Tool use imagery triggers tool incorporation in the body schema

Abstract

Tool-use has been shown to modify the way the brain represents the metrical characteristics of the effector controlling the tool. For example, the use of tools that elongate the physical length of the arm induces kinematic changes affecting selectively the transport component of subsequent free-hand movements. Although mental simulation of an action is known to involve -to a large extent- the same processes as those at play in overt motor execution, whether tool-use imagery can yield similar effects on the body representation remains unknown. Mentally simulated actions indeed elicit autonomic physiological responses and follow motor execution rules that are comparable to those associated with the correspondent overt performance. Therefore, here we investigated the effects of the mental simulation of actions performed with a tool on the body representation by studying subsequent free-hand movements. Subjects executed reach to grasp movements with their hand before and after an imagery task performed with either a tool elongating their arm length or, as a control, with their hand alone. Two main results were found: First, in agreement with previous studies, durations of imagined movements performed with the tool and the hand were similarly affected by task difficulty. Second, kinematics of free-hand movements was affected after tool-use imagery, but not hand-use imagery, in a way similar to that previously documented after actual tool-use. These findings constitute the first evidence that tool-use imagery is sufficient to affect the representation of the user's arm.

Keywords: action; body representation; kinematics; mental imagery; tool-use.

Figure 1

Schematic representation of the experimental set up from the subject's point of view (upper panel) and from above (lower panel). Subjects placed their right hand on a starting switch (purple) and the left hand on a response button (green). The target object was a cylinder, located 35 cm from the starting point. On its upper side were two colored dots indicating the location of the fingers (red for the thumb and yellow for the index); the line between these two dots constituted the opposition axis, which could be of three orientations: −22°, 0° and +22°. On the left an arrow indicated the height to which the object should be lifted.

Figure 2

Experimental design and procedure. The experiment took place during two consecutive days, each including three experimental sessions: Pre-imagery free-hand execution session (18 trials), Motor Imagery session (54 trials), Post-imagery free-hand execution session (18 trials).

Figure 3

Imagined movement duration. The graph displays the average imagined movement duration as a function of effector (hand and tool) and orientation of the Opposition Axis (−22°; 0°; +22°). Bar graphs illustrate mean values for each parameter ±1 s.e.m.

Figure 4

Tool-use imagery modifies free-hand movement kinematics. Bar graphs illustrate mean values for each parameter ±1 s.e.m. Asterisks denote significant differences from Newman-Keuls post-hoc.