Postural tracking of visual motion cues improves perception–action coupling in aging, yet the nature of the visual cues to be tracked is critical for the efficacy of such a paradigm. We investigated how well healthy older (72.45 ± 4.72 years) and young (22.98 ± 2.9 years) adults can follow with their gaze and posture horizontally moving visual target cues of different degree of complexity. Participants tracked continuously for 120 s the motion of a visual target (dot) that oscillated in three different patterns: a simple periodic (simulated by a sine), a more complex (simulated by the Lorenz attractor that is deterministic displaying mathematical chaos) and an ultra-complex random (simulated by surrogating the Lorenz attractor) pattern. The degree of coupling between performance (posture and gaze) and the target motion was quantified in the spectral coherence, gain, phase and cross-approximate entropy (cross-ApEn) between signals. Sway–target coherence decreased as a function of target complexity and was lower for the older compared to the young participants when tracking the chaotic target. On the other hand, gaze–target coherence was not affected by either target complexity or age. Yet, a lower cross-ApEn value when tracking the chaotic stimulus motion revealed a more synchronous gaze–target relationship for both age groups. Results suggest limitations in online visuo-motor processing of complex motion cues and a less efficient exploitation of the body sway dynamics with age. Complex visual motion cues may provide a suitable training stimulus to improve visuo-motor integration and restore sway variability in older adults.

Background We have previously shown that wearable technology and machine learning techniques can accurately discriminate between progressive supranuclear palsy (PSP), Parkinson's disease, and healthy controls. To date these techniques have not been applied in longitudinal studies of disease progression in PSP. Objectives We aimed to establish whether data collected by a body‐worn inertial measurement unit (IMU) network could predict clinical rating scale scores in PSP and whether it could be used to track disease progression. Methods We studied gait and postural stability in 17 participants with PSP over five visits at 3‐month intervals. Participants performed a 2‐minute walk and an assessment of postural stability by standing for 30 seconds with their eyes closed, while wearing an array of six IMUs. Results Thirty‐two gait and posture features were identified, which progressed significantly with time. A simple linear regression model incorporating the three features with the clearest progression pattern was able to detect statistically significant progression 3 months in advance of the clinical scores. A more complex linear regression and a random forest approach did not improve on this. Conclusions The reduced variability of the models, in comparison to clinical rating scales, allows a significant change in disease status from baseline to be observed at an earlier stage. The current study sheds light on the individual features that are important in tracking disease progression. © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society

Previously we have demonstrated that the effect of aging on posture and gaze active tracking of a visual target moving in the horizontal direction is dependent on target’s complexity. In this study, we asked whether a similar phenomenon is present when tracking a visual target moving with varying complexity in the vertical direction. Ten young (22.98 ± 2.9 years) and 10 older adults (72.45 ± 4.72 years) tracked for 120 s, a visual target moving vertically by shifting their bodyweight in the anterior-posterior direction. Three target motions were tested: a simple periodic (sine wave), a more complex (Lorenz attractor) and an ultra-complex random (Surrogated Lorenz attractor) pattern. Cross-spectral analysis revealed lower sway-target coherence as a function of age, regardless of target motion’s complexity. This age effect was significant for the sway-target gain but not for the phase index. Gaze-target analysis revealed age related differences only when tracking the more complex targets. Regardless of age, tracking of the complex target was associated with lower cross Approximate Entropy. It is concluded that tracking of visual targets oscillating in the vertical direction reveals age related constraints that are independent of visual motion’s complexity. These constraints are evident in the spatial and not temporal aspects of visuo-motor coupling, which suggests the presence of neuromuscular deficiencies in controlling visually guided postural sway in the anterior-posterior direction.

Visually guided weight shifting is widely employed in balance rehabilitation, but the underlying visuo-motor integration process leading to balance improvement is still unclear. In this study, we investigated the role of center of pressure (CoP) feedback on the entrainment of active voluntary sway to a moving visual target and on sway’s dynamic stability as a function of target predictability. Fifteen young and healthy adult volunteers (height 175 ± 7 cm, body mass 69 ± 12 kg, age 32 ± 5 years) tracked a vertically moving visual target by shifting their body weight antero-posteriorly under two target motion and feedback conditions, namely, predictable and less predictable target motion, with or without visual CoP feedback. Results revealed lower coherence, less gain, and longer phase lag when tracking the less predictable compared to the predictable target motion. Feedback did not affect CoP-target coherence, but feedback removal resulted in greater target overshooting and a shorter phase lag when tracking the less predictable target. These adaptations did not affect the dynamic stability of voluntary sway. It was concluded that CoP feedback improves spatial perception at the cost of time delays, particularly when tracking a less predictable moving target.

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