Jean Pailhous

Dopa-sensitive and Dopa-resistant gait parameters in Parkinson's disease

Quantitative analysis of gait was performed in 20 parkinsonians before and 1 h after the acute administration of L-Dopa in order to discriminate between the Dopa-sensitive and the Dopa-resistant kinematic gait parameters. The stride length and the kinematic parameters (swing velocity, peak velocity) related to the energy were Dopa-sensitive. The improvement of the bent forward posture by L-Dopa may explain the stride length increase. Temporal parameters (stride and swing duration, stride duration variability), related to rhythm, were Dopa-resistant. Experimental data argue for the importance of force control in maintaining the posture. The stride length variability, possibly related to the variability of force production shown to exist in parkinsonians was not significantly improved by L-Dopa. In Parkinsons disease different hypotheses might explain the inexorable aggravation of gait disorders along the course of the disease: (1) an advancing disorder of coordination between postural control and locomotion, (2) if some gait parameters like stride length and kinematic parameters are Dopa-sensitive, the others are Dopa-resistant and thus may involve other mechanisms than dopamine deficiency.

Stride variability in human gait: the effect of stride frequency and stride length

This study focused on spatial and temporal variability of the stride in human gait. We determined the role of stride frequency (F) and stride length (L) on those parameters. Eight healthy subjects walked on a treadmill using 25 different FL combinations (0.95<L<1.5 m, and 0.8<F<1.26 Hz). The results showed that spatial and temporal variabilities tend to increase in concert with respect to change in stride parameters. In addition, stride variability was found (1) to be minimal at F=1 Hz; and (2) to increase with smaller L. During additional trials, subjects walked freely at various speeds. Although it is generally hypothesized that freely chosen behaviors are optimal in terms of variability, our data show that this is not always the case in human gait.

Steady-state fluctuations of human walking

In steady-state walking, fluctuations in space-time behavior are observed for normal adult subjects. In the present study, the intrinsic fluctuations of gait have been analyzed when walking on a subject-driven treadmill (with adjustable inertial forces). Furthermore, these intrinsic fluctuations have been compared with those observed in natural overground locomotion which involves a real subjects displacement and thus an optical flow. Four adult subjects participated in both experimental sessions. It was found that the frequency and amplitude of the instantaneous fluctuations of leg movement were weak and of equal magnitude with or without optical flow. This was also the case for instantaneous fluctuations in displacement speed. Secondly, a low-frequency fluctuation in walking speed was observed when no optical flow information was available to the subject. This fluctuation results from the addition of a series of leg-movement fluctuations, whose values are all either positive or negative. As the optical flow provides information about the displacement speed, it allows the subject to avoid such addition, and thus plays a role in maintaining steady leg movement. Theoretical models linking space-time behavior of rhythmic movement with stiffness strongly suggest that the observed low-frequency fluctuations in speed result from fluctuations in stiffness.

Interaction between different sensory cues in the control of human gait

Abstract. This experiment investigates the interaction of different sensory cues in the control of propulsive forces in human gait which in turn allow the bodys forward progression to be regulated. The aim of this work was to determine how optic flow and leg-somatosensory feedback interact in this control. We therefore determined whether the responses to sinusoidal perturbations of optic flow were accentuated when leg-somatosensory feedback was modified by varying the support resistance. Subjects walked on a treadmill which was driven by their own locomotor activity (1) with a sinusoidal variation of optic flow velocity, (2) with a sinusoidal variation of support resistance which modified leg-somatosensory information and (3) with both visual and leg-somatosensory modification at different frequencies. The response of the subject was measured as changes in speed and propulsive power. The response to sinusoidal perturbations of optic flow was found to be increased and time delayed when visual perturbations are coupled with support perturbations in comparison with the response observed with visual perturbations only. This result shows the influence of leg-somatosensory feedback on the weighting of optic flow. Inversely, it was also found that the motor response to support perturbation was different when the flow was congruent (i.e., corresponding to the subjects virtual speed) and when it was not. This latter result shows the influence of optic flow on the weighting of leg-somatosensory feedback. The interaction between optic flow and leg-somatosensory feedback argues in favor of a multimodal sensory control of propulsive forces. This multimodal sensory control would be based on all the sensory feedback and all their mutual sensorial interaction. Therefore, the modification of one sensory input modifies not only this input but also the integration of the other inputs.

Spatiotemporal contrast sensitivity differs in normal aging and Parkinson's disease

We measured contrast sensitivity for static and laterally drifting vertical gratings in 12 young adults, 7 normal elderly adults, and 8 patients with Parkinsons disease (PD). We compared static and motion contrast sensitivity for spatial frequencies of 0.25,1, and 4 cycles per degree (cpd), and temporal frequencies of 1,3, and 9 Hz. Results show that normal aging leads to a reduction of motion sensitivity for the spatial frequency of 0.25 cpd. Compared with elderly controls, PD patients do not present specific abnormalities in this domain. However, for spatial frequencies of 1 and 4 cpd and temporal frequencies of 1 and 3 Hz, motion sensitivity is worse than static sensitivity in PD patients and not in elderly controls. These findings suggest a specific deficit of motion perception in PD, and possible dopaminergic involvement in the control of visuospatial behavior.

Dynamics of balancing space and time in memory: tau and kappa effects revisited.

In 3 experiments, the authors studied the organization of spatiotemporal information in memory. Stimuli consisted of configurations of dots, presented sequentially. The stimuli were either proportional, with interdot distances corresponding to interdot durations, or not proportional, with interdol distances not corresponding to interdot durations. After a learning phase, participants reproduced the spatial (Experiment 1), temporal (Experiment 2), or spatial and temporal (Experiment 3) characteristics of the target 60 times in succession. In the nonproportional conditions, effects of variable interdot durations or distances on the reproduction of, respectively, constant distances (tau effect) or durations (kappa effect) were observed, whereas no such effects were observed when variable distances or durations were to be produced. Tau and kappa effects influenced the accuracy but not the variability of responses. The results are discussed in light of the distinction between properties of the stabilized mental image and the process of stabilization.

