Processing of limb kinematics in the interpositus nucleus

Can EEG and MEG detect signals from the human cerebellum?

The cerebellum plays a key role in the regulation of motor learning, coordination and timing, and has been implicated in sensory and cognitive processes as well. However, our current knowledge of its electrophysiological mechanisms comes primarily from direct recordings in animals, as investigations into cerebellar function in humans have instead predominantly relied on lesion, haemodynamic and metabolic imaging studies. While the latter provide fundamental insights into the contribution of the cerebellum to various cerebellar-cortical pathways mediating behaviour, they remain limited in terms of temporal and spectral resolution. In principle, this shortcoming could be overcome by monitoring the cerebellum's electrophysiological signals. Non-invasive assessment of cerebellar electrophysiology in humans, however, is hampered by the limited spatial resolution of electroencephalography (EEG) and magnetoencephalography (MEG) in subcortical structures, i.e., deep sources. Furthermore, it has been argued that the anatomical configuration of the cerebellum leads to signal cancellation in MEG and EEG. Yet, claims that MEG and EEG are unable to detect cerebellar activity have been challenged by an increasing number of studies over the last decade. Here we address this controversy and survey reports in which electrophysiological signals were successfully recorded from the human cerebellum. We argue that the detection of cerebellum activity non-invasively with MEG and EEG is indeed possible and can be enhanced with appropriate methods, in particular using connectivity analysis in source space. We provide illustrative examples of cerebellar activity detected with MEG and EEG. Furthermore, we propose practical guidelines to optimize the detection of cerebellar activity with MEG and EEG. Finally, we discuss MEG and EEG signal contamination that may lead to localizing spurious sources in the cerebellum and suggest ways of handling such artefacts. This review is to be read as a perspective review that highlights that it is indeed possible to measure cerebellum with MEG and EEG and encourages MEG and EEG researchers to do so. Its added value beyond highlighting and encouraging is that it offers useful advice for researchers aspiring to investigate the cerebellum with MEG and EEG.

Sensory integration of a light touch reference in human standing balance

In upright stance, light touch of a space-stationary touch reference reduces spontaneous sway. Moving the reference evokes sway responses which exhibit non-linear behavior that has been attributed to sensory reweighting. Reweighting refers to a change in the relative contribution of sensory cues signaling body sway in space and light touch cues signaling finger position with respect to the body. Here we test the hypothesis that the sensory fusion process involves a transformation of light touch signals into the same reference frame as other sensory inputs encoding body sway in space, or vice versa. Eight subjects lightly gripped a robotic manipulandum which moved in a circular arc around the ankle joint. A pseudo-randomized motion sequence with broad spectral characteristics was applied at three amplitudes. The stimulus was presented at two different heights and therefore different radial distances, which were matched in terms of angular motion. However, the higher stimulus evoked a significantly larger sway response, indicating that the response was not matched to stimulus angular motion. Instead, the body sway response was strongly related to the horizontal translation of the manipulandum. The results suggest that light touch is integrated as the horizontal distance between body COM and the finger. The data were well explained by a model with one feedback loop minimizing changes in horizontal COM-finger distance. The model further includes a second feedback loop estimating the horizontal finger motion and correcting the first loop when the touch reference is moving. The second loop includes the predicted transformation of sensory signals into the same reference frame and a non-linear threshold element that reproduces the non-linear sway responses, thus providing a mechanism that can explain reweighting.

Control of voluntary and optogenetically perturbed locomotion by spike rate and timing of neurons of the mouse cerebellar nuclei

Neurons of the cerebellar nuclei (CbN), which generate cerebellar output, are inhibited by Purkinje cells. With extracellular recordings during voluntary locomotion in head-fixed mice, we tested how the rate and coherence of inhibition influence CbN cell firing and well-practiced movements. Firing rates of Purkinje and CbN cells were modulated systematically through the stride cycle (~200–300 ms). Optogenetically stimulating ChR2-expressing Purkinje cells with light steps or trains evoked either asynchronous or synchronous inhibition of CbN cells. Steps slowed CbN firing. Trains suppressed CbN cell firing less effectively, but consistently altered millisecond-scale spike timing. Steps or trains that perturbed stride-related modulation of CbN cell firing rates correlated well with irregularities of movement, suggesting that ongoing locomotion is sensitive to alterations in modulated CbN cell firing. Unperturbed locomotion continued more often during trains than steps, however, suggesting that stride-related modulation of CbN spiking is less readily disrupted by synchronous than asynchronous inhibition.

