Attentional cost in additional visual feedback protocols in healthy young subjects.

Additional visual feedback (VFB) is a technique allowing improved postural stability in young healthy individuals despite an increased muscular activity, the two trends being assessed through center-of-gravity (CG) and differences between CG and center-of-pressure (CP) movements (CP-CG), respectively. These two opposing effects are likely explained by the respective contribution of automatic and voluntary controls and in turn the neural circuits involved. To specify these specific contributions, a dual-task protocol was set up, consisting in adding to VFB a navigation task performed at the maximum cognitive capacities of the subjects who were evaluated beforehand. Overall, the protocol comprises six conditions: three visual tasks (eyes open without VFB, VFB based on body-weight distribution, VFB based on CP displacements) associated with or without a cognitive task. Variances of CP-CG and CG movements, along the mediolateral (ML) and anteroposterior (AP) axes, and parameters from fractional Brownian motion modeling (transition point coordinates and scaling regimes to assess the level of deterministic or stochastic activity) were used to assess the postural behaviors. The results show that during VFB, the dual tasks protocol infers a decreased contribution of deterministic activity in CP-CG movements, inducing decreased variances, and alters the correction of the CG over the longest Δt but nonetheless without changing CG variances. Disturbing the subject's attention during the VFB condition induces decreased CP-CG and CG movements along the ML and AP axes, respectively. These data demonstrate the high level of attention induced by VFB protocols. If the tonic postural activity, expressed through CP-CG movements, decreases whatever VFB condition along both the ML and AP axes, the effects on CG movement appear to be mostly related to the additional information (BW or CP) provided. Overall, if too much voluntary control in upright stance maintenance is detrimental for the magnitudes of the CP-CG movements, it appears beneficial for those of the CG movements. By emphasizing the role of automatic and voluntary controls in VFB protocols, these insights document the neural circuits involved in such protocols and specify their conditions of use.