Martin Schubert

Visual influence on human locomotion: Modulation to changes in optic flow

Abstract The effect of an optic flow pattern on human locomotion was studied in subjects walking on a self-driven treadmill. During walking an optic flow pattern was presented, which gave subjects the illusion of walking in a tunnel. Visual stimulation was achieved by a closed-loop system in which optic flow and treadmill velocity were automatically adjusted to the intended walking velocity (WV). Subjects were instructed to keep their WV constant. The presented optic flow velocity was sinusoidally varied relative to the WV with a cycle period of 2 min. The independent variable was the relative optic flow (rOF), ranging from −1, i.e., forward flow of equal velocity as the WV, and 3, i.e., backward flow 3 times faster than WV. All subjects were affected by rOF in a similar way. The results showed, firstly, an increase in stride-cycle variability that suggests a larger instability of the walking pattern than in treadmill walking without optic flow; and, secondly, a significant modulating effect of rOF on the self-chosen WV. Backward flow resulted in a decrease, whereas forward flow induced an increase of WV. Within the analyzed range, a linear relationship was found between rOF and WV. Thirdly, WV-related modulations in stride length (SL) and stride frequency (SF) were different from normal walking: whereas in the latter a change in WV is characterized by a stable linear relationship between SL and SF (i.e., an approximately constant SL to SF ratio), optic flow-induced changes in WV are closely related to a modulation of SL (i.e., a change of SL-SF ratio). Fourthly, this effect of rOF diminished by about 45% over the entire walking distance of 800 m. The results suggest that the adjustment of WV is the result of a summation of visual and leg-proprioceptive velocity informations. Visual information about ego-motion leads to an unintentional modulation of WV by affecting specifically the relationship between SL and SF. It is hypothesized that the space-related parameter (SL) is influenced by visually perceived motion information, whereas the temporal parameter (SF) remains stable. The adaptation over the entire walking distance suggests that a shift from visual to leg-proprioceptive control takes place.

Corticospinal input in human gait: modulation of magnetically evoked motor responses

Abstract Transcranial magnetic stimulation (TMS) of the motor cortex was applied during locomotion to investigate the significance of corticospinal input upon the gait pattern. Evoked motor responses (EMR) were studied in the electromyogram (EMG) of tibialis anterior (TA), gastrocnemius (GM) and, for reference, abductor digiti minimi (AD) muscles by applying below-threshold magnetic stimuli during treadmill walking in healthy adults. Averages of 15 stimuli introduced randomly at each of 16 phases of the stride cycle were analysed. Phase-dependent amplitude modulation of EMR was present in TA and GM which did not always parallel the gait-associated modulation of the EMG activity. No variation of onset latency of the EMR was observed. The net modulatory response was calculated by comparing EMR amplitudes during gait with EMR amplitudes obtained (at corresponding background EMG activities) during tonic voluntary muscle contraction. Large net responses in both muscles occurred prior to or during phasic changes of EMG activity in the locomotor pattern. This facilitation of EMR was significantly higher in leg flexor than extensor muscles, with maxima in TA prior to and during late swing phase. A comparison of this facilitation of TA EMR prior to swing phase and prior to a phasic voluntary foot dorsiflexion revealed a similar onset but an increased amount of early facilitation in the gait condition. The modulated facilitation of EMR during locomotion could in part be explained by spinal effects which are different under dynamic and static motor conditions. However, we suggest that changes in corticospinal excitability during gait are also reflected in this facilitation. This suggestion is based on: (1) the similar onset yet dissimilar size of facilitatory effects in TA EMR prior to the swing phase of the stride cycle and during a voluntary dynamic activation, (2) the inverse variation of EMR and EMG amplitudes during this phase, and (3) the occurrence of this inversion at stimulation strengths below motor threshold (motor threshold was determined during weak tonic contraction and EMR were facilitated during gait). It is hypothesized that the facilitation is phase linked to ensure postural stability and is most effective during the phases prior to and during rhythmical activation of the leg muscles resulting in anticipatory adjustment of the locomotor pattern.

Cortical and spinal adaptations induced by balance training: correlation between stance stability and corticospinal activation

Aim:  To determine the sites of adaptation responsible for improved stance stability after balance (=sensorimotor) training, changes in corticospinal and spinal excitability were investigated in 23 healthy subjects.