Attentive processes, blood lactate and CrossFit®

OBJECTIVES

To analyze the influences of blood lactate produced during a specific session of CrossFit® on intensity and selectivity of attention. The first was evaluated by measuring the reaction time and the second by analyzing divided attention with a dual task.

METHODS

Fifteen male professionals of CrossFit® volunteered in the study. The training session was the Workout Of the Day (WOD) called 15.5, marked as: 27-21-15-9 repetitions (without recovery) in term of calories measured by using a rowing ergometer (e.g. 27 rowed calories) and in term of barbell full squats (raising a weight of 43 kg for men and of 29.5 kg for women). Blood lactate, blood glucose, reaction time, execution time of a dual task, number of errors and number of omissions were measured at rest, at the conclusion of the session and 15 minutes after its end.

RESULTS

The levels of the blood lactate before the start of the session were considerably higher than those which normally occur at rest (<2 mmol /L), with a mean value of 4.5 mmol /l (± 1.99 SD). At the end of the workout session the blood lactate exhibited a significant increase, reaching a mean value of 13.8 mmol /l (± 1.18 SD) and then returning to values similar to the initial ones after 15 minutes. Blood glucose did not exhibit any statistically significant differences during the session. Reaction time, execution time, number of errors and number of omissions exhibited a significant worsening concomitantly with the increase in blood lactate.

CONCLUSION

Athletes practicing CrossFit®, with high levels of blood lactate even at rest, should consequently have attentional performances somewhat limited.

How the credit assignment problems in motor control could be solved after the cerebellum predicts increases in error

We present a cerebellar architecture with two main characteristics. The first one is that complex spikes respond to increases in sensory errors. The second one is that cerebellar modules associate particular contexts where errors have increased in the past with corrective commands that stop the increase in error. We analyze our architecture formally and computationally for the case of reaching in a 3D environment. In the case of motor control, we show that there are synergies of this architecture with the Equilibrium-Point hypothesis, leading to novel ways to solve the motor error and distal learning problems. In particular, the presence of desired equilibrium lengths for muscles provides a way to know when the error is increasing, and which corrections to apply. In the context of Threshold Control Theory and Perceptual Control Theory we show how to extend our model so it implements anticipative corrections in cascade control systems that span from muscle contractions to cognitive operations.

Visual contribution to human standing balance during support surface tilts

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Contributions of visual position and velocity cues to standing balance are analyzed.

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Both visual cues reduce sway responses to support surface tilt and sway variability.

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Model simulations are used for data interpretation and data reproduction.

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Visual cues improve disturbance estimates by reduction of estimation thresholds.

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Reduction of noise by visual cues appears to be an instrumental factor.

Visual position and velocity cues improve human standing balance, reducing sway responses to external disturbances and sway variability. Previous work suggested that human balancing is based on sensory estimates of external disturbances and their compensation using feedback mechanisms (Disturbance Estimation and Compensation, DEC model). This study investigates the visual effects on sway responses to pseudo-random support surface tilts, assuming that improvements result from lowering the velocity threshold in a tilt estimate and the position threshold in an estimate of the gravity disturbance. Center of mass (COM) sway was measured with four different tilt amplitudes, separating the effect of visual cues across the conditions ‘Eyes closed’ (no visual cues), ‘4 Hz stroboscopic illumination’ (visual position cues), and ‘continuous illumination’ (visual position and velocity cues). In a model based approach, parameters of disturbance estimators were identified. The model reproduced experimental results and showed a specific reduction of the position and velocity threshold when adding visual position and velocity cues, respectively. Sway variability was analyzed to explore a hypothesized relation between estimator thresholds and internal noise. Results suggest that adding the visual cues reduces the contribution of vestibular noise, thereby reducing sway variability and allowing for lower thresholds, which improves the disturbance compensation.

The activity in the contralateral primary motor cortex, dorsal premotor and supplementary motor area is modulated by performance gains

There is growing experimental evidence that the engagement of different brain areas in a given motor task may change with practice, although the specific brain activity patterns underlying different stages of learning, as defined by kinematic or dynamic performance indices, are not well understood. Here we studied the change in activation in motor areas during practice on sequences of handwriting-like trajectories, connecting four target points on a digitizing table “as rapidly and as accurately as possible” while lying inside an fMRI scanner. Analysis of the subjects' pooled kinematic and imaging data, acquired at the beginning, middle, and end of the training period, revealed no correlation between the amount of activation in the contralateral M1, PM (dorsal and ventral), supplementary motor area (SMA), preSMA, and Posterior Parietal Cortex (PPC) and the amount of practice per-se. Single trial analysis has revealed that the correlation between the amount of activation in the contralateral M1 and trial mean velocity was partially modulated by performance gains related effects, such as increased hand motion smoothness. Furthermore, it was found that the amount of activation in the contralateral preSMA increased when subjects shifted from generating straight point-to-point trajectories to their spatiotemporal concatenation into a smooth, curved trajectory. Altogether, our results indicate that the amount of activation in the contralateral M1, PMd, and preSMA during the learning of movement sequences is correlated with performance gains and that high level motion features (e.g., motion smoothness) may modulate, or even mask correlations between activity changes and low-level motion attributes (e.g., trial mean velocity).

