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The sense of agency (SoA) is part of psychophysiological modules related to the self. Disturbed SoA is found in several clinical conditions, hence understanding the neural correlates of the SoA is useful for the diagnosis and determining the proper treatment strategies. Although there are several neuroimaging studies on SoA, it is desirable to translate the knowledge to more accessible and inexpensive EEG-based biomarkers for the sake of applicability. However, SoA has not been widely investigated using EEG. To address this issue, we designed an EEG experiment on healthy adults (n = 15) to determine the sensitivity of EEG on the SoA paradigm using hand movement with parametrically delayed visual feedback. We calculated the power spectral density over the traditional EEG frequency bands for ten delay conditions relative to no delay condition. Independent component analysis and equivalent current dipole modeling were applied to address artifact rejection, volume conduction, and source localization to determine the effect of interest. The results revealed that the alpha and low-beta EEG power increased in the parieto-occipital regions in proportion to the reduced SoA reported by the subjects. We conclude that the parieto-occipital alpha and low-beta EEG power reflect the sense of agency.

Objective: We investigated the effects of a computer-generated interactive visual feedback training program on the recovery from pusher syndrome in stroke patients. Design: Assessor-blinded, pilot randomized controlled study. Participants: A total of 12 stroke patients with pusher syndrome were randomly assigned to either the experimental group (N = 7, computer-generated interactive visual feedback training) or control group (N = 5, mirror visual feedback training). Main outcome measures: The scale for contraversive pushing for severity of pusher syndrome, the Berg Balance Scale for balance performance, and the Fugl-Meyer assessment scale for motor control were the outcome measures. Patients were assessed pre- and posttraining. Results: A comparison of pre- and posttraining assessment results revealed that both training programs led to the following significant changes: decreased severity of pusher syndrome scores (decreases of 4.0 ±1.1 and 1.4 ±1.0 in the experimental and control groups, respectively); improved balance scores (increases of 14.7 ±4.3 and 7.2 ±1.6 in the experimental and control groups, respectively); and higher scores for lower extremity motor control (increases of 8.4 ±2.2 and 5.6 ±3.3 in the experimental and control groups, respectively). Furthermore, the computer-generated interactive visual feedback training program produced significantly better outcomes in the improvement of pusher syndrome (p < 0.01) and balance (p < 0.05) compared with the mirror visual feedback training program. Conclusions: Although both training programs were beneficial, the computer-generated interactive visual feedback training program more effectively aided recovery from pusher syndrome compared with mirror visual feedback training.

Perception of the size of body parts, for instance the hand, has been shown to be distorted in healthy participants, with over- and underestimations of width and length, respectively. Illusory manipulations of body shape and size have highlighted the flexibility of the body representation and have also been found to update immediate perceptions of body size and surrounding objects. Here, we examined whether underlying misperceptions of hand width and length can be modified through exposure to illusory changes in hand size using a mirror visual feedback (MVF) paradigm. While questionnaire responses indicated subjective susceptibility to both magnified and minified manipulations, objective hand size estimates only showed significant differences following exposure to minifying mirrors. These variations might reflect differences in the way that stored representations are accessed or updated in response to size manipulations. Secondly, the findings further reinforce differences between subjective and objective outcomes of illusions on subsequent body perception.

Mirror visual feedback is used for reducing pain and visually distorting the size of the reflection may improve efficacy. The findings of studies investigating size distortion are inconsistent. The influence of the size of the reflected hand on embodiment of the mirror reflection is not known. The aim of this study was to compare the effect of magnifying and minifying mirror reflections of the hand on embodiment measured using an eight-item questionnaire and on proprioceptive drift. During the experiment, participants ( n  = 45) placed their right hand behind a mirror and their left hand in front of a mirror. Participants watched a normal-sized, a magnified and a minified reflection of the left hand while performing synchronised finger movements for 3 min (adaptive phase). Measurements of embodiment were taken before (pre) and after (post) synchronous movements of the fingers of both hands (embodiment adaptive phase). Results revealed larger proprioceptive drift post-adaptive phase ( p  = 0.001). Participants agreed more strongly with questionnaire items associated with location, ownership and agency of the reflection of the hand post-adaptive phase ( p  < 0.001) and when looking at the normal-sized reflection ( p  < 0.001). In conclusion, irrespective of size, watching a reflection of the hand while performing synchronised movements enhances the embodiment of the reflection of the hand. Magnifying and minifying the reflection of the hand has little effect on proprioceptive drift, but it weakens the subjective embodiment experience. Such factors need to be taken into account in future studies using this technique, particularly when assessing mirror visual feedback for pain management.

Direct touch finger interaction on a smartphone or a tablet is now ubiquitous. However, the latency inherent in digital computation produces an average feedback delay of ~ 75 ms between the action of the hand and its visible effect on digital content. This delay has been shown to affect users’ performance, but it is unclear whether users adapt to this delay and whether it influences skill learning. Previous work studied adaptation to feedback delays but only for longer delays, with hidden hand or indirect devices. This paper addresses adaptation to touchscreen delay in two empirical studies involving the tracking of a target moving along an elliptical path. Participants were trained for the task either at the minimal delay the system allows (~ 9 ms) or at a longer delay equivalent to commercialized touch devices latencies (75 ms). After 10 training sessions over a minimum of 2 weeks (Experiment 1), participants adapt to the delay. They also display long-term retention 7 weeks after the last training session. This adaptation generalizes to a similar tracking path (e.g., infinity symbol). We also observed generalization of learning from the longer delay to the minimal-delay condition (Experiment 2). The delay thus does not prevent the learning of tracking skill, which suggests that delay adaptation and tracking skill could be two separate components of learning.

