Predictability of Sinusoidal Stimulus Movement in Accommodative Demand Does Not Improve Accommodation

Previous research work suggests that predictable target motion such as sinusoidal movement can be anticipated by the visual system, thereby improving the accommodative response. The validity of predictable motion for studying human dynamic accommodation is sometimes put into question. The aim of this work was to assess from a practical perspective the effect of anticipation along with learning (and motivation, etc.) and fatigue (and boredom, loss of attention, etc.) on dynamic accommodation experiments. Specifically, changes in amplitude and temporal phase were estimated within and between trials as 9 adult observers were instructed to focus on a stimulus that oscillated in distance at specific temporal frequencies. On average, amplitude decreased whereas phase increased within trials. No evidence of anticipation or learning was observed either within or between trials. Fatigue consistently dominated anticipation and learning within the course of each trial. Even if the eye is equipped by a “prediction operator”, in practice, it is fatigue, and not anticipation or learning, that seems to muddle the results from dynamic accommodation experiments.

Superentrainment of muscle sympathetic nerve activity during sinusoidal galvanic vestibular stimulation

Sinusoidal galvanic vestibular stimulation (sGVS), delivered at frequencies ranging from 0.08 to 2.0 Hz, induces vestibular illusions of side-to-side motion and robust modulation of muscle sympathetic nerve activity (MSNA) to the lower legs. We have previously documented, in seated subjects, de novo synthesis of bursts of MSNA that are temporally locked to the sinusoidal stimulus rather than to the cardiac-related rhythm. Here we tested the hypothesis that this vestibular entrainment of MSNA is higher in the upright than in the supine position. MSNA was recorded from the common peroneal nerve in 10 subjects lying on a tilt table. Bipolar binaural sGVS (±2 mA, 200 cycles) was applied to the mastoid processes at 0.2, 0.8, and 1.4 Hz in the supine and upright (75°) positions. In four subjects, “superentrainment” of MSNA occurred during sGVS, with strong bursts locked to one phase of the sinusoidal stimulus. This occurred more prominently in the upright position. On average, cross-correlation analysis revealed comparable vestibular modulation of MSNA in both positions at 0.2 Hz (84.9 ± 3.6% and 78.7 ± 5.7%), 0.8 Hz (77.4 ± 3.9% and 74.4 ± 8.9%), and 1.4 Hz (69.8 ± 4.6% and 80.2 ± 7.4%). However, in the supine position there was a significant linear fall in the magnitude of vestibular modulation with increasing frequency, whereas this was not present in the upright position. We conclude that vestibular contributions to the control of blood pressure are higher in the upright position.

Analysis of Human Standing Balance by Largest Lyapunov Exponent

The purpose of this research is to analyse the relationship between nonlinear dynamic character and individuals’ standing balance by the largest Lyapunov exponent, which is regarded as a metric for assessing standing balance. According to previous study, the largest Lyapunov exponent from centre of pressure time series could not well quantify the human balance ability. In this research, two improvements were made. Firstly, an external stimulus was applied to feet in the form of continuous horizontal sinusoidal motion by a moving platform. Secondly, a multiaccelerometer subsystem was adopted. Twenty healthy volunteers participated in this experiment. A new metric, coordinated largest Lyapunov exponent was proposed, which reflected the relationship of body segments by integrating multidimensional largest Lyapunov exponent values. By using this metric in actual standing performance under sinusoidal stimulus, an obvious relationship between the new metric and the actual balance ability was found in the majority of the subjects. These results show that the sinusoidal stimulus can make human balance characteristics more obvious, which is beneficial to assess balance, and balance is determined by the ability of coordinating all body segments.