Consensus paper: current views on the role of cerebellar interpositus nucleus in movement control and emotion

In the present paper, we examine the role of the cerebellar interpositus nucleus (IN) in motor and non-motor domains. Recent findings are considered, and we share the following conclusions: IN as part of the olivo-cortico-nuclear microcircuit is involved in providing powerful timing signals important in coordinating limb movements; IN could participate in the timing and performance of ongoing conditioned responses rather than the generation and/or initiation of such responses; IN is involved in the control of reflexive and voluntary movements in a task- and effector system-dependent fashion, including hand movements and associated upper limb adjustments, for quick effective actions; IN develops internal models for dynamic interactions of the motor system with the external environment for anticipatory control of movement; and IN plays a significant role in the modulation of autonomic and emotional functions.

Automatic online control of motor adjustments in reaching and grasping

Following the princeps investigations of Marc Jeannerod on action-perception, specifically, goal-directed movement, this review article addresses visual and non-visual processes involved in guiding the hand in reaching or grasping tasks. The contributions of different sources of correction of ongoing movements are considered; these include visual feedback of the hand, as well as the often-neglected but important spatial updating and sharpening of goal localization following gaze-saccade orientation. The existence of an automatic online process guiding limb trajectory toward its goal is highlighted by a series of princeps experiments of goal-directed pointing movements. We then review psychophysical, electrophysiological, neuroimaging and clinical studies that have explored the properties of these automatic corrective mechanisms and their neural bases, and established their generality. Finally, the functional significance of automatic corrective mechanisms-referred to as motor flexibility-and their potential use in rehabilitation are discussed.

Noradrenergic control of neuronal firing in cerebellar nuclei: modulation of GABA responses

The effects of noradrenaline (NA) on inhibitory responses to gamma aminobutyric acid (GABA) in neurones of the deep cerebellar nuclei were studied in vivo in rats, using extracellular single-unit recordings and microiontophoretic drug application. NA application altered GABA-evoked responses in 95 % of the neurones tested, but the effects differed between nuclei. Application of NA depressed GABA responses in the medial (MN) and posterior interpositus (PIN) nuclei, but enhanced GABA responses in the anterior interpositus nucleus (AIN). Comparable proportions of enhancing (57 %) and depressive (43 %) effects were found in the lateral nucleus (LN). The alpha2 noradrenergic receptor agonist clonidine mimicked the depressive effect of NA on GABA responses in MN and PIN and its enhancing effects in AIN and LN, while the alpha2 antagonist yohimbine partially blocked these effects. The beta-adrenergic agonist isoproterenol and antagonist timolol respectively induced and partially blocked enhancements of GABA responses in all nuclei except for LN, where isoproterenol had a weak depressive effect. It is concluded that NA modulates GABA responses by acting on both alpha2 and beta receptors. Activation of these receptors appears to be synergistic in the AIN and opposite in the remaining deep nuclei. These results support the hypothesis that the noradrenergic system participates in all the regulatory functions involving the cerebellum in a specific and differential manner, and suggest that any change in NA content, as commonly observed in ageing or stress, could influence cerebellar activity.

Timing of muscle response to a sudden leg perturbation: comparison between adolescents and adults with Down syndrome