Virtual error amplification (VEA) in visual feedback enhances attentive control over postural stability, although the neural mechanisms are still debated. This study investigated the distinct cortical control of unsteady stance in older adults using VEA through cross-frequency modulation of postural fluctuations and scalp EEG. Thirty-seven community-dwelling older adults (68.1 ± 3.6 years) maintained an upright stance on a stabilometer while receiving either VEA or real error feedback. Along with postural fluctuation dynamics, phase–amplitude coupling (PAC) and amplitude–amplitude coupling (AAC) were analyzed for postural fluctuations under 2 Hz and EEG sub-bands (theta, alpha, and beta). The results revealed a higher mean frequency of the postural fluctuation phase ( p  = .005) and a greater root mean square of the postural fluctuation amplitude ( p  = .003) with VEA compared to the control condition. VEA also reduced PAC between the postural fluctuation phase and beta-band EEG in the left frontal ( p  = .009), sensorimotor ( p  = .002), and occipital ( p  = .018) areas. Conversely, VEA increased the AAC of posture fluctuation amplitude and beta-band EEG in FP2 ( p  = .027). Neither theta nor alpha band PAC or AAC were affected by VEA. VEA optimizes postural strategies in older adults during stabilometer stance by enhancing visuospatial attentive control of postural responses and facilitating the transition of motor states against postural perturbations through a disinhibitory process. Incorporating VEA into virtual reality technology is advocated as a valuable strategy for optimizing therapeutic interventions in postural therapy, particularly to mitigate the risk of falls among older adults.

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 (CGv) and differences between CGv and center-of-pressure (CP) movements (CP–CGv), 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, VFBBW based on body-weight distribution, VFBCP based on CP displacements) associated with or without a cognitive task. Variances of CP–CGv and CGv 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 VFBCP, the dual tasks protocol infers a decreased contribution of deterministic activity in CP–CGv movements, inducing decreased variances, and alters the correction of the CGv over the longest Δt but nonetheless without changing CGv variances. Disturbing the subject’s attention during the VFBBW condition induces decreased CP–CGv and CGv 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–CGv movements, decreases whatever VFB condition along both the ML and AP axes, the effects on CGv 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–CGv movements, it appears beneficial for those of the CGv 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.

Temporal congruency between the efference copy of a motor command and the reafferent sensory feedback is crucial for identifying self-generated body movements. We investigated how temporal discrepancy between the efference copy and visual feedback affects the self-body movement recognition process. Subjects experienced active and passive hand movements under conditions of delayed visual feedback (118-352 ms) and judged whether observed hand movements were delayed with respect to the felt movement. The results showed that the discrimination threshold of visual feedback delay (50% detection rate) was not significantly different between active and passive movements. In contrast, the judgment probability curve was significantly steeper for active than passive movements. This indicates that the efference copy enhances the contrast between synchronous and asynchronous movements but does not narrow the time window in this process. We discuss processing of active and passive movements in relation to the senses of self-agency and self-ownership.

Forward walking (FW) is a common balance assessment tool. However, its sensitivity is limited by the ceiling effect. Reverse gait, such as backward walking (BW), has been reported to have more advantages than FW for balance assessment. Three factors related to postural instability (i.e., increased speeds, restricted arm swing, and reduced visual feedback) during BW were investigated to determine BW conditions that have the potential to predict falls. Three-dimensional analyses were used to analyze seven walking conditions. FW and BW at self-selected and fast speeds were analyzed to identify the effects of speed. Walking with normal arm swings, crossed arms, and abducted arms during BW was tested to determine the effects of arm position. BW with closed and open eyes was compared to investigate the effects of visual feedback. BW had a significantly shorter step length than FW at high speeds. When the arms were abducted, the stance phase (%) was significantly lower compared to when arms were crossed during BW. Moreover, BW with closed eyes revealed significantly higher mediolateral center of mass (COM) displacements than with open eyes. We observed that BW with fast speeds, a crossed arm position, and closed eyes has the potential to help assess fall risk because it requires higher balance ability through spatiotemporal and COM adjustment.

Chronic stroke often results in balance and gait impairments, significantly impacting patients’ quality of life. The purpose of this study was to investigate whether the combined effect of unstable surface balance training and visual feedback, based on proprioceptive neuromuscular stimulation in patients with chronic stroke, is effective in restoring balance and gait ability. A total of 39 chronic stroke patients were randomly assigned to a visual feedback combined with unstable surface balance training group (VUSBG), an unstable surface balance training group (USBG), or a conventional physical therapy group (CG). This study was conducted with the Trunk Impairment Scale, the Bug Balance Scale, the Timed Get Up and Go Test, and Gait Analysis. VUSBG and USBG improved function and gait (stride length and hip/knee flexion angle), but there was no significant difference in the CG group. Specific results showed that the stride length in the VUSBG improved by 25% (p < 0.05), and the hip/knee flexion angle improved by 18% (p < 0.05). The post-hoc analysis revealed that VUSBG had a greater impact on the hip/knee flexion angle relative to the other two groups, as well as gait velocity and stride length relative to CG. Visual feedback complex exercise based on the principle of proprioceptive neuromuscular facilitation could be an intervention strategy to improve gait speed, trunk stability, and mobility in chronic stroke patients.