Detecting deception in movement: the case of the side-step in rugby

Abstract

Although coordinated patterns of body movement can be used to communicate action intention, they can also be used to deceive. Often known as deceptive movements, these unpredictable patterns of body movement can give a competitive advantage to an attacker when trying to outwit a defender. In this particular study, we immersed novice and expert rugby players in an interactive virtual rugby environment to understand how the dynamics of deceptive body movement influence a defending player's decisions about how and when to act. When asked to judge final running direction, expert players who were found to tune into prospective tau-based information specified in the dynamics of 'honest' movement signals (Centre of Mass), performed significantly better than novices who tuned into the dynamics of 'deceptive' movement signals (upper trunk yaw and out-foot placement) (p<.001). These findings were further corroborated in a second experiment where players were able to move as if to intercept or 'tackle' the virtual attacker. An analysis of action responses showed that experts waited significantly longer before initiating movement (p<.001). By waiting longer and picking up more information that would inform about future running direction these experts made significantly fewer errors (p<.05). In this paper we not only present a mathematical model that describes how deception in body-based movement is detected, but we also show how perceptual expertise is manifested in action expertise. We conclude that being able to tune into the 'honest' information specifying true running action intention gives a strong competitive advantage.

Figure 1. Dynamics of deception.

The skeletal representations in the top two panels show how a deceptive (DM – left panel) and non-deceptive (NDM – right panel) movement unfold during the attacking player’s approach run. Each image represents a given moment in time during the unfolding movement and shows how the honest (blue) and deceptive (red) signals evolve during the movement. The graphs below show how during a deceptive movement the displacement of the honest signal (Centre of Mass (COM) displacement) is minimised whilst the displacement of the deceptive signals (i.e. Upper trunk yaw, Out Foot (OF) displacement and Head yaw) are all maximised. The non-deceptive movement (NDM) has a very different profile with all key body signals moving in a similar direction as the movement unfolds.

Figure 2. Example of the Tau COM displacement medio-lateral (M/L) for a DM to the right.

This figure shows an example of how Tau of closure of the COM M/L displacement gap is calculated to detect the point of reorientation. In this example, the closing motion-gap (top panel) is defined as the difference between the initial running direction and the maximal medio-lateral displacement (which corresponds to the reorientation peak symbolized by the vertical red dotted line). For the other angle parameters, such as upper trunk yaw, the same procedure is used but the gap is closed from the initial orientation before the DM (straight run ∼0°) to the point of maximal orientation (i.e. the reorientation peak for this parameter). The middle panel shows the rate of change of the COM displacement towards the point of reorientation and the bottom panel shows the tau of the COM displacement. On the bottom graph the critical values (CVs) are presented for both experts (Exp. CV) and novices (Nov. CV). These values represent the time when the information becomes most important. Note how it is much sooner for the experts than the novices.

Figure 3. Top panel. Real movement data recorded from attacker versus defender duels are used to form the basis of the movement of the animated virtual attacker.

Bottom panel: The two immersive tasks in a virtual rugby environment used a Head Mounted Display with a wireless motion tracker. This gave the participants a fully (360 degree visual field) immersive experience. In the Perception Only task, a gamepad was used to record the participant’s predictions about final running direction (by pressing left or right buttons) at the different occlusion times. In the Perception and Action experiment, participants wore a backpack containing the control unit for the HMD so that they not only had a fully immersive experience but that they were also free to move (up to 3 m to the left or right) to intercept the virtual attacker. Their movements were recorded using the Qualisys motion capture system.

Figure 4. Protocol for Determining Occlusions.

Footstep patterns for a Deceptive Movement (DM - grey - Movement towards the right, reorientation back towards the left) and a Non-Deceptive Movement (NDM - black - Movement towards the right with no reorientation). The first occlusion time (T0) is defined as the moment the attacker’s foot makes contact with the ground before the first directional change in the movement (towards the right in this instance).

Figure 5. Overview of correct responses for both Novice and Expert participants.

Mean percentage of correct responses for both novice (grey line) and expert (back line) groups when presented with deceptive (DM – solid line) and non-deceptive (NDM – dashed line) movements at the four different occlusion times (T0, T1, T2 and T3). The stick figures below represent the differences in static body configuration at each occlusion time.

Figure 6. Relationship between the displacement of the attacker and a novice defender.

This schematic diagram shows the reorientation point for the attacker and the distance gap that needs to be closed so that the defender can intercept the attacker (blue arrows). The interception zone shows where this took place. The panel on the right shows an early movement bias, that is a movement in the wrong direction caused by the deceptive movement of the attacker. The tau-coupling analysis looked at how the information embedded in the movement kinematics of the attacker from the point of reorientation to the interception zone (tau perception), influenced the way the defender moved to close the gap between them and the attacker (tau action).

Figure 7. Effects of expertise on movement initiation and displacement.

Four examples of how the virtual attacker’s movements (dark grey - DM-R (deceptive movement right), DM-L (deceptive movement left), NDM-L (non-deceptive movement left) and NDM-R (non-deceptive movement right)) influence the movements of an expert (purple) and a novice (yellow) defender. Displacements represent the lateral movement (cm) of the COM (centre of mass) over time (0 s corresponds to T0 in the Perception Only experiment – Figure 4). Note how the novice (yellow line) moves in the wrong direction and initiates his movements earlier than the expert in the DM conditions.