Movement disturbances associated with Down syndrome reduce mechanical stability, worsening the execution of important tasks such as walking and upright standing. To compensate these deficits, persons with Down syndrome increase joint stability modulating the level of activation of single muscles or producing an agonist-antagonist co-activation. Such activations are also observed when a relaxed, extended leg is suddenly released and left to oscillate passively under the influence of gravity (Wartenberg test). In this case, the Rectus femoris of adults with Down syndrome displayed peaks of activation after the onset of the first leg flexion. With the aim to verify if these muscular reactions were acquired during the development time and to find evidences useful to give them a functional explanation, we used the Wartenberg test to compare the knee joint kinematics and the surface electromyography of the Rectus femoris and Biceps femoris caput longus between adolescents and adults with Down syndrome. During the first leg flexion, adolescents and adults showed single Rectus femoris activations while, a restricted number of participants exhibited agonist-antagonist co-activations. However, regardless the pattern of activation, adults initiated the muscle activity significantly later than adolescents. Although most of the mechanical parameters and the total movement variability were similar in the two groups, the onset of the Rectus femoris activation was well correlated with the time of the minimum acceleration variability. Thus, in adolescents the maximum mechanical stability occurred short after the onset of the leg fall, while adults reached their best joint stability late during the first flexion. These results suggest that between the adolescence and adulthood, persons with Down syndrome explore a temporal window to select an appropriate timing of muscle activation to overcome their inherent mechanical instability.

Dynamic changes in the cerebellar-interpositus/red-nucleus-motoneuron pathway during motor learning

Understanding the role played by the cerebellum in the genesis and control of learned motor responses requires a precise knowledge of interdependent relationships between kinetic neural commands and the performance (kinematics) of the acquired movements. The eyelid motor system is a useful model for studying how simple motor responses are generated and performed. Here, we recorded the activity of interpositus, red nucleus, and/or facial motor neurons during classical eyeblink conditioning, using a delay paradigm. Experiments were carried out in behaving cats, and in conscious wild-type and (Purkinje cell devoid) Lurcher mice. Kinetic variables were determined by recording the firing activities of identified neurons at the mentioned nuclei, whilst kinematic variables were selected from the electromyographic activity of the orbicularis oculi muscle and/or from eyelid position recorded during the conditioned-stimulus/unconditioned-stimulus interval. Whereas motoneurons encoded eyelid kinematics for acquired eyelid responses, interpositus, and red nucleus neurons did not directly encode eyelid performance, and the dynamic association between their neuronal activities was barely significant (from moderate to weak correlation, nonlinear coupling with high asymmetry, and neural firing activities that always lagged the beginning of the conditioned response). Nevertheless, interpositus and red nucleus neurons seem to play a modulating role in the dynamic control of this type of learned motor response, and present interesting adaptive properties in Lurcher mice. The analytical procedures proposed here could be very helpful in defining the functional state corresponding to each stage across the acquisition of new motor and cognitive abilities.

A biomechanical model for encoding joint dynamics: applications to transfemoral prosthesis control

This paper presents and tests a framework for encoding joint dynamics into energy states using kinematic and kinetic knee joint sensor data and demonstrates how to use this information to predict the future energy state (torque and velocity requirements) of the joint without a priori knowledge of the activity sequence. The intended application is for enhancing micro-controlled prosthetics by making use of the embedded sensory potential of artificial limbs and classical mechanical principles of a prosthetic joint to report instantaneous energy state and most probable next energy state. When applied to the knee during preferred and fast speed walking in 8 human subjects (66 preferred-speed trials and 50 fast-speed trials), it was found that joint energy states could be consistently sequenced (75% consensus) according to mechanical energy transference conditions and subsequences appeared to reflect the stability and energy dissipation requirements of the knee during gait. When simple constraints were applied to the energy transfer input conditions (their signs), simulations indicated that it was possible to predict the future energy state with an accuracy of >80% when 2% cycle in advance (∼20 ms) of the switch and >60% for 4% (∼40 ms) in advance. This study justifies future research to explore whether this encoding algorithm can be used to identify submodes of other human activity that are relevant to TFP control, such as chair and stair activities and their transitions from walking, as well as unexpected perturbations.

Evaluating the adaptive-filter model of the cerebellum

The adaptive-filter model of the cerebellar microcircuit is in widespread use, combining as it does an explanation of key microcircuit features with well-specified computational power. Here we consider two methods for its evaluation. One is to test its predictions concerning relations between cerebellar inputs and outputs. Where the relevant experimental data are available, e.g. for the floccular role in image stabilization, the predictions appear to be upheld. However, for the majority of cerebellar microzones these data have yet to be obtained. The second method is to test model predictions about details of the microcircuit. We focus on features apparently incompatible with the model, in particular non-linear patterns in Purkinje cell simple-spike firing. Analysis of these patterns suggests the following three conclusions. (i) It is important to establish whether they can be observed during task-related behaviour. (ii) Highly non-linear models based on these patterns are unlikely to be universal, because they would be incompatible with the (approximately) linear nature of floccular function. (iii) The control tasks for which these models are computationally suited need to be identified. At present, therefore, the adaptive filter remains a candidate model of at least some cerebellar microzones, and its evaluation suggests promising lines for future enquiry.