synchronous condition (>1). In contrast, sense of ownership was not positively affirmed. Asterisk indicates that agency category score (the mean of the three statements related to agency) was significantly greater than its respective control category in the synchronous condition and grater in the synchronous than asynchronous condition. Error bars indicate standard error of the mean.