Grasp aperture corrections in reach-to-grasp movements do not reliably alter size perception

When grasping an object, the opening between the fingertips (grip aperture) scales with the size of the object. If an object changes in size, the grip aperture has to be corrected. In this study, it was investigated whether such corrections would influence the perceived size of objects. The grasping plan was manipulated with a preview of the object, after which participants initiated their reaching movement without vision. In a minority of the grasps, the object changed in size after the preview and participants had to adjust their grasping movement. Visual feedback was manipulated in two experiments. In experiment 1, vision was restored during reach and both visual and haptic information was available to correct the grasp and lift the object. In experiment 2, no visual information was provided during the movement and grasps could only be corrected using haptic information. Participants made reach-to-grasp movements towards two objects and compared these in size. Results showed that participants adjusted their grasp to a change in object size from preview to grasped object in both experiments. However, a change in object size did not bias the perception of object size or alter discrimination performance. In experiment 2, a small perceptual bias was found when objects changed from large to small. However, this bias was much smaller than the difference that could be discriminated and could not be considered meaningful. Therefore, it can be concluded that the planning and execution of reach-to-grasp movements do not reliably affect the perception of object size.

Costs of position, velocity, and force requirements in optimal control induce triphasic muscle activation during reaching movement

The nervous system activates a pair of agonist and antagonist muscles to determine the muscle activation pattern for a desired movement. Although there is a problem with redundancy, it is solved immediately, and movements are generated with characteristic muscle activation patterns in which antagonistic muscle pairs show alternate bursts with a triphasic shape. To investigate the requirements for deriving this pattern, this study simulated arm movement numerically by adopting a musculoskeletal arm model and an optimal control. The simulation reproduced the triphasic electromyogram (EMG) pattern observed in a reaching movement using a cost function that considered three terms: end-point position, velocity, and force required; the function minimised neural input. The first, second, and third bursts of muscle activity were generated by the cost terms of position, velocity, and force, respectively. Thus, we concluded that the costs of position, velocity, and force requirements in optimal control can induce triphasic EMG patterns. Therefore, we suggest that the nervous system may control the body by using an optimal control mechanism that adopts the costs of position, velocity, and force required; these costs serve to initiate, decelerate, and stabilise movement, respectively.

Reaching articular limits can negatively impact embodiment in virtual reality.

Providing Virtual Reality(VR) users with a 3D representation of their body complements the experience of immersion and presence in the virtual world with the experience of being physically located and more personally involved. A full-body avatar representation is known to induce a Sense of Embodiment (SoE) for this virtual body, which is associated with improvements in task performance, motivation and motor learning. Recent experimental research on embodiment provides useful guidelines, indicating the extent of discrepancy tolerated by users and, conversely, the limits and disruptive events that lead to a break in embodiment (BiE). Based on previous works on the limit of agency under movement distortion, this paper describes, studies and analyses the impact of a very common yet overlooked embodiment limitation linked to articular limits when performing a reaching movement. We demonstrate that perceiving the articular limit when fully extending the arm provides users with an additional internal proprioceptive feedback which, if not matched in the avatar's movement, leads to the disruptive realization of an incorrect posture mapping. This study complements previous works on self-contact and visuo-haptic conflicts and emphasizes the risk of disrupting the SoE when distorting users’ movements or using a poorly-calibrated avatar.

HAPPY FACIAL EXPRESSIONS IMPAIR INHIBITORY CONTROL WITH RESPECT TO FEARFUL FACIAL EXPRESSIONS BUT ONLY WHEN TASK-RELEVANT

The ability to generate appropriate responses, especially in social contexts, requires integrating emotional information with ongoing cognitive processes. In particular, inhibitory control plays a crucial role in social interactions, preventing the execution of impulsive and inappropriate actions. In this study, we focused on the impact of facial emotional expressions on inhibition. Research in this field has provided highly mixed results. In our view, a crucial factor explaining such inconsistencies is the task-relevance of the emotional content of the stimuli. To clarify this issue, we gave two versions of a Go/No-go task to healthy participants. In the emotional version, participants had to withhold a reaching movement at the presentation of emotional facial expressions (fearful or happy) and move when neutral faces were shown. The same pictures were displayed in the other version, but participants had to act according to the actor's gender, ignoring the emotional valence of the faces. We found that happy expressions impaired inhibitory control with respect to fearful expressions, but only when they were relevant to the participants' goal. We interpret these results as suggesting that facial emotions do not influence behavioral responses automatically. They would instead do so only when they are intrinsically germane for ongoing goals.

The costs of positon, velocity and force requirements in optimal control induce triphasic muscle activation during reaching movement

The nervous system activates a pair of agonist and antagonist muscles to determine the muscle activation pattern for a desired movement. Although there is a problem with redundancy, it is solved immediately, and movements are generated with characteristic muscle activation patterns in which antagonistic muscle pairs show alternate bursts with a triphasic shape. To investigate the requirements for deriving this pattern, this study simulated arm movement numerically by adopting a musculoskeletal arm model and an optimal control based on the minimization of neural input. The simulation reproduced the triphasic electromyogram (EMG) pattern observed in a reaching movement using a cost function that considered three terms: end-point position, velocity, and force required. The first, second and third bursts of muscle activity were generated by the cost terms of position, velocity and force, respectively. Thus we concluded that the costs of position, velocity and force requirements in optimal control can induce triphasic EMG patterns. Therefore we suggest that the nervous system may control the body by using an optimal control mechanism that adopts the costs of position, velocity and force required, which serve to initiate, decelerate and stabilize movement, respectively.