Faster but Less Careful Prehension in Presence of High, Rather than Low, Social Status Attendees

Abstract

Ample evidence attests that social intention, elicited through gestures explicitly signaling a request of communicative intention, affects the patterning of hand movement kinematics. The current study goes beyond the effect of social intention and addresses whether the same action of reaching to grasp an object for placing it in an end target position within or without a monitoring attendee's peripersonal space, can be moulded by pure social factors in general, and by social facilitation in particular. A motion tracking system (Optotrak Certus) was used to record motor acts. We carefully avoided the usage of communicative intention by keeping constant both the visual information and the positional uncertainty of the end target position, while we systematically varied the social status of the attendee (a high, or a low social status) in separated blocks. Only thirty acts performed in the presence of a different social status attendee, revealed a significant change of kinematic parameterization of hand movement, independently of the attendee's distance. The amplitude of peak velocity reached by the hand during the reach-to-grasp and the lift-to-place phase of the movement was larger in the high rather than in the low social status condition. By contrast, the deceleration time of the reach-to-grasp phase and the maximum grasp aperture was smaller in the high rather than in the low social status condition. These results indicated that the hand movement was faster but less carefully shaped in presence of a high, but not of a low social status attendee. This kinematic patterning suggests that being monitored by a high rather than a low social status attendee might lead participants to experience evaluation apprehension that informs the control of motor execution. Motor execution would rely more on feedforward motor control in the presence of a high social status human attendee, vs. feedback motor control, in the presence of a low social status attendee.

Fig 1. Experimental setting.

A schematic of the bird's-eye view of experimental setting, with superposed the average trajectories (for the high and low social status conditions and the familiarization block), and the minimal path solution (dashed line) composed by connecting two straight segments both orthogonal to the working surface: one spanning from the IRED of the wrist in starting position to the IRED of the wrist in initial position, the other spanning from the IRED of the object in initial position to the IRED of the object in end position. The initial and the end target position as well as the respective distances from the starting agent's hand position are illustrated. The attendee is shown in the two tested conditions of distance (close and far) together with the distance of the far condition from the workspace (i.e. 1000 mm), and the corresponding peripersonal spaces (dotted semicircles).

Fig 2. Relevant kinematic profiles for the hand movements performed by the agent in our task.

The colour codes for the block (legend): high (red) social status, low (green) social status human and familiarization (blue). In A the average wrist velocity profile: shaded colored regions represent ± 1 standard error of the mean. The overall movement is subdivided into two phases: a reach-to-grasp phase from the time elapsed when the wrist marker exceeded the threshold velocity after the go-signal (t0) and the time the wrist marker dropped below the threshold velocity (t1); a lift-to-place phase from t1 to the time the object marker dropped below the threshold velocity (t3). The lift-to-place phase of the movement included a stop period occurring after contact of fingers with the object with the wrist IRED remaining below threshold velocity for a certain dwell time until t2. The icons above panel A provide a schematic representation of the shaping of the hand relative to the target object in the different phases of the hand movement ordered as follows: start position → reach-to-grasp phase with the wrist IRED reaching peak velocity (peak wrist velocity) → stop period → lift-to-place phase with the object IRED reaching peak velocity (peak lift velocity). Grey arrows represent velocity vectors as registered by the IREDs in real time for the encoding of the index, thumb, wrist and object 3D position over time, during the execution of the movement. Panel in B depicts the profile for the grip aperture, as occurring during the reach-to-grasp phase of the movement: shaded coloured regions represent ± 1 standard error of the mean. The maximum positive value along this curve corresponds to the MGA. In both panels A and B, the blue curve is a reference for evaluating the biasing effect of social status (red and green curves) relative to a condition with a lower level of familiarization with the motor task. In particular, the motor performance during the familiarization phase (blue curve) which relies on a significant degree on online feedback control (being the agent poorly familiarized with the task), differs to a greater extent from the motor performance in presence of the high (which relies on a significant degree on planning—red curves), than the low (as relying less on planning—green curves) social attendee.

Fig 3. Kinematic parameters of prehensile movements are biased by the social status of the attendee.

In A the average temporal parameters ± SEM, associated with the relevant events characterizing the motor act performed by the agent in our task are represented as staked segment for the two tested social status conditions (red segment for high social, green segment for low social) and for the familiarization phase (blue segment). The length of each segment is proportional to the time from the go-signal (ms). Each point along the segment represents a relevant kinematic event (see the topmost labels for the ordering and type of event). Each segment is subdivided by the points into six successive temporal intervals each representing the temporal duration of a relevant part of the motor act. A high social status attendee evoked the longest planning of the movement (longest first temporal interval), followed by the shortest execution of the act (all intervals successive to the first are considerably shorter for the high social status condition, relative to the low social status condition, and even more to the familiarization phase). (B, C,) Average spatio-temporal kinematic parameters ± SEM of the reach (B, amplitude of peak wrist velocity), and lifting (C, amplitude of peak object velocity) components of the movement, and a schematic view of the corresponding measure associated with each parameter (topmost insets). The panel in D shows the average spatial kinematic parameters ± SEM of the grasp (maximum grip aperture, MGA). In B, C and D horizontal blue lines represent the average values on the familiarization phase, ± SEM. These values are a reference for evaluating the biasing effects of social status relative to a condition with a lower level of familiarization with the motor task.