Estimation of muscle spindle information rate by pattern matching and the effect of gamma system activity on parallel spindles

Improved proprioceptive function by application of subsensory electrical noise: Effects of aging and task-demand

The application of subsensory noise stimulation over the lower limbs has been shown to improve proprioception and postural control under certain conditions. Whereas the effect specificity seems to depend on several factors, studies are still needed to determine the appropriate method for training and rehabilitation purposes. In the current study, we investigated whether the application of subsensory electrical noise over the legs improves proprioceptive function in young and older adults. We aimed to provide evidence that stronger and age-related differential effects occur in more demanding tasks. Proprioceptive function was initially assessed by testing the detection of passive ankle movement (kinesthetic perception) in twenty-eight subjects (14 young and 14 older adults). Thereafter, postural control was assessed during tasks with different sensory challenges: i) by removing visual information (eyes closed) and; ii) by moving the visual scene (moving room paradigm). Tests performed with the application of electrical noise stimulation were compared to those performed without noise. The results showed that electrical noise applied over the legs led to a reduction in the response time to kinesthetic perception in both young and older adults. On the other hand, the magnitude of postural sway was reduced by noise stimulation only during a more challenging task, namely, when the optical flow was changing in an unpredictable (nonperiodic) manner. No differential effects of stimulation between groups were observed. These findings suggest that the relevance of proprioceptive inputs in tasks with different challenges, but not the subjects' age, is a determining factor for sensorimotor improvements due to electrical noise stimulation.

Noise-enhanced kinaesthesia: a psychophysical and microneurographic study

We first explored whether the ability of subjects to detect the direction of slow ramp imposed movements may be improved by the application of mechanical noise to muscle tendons. Movements were plantar/dorsal flexion of the ankle at 0.04°/s, and the amplitude was just sub-threshold for each subject. A white noise signal (random vibration), low-pass filtered to 100 Hz and distributed uniformly in amplitude, was applied to both the extensor and the flexor ankle muscle tendons with four different mean amplitudes (20, 30, 100, 280 μm). The population of subjects was observed to exhibit clear stochastic-type behaviour: their ability to determine the direction of sub-threshold movements significantly increased when the two lower levels of noise were added and subsequently decreased when the noise magnitude was enhanced. Second, using microneurography, we explored the response of 9 primary muscle spindle afferents and 8 cutaneous afferents to the same imposed movements with and without noise application. While these conditions of ankle mobilisation were too small to induce a response in most of the recorded afferents, two muscle afferents exhibited responses that were characteristic of aperiodic stochastic resonance behaviour: the unit movement response was either triggered or improved by the application of an optimal level of noise. All cutaneous afferents were unresponsive to the imposed movements with or without noise application. We conclude that ankle movement sense can be significantly improved by adding an optimal level of mechanical noise to ankle muscle tendons and discuss the optimisation of the response of movement-encoding receptors that may account for this improvement. The application of a mechanical noise on ankle muscle tendons may constitute a means of improving postural stability in subjects with sensory deficits.

Computational motor control: feedback and accuracy

Speed/accuracy trade-off is a ubiquitous phenomenon in motor behaviour, which has been ascribed to the presence of signal-dependent noise (SDN) in motor commands. Although this explanation can provide a quantitative account of many aspects of motor variability, including Fitts' law, the fact that this law is frequently violated, e.g. during the acquisition of new motor skills, remains unexplained. Here, we describe a principled approach to the influence of noise on motor behaviour, in which motor variability results from the interplay between sensory and motor execution noises in an optimal feedback-controlled system. In this framework, we first show that Fitts' law arises due to signal-dependent motor noise (SDN(m)) when sensory (proprioceptive) noise is low, e.g. under visual feedback. Then we show that the terminal variability of non-visually guided movement can be explained by the presence of signal-dependent proprioceptive noise. Finally, we show that movement accuracy can be controlled by opposite changes in signal-dependent sensory (SDN(s)) and SDN(m), a phenomenon that could be ascribed to muscular co-contraction. As the model also explains kinematics, kinetics, muscular and neural characteristics of reaching movements, it provides a unified framework to address motor variability.

Changes in human muscle spindle sensitivity during a proprioceptive attention task

The aim of the present study was to test whether fusimotor control of human muscle spindle sensitivity changed when attention was selectively directed to the recognition of an imposed two-dimensional movement in the form of a written symbol. The unitary activities of 32 muscle spindle afferents (26 Ia, 6 II) were recorded by microneurography at the level of the common peroneal nerve. The patterns of firing rate in response to passive movements of the ankle, forming different letters or numbers, were compared in two conditions: control and recognition. No visual cues were given in either condition, but subjects had to recognize and name the character in one condition compared with not paying attention in the control condition. The results showed that 58% of the tested Ia afferents presented modified responses to movements when these had to be recognized. Changes in Ia afferent responses included decreased depth of modulation, increased variability of discharge, and changes in spontaneous activity. Not all changes were evident in the same afferent. Furthermore, the percentage of correctly recognized movements amounted to 63% when changes were observed, but it was only 48% when the primary ending sensitivity was unaltered. The responses of group II afferents were only weakly changed or unchanged. It is suggested that the altered muscle spindle sensitivity is because of selective changes in fusimotor control, the consequence of which might be to feed the brain movement trajectory information that is more accurate.