Short Intracortical and Surround Inhibition Are Selectively Reduced during Movement Initiation in Focal Hand Dystonia

In patients with focal hand dystonia (FHD), pathological overflow activation occurs in muscles not involved in the movement. Surround inhibition is a neural mechanism that can sharpen desired movement by inhibiting unwanted movement in adjacent muscles. To further establish the phenomenon of surround inhibition and to determine whether short intracortical inhibition (SICI) reflecting inhibition from the local interneurons in primary motor cortex (M1), might play a role in its genesis, single- and paired-pulse transcranial magnetic stimulation (TMS), and Hoffmann reflex testing were applied to evaluate the excitability of the relaxed abductor pollicis brevis muscle (APB) at various intervals during a movement of the index finger in 16 patients with FHD and 20 controls. Whereas controls showed inhibition of APB motor-evoked potential (MEP) size during movement initiation and facilitation of APB MEP size during the maintenance phase, FHD patients did not modulate APB MEP size. In contrast, SICI remained constant in controls, but FHD patients showed reduced SICI during movement initiation. The Hmax/Mmax ratio in control subjects increased during movement initiation. The results provide additional evidence for the presence of surround inhibition in M1, where it occurs only during movement initiation, indicating that different mechanisms underlie movement initiation and maintenance. Thus, surround inhibition is sculpted both in time and space and may be an important neural mechanism during movement initiation to counteract increased spinal excitability. SICI may contribute to its generation, because in patients with FHD, the lack of depression of APB MEP size is accompanied by a reduction in SICI.

Task-specific changes in motor evoked potentials of lower limb muscles after different training interventions.

This study aimed to identify sites and mechanisms of long-term plasticity following lower limb muscle training. Two groups performing either a postural stability maintenance training (SMT) or a ballistic ankle strength training (BST) were compared to a non-training group. The hypothesis was that practicing of a self-initiated voluntary movement would facilitate cortico-spinal projections, while practicing fast automatic adjustments during stabilization of stance would reduce excitatory influence from the primary motor cortex. Training effects were expected to be confined to the practiced task. To test for training specificity, motor evoked potentials (MEP) induced by transcranial magnetic stimulation (TMS) were recorded at rest and during motor tasks that were similar to each training. Intracortical, cortico-spinal, as well as spinal parameters were assessed at rest and during these tasks. The results show high task and training specificity. Training effects were only observable during performance of the trained task. While MEP size was decreased in the SMT group for the trained tasks, MEP recruitment was increased in the BST group in the trained task only. The control group did not show any changes. Background electromyogram levels, M. soleus H-reflex amplitudes and intracortical parameters were unaltered. In summary, it is suggested that the changes of MEP parameters in both training groups, but not in the control group, reflect cortical motor plasticity. While cortico-spinal activation was enhanced in the BST group, SMT may be associated with improved motor control through increased inhibitory trans-cortical effects. Since spinal excitability remained unaltered, changes most likely occur on the supraspinal level.

Balance training and ballistic strength training are associated with task-specific corticospinal adaptations

The aim of this study was to investigate the role of presumably direct corticospinal pathways in long‐term training of the lower limb in humans. It was hypothesized that corticospinal projections are affected in a training‐specific manner. To assess specificity, balance training was compared to training of explosive strength of the shank muscles and to a nontraining group. Both trainings comprised 16 1‐h sessions within 4 weeks. Before and after training, the maximum rate of force development was monitored to display changes in motor performance. Neurophysiological assessment was performed during rest and two active tasks, each of which was similar to one type of training. Hence, both training groups were tested in a trained and a nontrained task. H‐reflexes in soleus (SOL) muscle were tested in order to detect changes at the spinal level. Corticospinal adaptations were assessed by colliding subthreshold transcranial magnetic stimulation to condition the SOL H‐reflex. The short‐latency facilitation of the conditioned H‐reflex was diminished in the trained task and enhanced in the nontrained task. This was observable in the active state only. On a functional level, training increased the rate of force development suggesting that corticospinal projections play a role in adaptation of leg motor control. In conclusion, long‐term training of shank muscles affected fast corticospinal projections. The significant interaction of task and training indicates context specificity of training effects. The findings suggest reduced motor cortical influence during the trained task but involvement of direct corticospinal control for new leg motor tasks in humans.

Chronic Cervical Spinal Cord Injury: DTI Correlates with Clinical and Electrophysiological Measures

Diffusion tensor imaging (DTI) is rarely applied in spinal cord injury (SCI). The aim of this study was to correlate diffusion properties after SCI with electrophysiological and neurological measures. Nineteen traumatic cervical SCI subjects and 28 age-matched healthy subjects participated in this study. DTI data of the spinal cord were acquired with a Philips Achieva 3 T MR scanner using an outer volume suppressed, reduced field of view (FOV) acquisition with oblique slice excitation and a single-shot EPI readout. Neurological and electrophysiological measures, American Spinal Injury Association (ASIA) impairment scale scores, and motor (MEP) and somatosensory evoked potentials (SSEP) were assessed in SCI subjects. Fractional anisotropy (FA) values were decreased in the SCI subjects compared to the healthy subjects. In upper cervical segments, the decrease in FA was significant for the evaluation of the entire cross-sectional area of the spinal cord, and for corticospinal and sensory tracts. A decreasing trend was also found at the thoracic level for the corticospinal tracts. The decrease of DTI values correlated with the clinical completeness of SCI, and with SSEP amplitudes. The reduced DTI values seen in the SCI subjects are likely due to demyelination and axonal degeneration of spinal tracts, which are related to clinical and electrophysiological measures. A reduction in DTI values in regions remote from the injury site suggests their involvement with wallerian axonal degeneration. DTI can be used for the quantitative evaluation of the extent of spinal cord damage, and eventually to monitor the effects of future regeneration-inducing treatments.

