The influence of adequate vestibular stimulation on evoked locomotor muscle activity in the decerebrated guinea-pig

Extracting phase-dependent human vestibular reflexes during locomotion using both time and frequency correlation approaches

Daily activities, such as walking, may require dynamic modulation of vestibular input onto motoneurons. This dynamic modulation is difficult to identify in humans due to limitations in the delivery and analysis of current vestibular probes, such as galvanic vestibular stimulation. Stochastic vestibular stimulation, however, provides an alternative method to extract human vestibular reflexes. Here, we used time-dependent coherence and time-dependent cross-correlation, coupled with stochastic vestibular stimulation, to investigate the phase dependency of human vestibular reflexes during locomotion. We found that phase-dependent activity from the medial gastrocnemius muscles is correlated with the vestibular signals over the 2- to 20-Hz bandwidth during the stance phase of locomotion. Vestibular-gastrocnemius coherence and time-dependent cross-correlations reached maximums at 21 ± 4 and 23 ± 8% of the step cycle following heel contact and before the period of maximal electromyographic activity (38 ± 5%). These results demonstrate 1) the effectiveness of these techniques in extracting the phase-dependent modulation of vestibulomuscular coupling during a cyclic task; 2) that vestibulomuscular coupling is phasically modulated during locomotion; and 3) that the period of strongest vestibulomuscular coupling does not correspond to the period of maximal electromyographic activity in the gastrocnemius. Therefore, we have shown that stochastic vestibular stimulation, coupled with time-frequency decomposition, provides an effective tool to assess the contribution of vestibular ex-afference to the muscular control during locomotion.

Walking performance of vestibular-defective patients before and after unilateral vestibular neurotomy

The present study investigated goal-directed linear locomotion in nine Menière's patients before and after (1 week, 1 and 3 months) a curative unilateral vestibular neurotomy (UVN). Experiments were done using a 3D motion analysis system in subjects walking eyes open (EO) and eyes closed (EC) towards a real or memorized target, respectively. Locomotor pattern (velocity, step length, step frequency and walk ratio) and walking trajectory deviations were evaluated for normal and fast speeds of locomotion and compared to those recorded in 10 healthy subjects. Before UVN, patients showed no walking deviation but gait pattern changes characterized by slower walks compared to the controls, mainly due to step length and step frequency reductions for both visual conditions and locomotion speeds. In the acute stage after UVN, locomotor pattern impairments were significantly accentuated. On the other hand, patients showed strong walking deviations towards the lesioned side with EC. Opposite lateral deviation towards the intact side were observed with EO for normal speed only. Recovery from impaired locomotor pattern was achieved within 1 month for normal speed but remained uncompensated 3 months post-lesion for fast speed particularly in EC condition. Finally, the walking trajectory deviation towards the lesioned side in the dark was maintained up to 3 months after UVN. The results show that central processing of visual and vestibular cues contributes to an accurate locomotor pointing. They argue for an increased weight of visual reference frame on locomotor functions when vestibular function is unilaterally impaired.

Vestibulospinal and reticulospinal neuronal activity during locomotion in the intact cat. I. Walking on a level surface

To examine the function of descending brain stem pathways in the control of locomotion, we have characterized the discharge patterns of identified vestibulo- and reticulospinal neurons (VSNs and RSNs, respectively) recorded from the lateral vestibular nucleus (LVN) and the medullary reticular formation (MRF), during treadmill walking. Data during locomotion were obtained for 44 VSNs and for 63 RSNs. The discharge frequency of most VSNs (42/44) was phasically modulated in phase with the locomotor rhythm and the averaged peak discharge frequency ranged from 41 to 165 Hz (mean = 92.8 Hz). We identified three classes of VSNs based on their discharge pattern. Type A, or double peak, VSNs (20/44 neurons, 46%) showed two peaks and two troughs of activity in each step cycle. One of the peaks was time-locked to the activity of extensor muscles in the ipsilateral hindlimb while the other occurred anti-phase to this period of activity. Type B, or single pause, neurons (13/44 neurons, 30%) were characterized by a tonic or irregular discharge that was interrupted by a single pronounced and brief period of decreased activity that occurred just before the onset of swing in the ipsilateral hindlimb; some type B VSNs also exhibited a brief pulse of activity just preceding this decrease. Type C, or single peak, neurons (9/44 neurons, 23%) exhibited a single period of increased activity that, in most cells, was time-locked to the burst of activity of either extensor or flexor muscles of a single limb. The population of RSNs that we recorded included neurons that showed phasic activity related to the activity of flexor or extensor muscles [electromyographically (EMG) related, 26/63, 41%], those that were phasically active but whose activity was not time-locked to the activity of any of the recorded muscles (13/63, 21%) and those that were completely unrelated to locomotion (24/63, 38%). Most of the EMG-related RSNs showed one (15/26) or two (11/26) clear phasic bursts of activity that were temporally related to either flexor or extensor muscles. The discharge pattern of double-burst RSNs covaried with ipsilateral and contralateral flexor muscles. Peak averaged discharge activity in these EMG-related RSNs ranged from 4 to 98 Hz (mean = 35.2 Hz). We discuss the possibility that most VSNs regulate the overall activity of extensor muscles in the four limbs while RSNs provide a more specific signal that has the flexibility to modulate the activity of groups of flexor and extensor muscles, in either a single or in multiple limbs.

The effect of procaine injection into the medullary reticular formation on forelimb muscle activity evoked by mesencephalic locomotor region and vestibular stimulation in the decerebrated guinea-pig

The effect of procaine microinjection into the ventromedial portion of the medullary reticular formation on forelimb muscle activity evoked by electrical mesencephalic locomotor region and natural vestibular stimulation has been investigated in the decerebrated guinea-pig. This injection is followed by a reversible increase of the threshold of mesencephalic locomotor region stimulation necessary for the activation of muscle rhythmic activity. Procaine injection is accompanied by reduction of vestibular influence on flexor muscle activity evoked by electrical cutaneous and mesencephalic locomotor region stimulation. Vestibular influence on extensor muscle activity remains unchanged after the injection. The results indicate that medial medullary reticular formation is the site of the convergence of mesencephalic locomotor region and vestibular activity. It is suggested that the vestibular system contributes to the modulation of reticulospinal activity coupled with the initiation and control of locomotion.