Prior intention can locally tune inhibitory processes in the primary motor cortex: direct evidence from combined TMS-EEG

Human subjects are able to prepare cognitively to resist an involuntary movement evoked by a suprathreshold transcranial magnetic stimulation (TMS) applied over the primary motor cortex (M1) by anticipatory selective modulation of corticospinal excitability. Uncovering how the sensorimotor cortical network is involved in this process could reveal directly how a prior intention can tune the intrinsic dynamics of M1 before any peripheral intervention. Here, we used combined TMS‐EEG to study the cortical integrative processes that are engaged both in the preparation to react to TMS (Resist vs. Assist) and in the subsequent response to it. During the preparatory period, the contingent negative variation (CNV) amplitude was found to be smaller over central electrodes (FC1, C1, Cz) when preparing to resist compared with preparing to assist the evoked movement whereas α‐oscillation power was similar in the two conditions. Following TMS, the amplitude of the TMS evoked‐N100 component was higher in the Resist than in the Assist condition for some central electrodes (FCz, C1, Cz, CP1, CP3). Moreover, for six out of eight subjects, a single‐trial‐based analysis revealed a negative correlation between CNV amplitude and N100 amplitude. In conclusion, prior intention can tune the excitability of M1. When subjects prepare to resist a TMS‐evoked movement, the anticipatory processes cause a decreased cortical excitability by locally increasing the inhibitory processes.

Intentional on-line adaptation of stride length in human walking.

Abstract. The intentional control of stride length is a fundamental basis for the adaptation of the stride to environmental constraints (obstacle avoidance, for example). Controlling the propulsive forces during the stance and/or controlling the pendular movement of the oscillating leg constitute the two potential and non-exclusive mechanisms underlying intentional stride length modulation. The present experiment was conducted in order to determine if these two mechanisms contribute to voluntary length modulation and, if so, how they cooperate according to whether the subject has to lengthen or shorten a stride and how these mechanisms are implemented at the neuromuscular level. Subjects had to produce a temporarily modulated stride of the same length, but originating from two different initial steady-states: one from shorter stride length and one from longer stride length. We found that the shortening was essentially realized by a swing-duration decrease (an increased activity in the hip extensor – biceps femoris – during the swing of the ipsilaterally shortened stride stopped the pendular leg movement earlier). The lengthening was realized by two mechanisms: (1) an increase in the propulsive forces (via an increased activity of the ankle extensor muscles –soleus – and the hip extensors – biceps femoris – from the stance of the ipsilaterally modulated stride, which was prolonged during the following stance of the contralateral leg), and (2) an increase in swing duration on the ipsilateral leg (an increased activity in hip and ankle flexors –rectus femoris and tibialis anterior – maintained the ipsilateral leg in flexion during the lengthened swing so that the foot landed later). In this experiment, the subjects were faced with a spatial constraint of the same magnitude in the direction of stride lengthening and stride shortening. However, under these conditions, subjects used a different balance between swing control (that directly modifies the foot trajectory without affecting the trajectory of the head-arm-trunk system) and/or the control of propulsive forces (that indirectly influences foot trajectory by modifying the trajectory of the head-arm-trunk system). In the first case, this concerns a voluntary control of gesture produced by the legs and usually implicated in the locomotor pointing; in the second case, this concerns a voluntary control of propulsive forces.

Task-induced modulation of motor evoked potentials in upper-leg muscles during human gait: a TMS study

The aim of this study was to determine the relative involvement of the corticospinal (CS) pathway in voluntarily controlled walking compared to unconstrained walking. In the voluntarily controlled walking condition, subjects had to walk at the same speed as in unconstrained walking with a mechanical constraint, which is known to affect specifically the upper‐leg muscles. The motor cortex was activated transcranially using a focal magnetic stimulation coil in order to elicit motor evoked potentials (MEPs) in the rectus femoris (RF) and the biceps femoris (BF). The magnetic stimulation was delivered at the end of the swing (at 90% of the cycle duration), when the EMG backgrounds were similar in the two experimental conditions. For each subject in each condition, MEPs were measured for several stimulus intensities in order to establish the input/output (I/O) curve (MEPs amplitude plotted against stimulus strength). The results showed a significant increase in the MEPs amplitude of both the RF and BF in voluntarily controlled walking compared to unconstrained walking, which is the first evidence of cofacilitation of MEPs in antagonist upper‐leg muscles during human gait. In conclusion, although a lot of studies have emphasized a privileged input of the corticospinal pathway to the distal lower‐leg muscles, this study shows that, if a locomotory task requires fine control of the proximal upper‐leg muscles, a selective facilitation of MEPs is observed in these muscles.

A note on modulation of gait in man: Effects of constraining stride length and frequency

Abstract In this study, our aim was to investigate the effect of an imposed stride-length on walking speed and stride-frequency, and the effect of an imposed stride-frequency on walking speed and stride-length. These variations were determined in relation to the values obtained by analysing the subjects preferred pace. In the first case, the subject had to step on transversal stripes on the floor; in the second case, he had to synchronize his stride with a regular auditory signal. The results obtained with 8 subjects show that speed was the most variable factor. Variations in speed were correlated with variations in the imposed parameter: whenever the subject lengthened his stride or increased his stride-frequency, his walking speed increased proportionally. These results point to a relative independence between stride-length and stride-frequency, and a strong correlation between each of these parameters and speed.