Voluntary control of human gait: conditioning of magnetically evoked motor responses in a precision stepping task

Abstract The aim of this study was to investigate visuomotor control during human gait. It was assumed that visual input should modulate transcranially evoked motor potentials (EMPs) during walking. The effect of transcranial magnetic stimulation (TMS) in a visually guided precision stepping task was compared with that during normal gait. EMPs were studied in tibialis anterior (TA), gastrocnemius (GM), and abductor digiti minimi (AD) muscles during treadmill walking. In both stepping tasks, a facilitation of EMPs was observed prior to activation of the respective leg muscle. EMP facilitation proved to be modulated throughout the stride cycle when normalising EMP with respect to the underlying electromyogram (EMG). Facilitation was strongest in TA prior to the swing phase. Significant differences of EMP facilitation between the visual and control tasks were present. In the visual task, maximal facilitation of TA EMPs prior to and during the swing phase was decreased compared to the control task. Conversely, there was increased facilitation of GM EMPs during swing phase of the visual task, prior to the heel strike and prior to the plantarflexion, which was the moment when the target was hit. Thus, the effect of visual input upon EMPs in TA and GM was differential and reciprocal according to the respective functional state. The results support the hypothesis of a conditioning effect of visual or, alternatively volitional, drive on EMPs during stepping.

Visual kinesthesia and locomotion in Parkinson's disease.

We investigated predominance of visual control in Parkinsons disease (PD) gait regulation and whether visual kinesthesia has systematic effects on gait parameters. Effects of artificial optic flow were studied on walking velocity (WV), stride length (SL), and stride frequency (SF) during treadmill walking in PD patients and young and elderly adults. The independent variable was relative optic flow (rOF), ranging from −1 times (forward flow, i.e., in walking direction) to 3 times WV (backward flow, natural direction). All walkers were influenced similarly by rOF, inducing systematic changes of WV. Backward flow caused a decrease and forward flow an increase of WV. Without effect of rOF, PD patients on average walked at 0.89 meters per second compared to 1.31 meters per second in the age‐matched healthy group. The rOF‐induced mean changes of WV in all PD patients amounted to 0.45 meters per second (50.4%), with 45.1% due to changes in SL and 5.3% to SF. In the age‐matched, rOF‐induced WV changes reached 0.18 meters per second (13.8%), with 10.8% due to SL and 3.2% to SF. Thus, compared to the results of the age‐matched group, effects of rOF in PD patients were stronger, which increased WV to a normal level by normalization of SL. Contrary to the healthy subjects, no attenuation of optic flow effects over time was observed in the PD patients. Predominance of visual control in PD gait is suggested due to deficits in proprioception compensated by visual kinesthesia, causing exaggerated reaction to visual feedback. The results extend beyond earlier findings, generally stating improvement of PD gait by presence of visual feedback but show systematic effects on gait parameters due to reweighting of visual kinesthesia.

Surround inhibition depends on the force exerted and is abnormal in focal hand dystonia.

There is evidence that surround inhibition (SI), a neural mechanism to enhance contrast between signals, may play a role in primary motor cortex during movement initiation, while it is deficient in patients with focal hand dystonia (FHD). To further characterize SI with respect to different force levels, single- and paired-pulse transcranial magnetic stimulation was applied at rest and during index finger movement to evoke potentials in the nonsynergistic, abductor policis muscle. In Experiment 1, in 19 healthy volunteers, SI was tested using single-pulse transcranial magnetic stimulation. Motor-evoked potentials at rest were compared with those during contraction using four different force levels [5, 10, 20, and 40% of maximum force (F(max))]. In Experiments 2 and 3, SI and short intracortical inhibition (SICI) were tested, respectively, in 16 patients with FHD and 20 age-matched controls for the 10% and 20% F(max) levels. SI was most pronounced for 10% F(max) and abolished for the 40% F(max) level in controls, whereas FHD patients had no SI at all. In contrast, a loss of SICI was observed in FHD patients, which was more pronounced for 10% F(max) than for 20% F(max). Our results suggest that SI is involved in the generation of fine finger movements with low-force levels. The greater loss of SICI for the 10% F(max) level in patients with FHD than for the 20% F(max) level indicates that this inhibitory mechanism is more abnormal at lower levels of force.