Michel J. A. Gauthier

Voluntary and reflex control of human back muscles during induced pain

1 Back pain is known to change motor patterns of the trunk. The purpose of this study was to examine the motor output of the erector spinae (ES) muscles during pain in the lumbar region. First, their voluntary activation was assessed during flexion and re‐extension of the trunk. Second, effects of cutaneous and muscle pain on the ES stretch reflex were measured, since increased stretch reflex gain has been suggested to underlie increased muscle tone in painful muscles. 2 The trunk movement and electromyographical (EMG) signals from the right and left ES during pain were compared with values before pain. Controlled muscle pain was induced by infusion of 5% saline into the right lumbar ES. Cutaneous pain was elicited by mechanical or electrical stimulation of the dorsal lumbar skin. The stretch reflex was evoked by rapidly indenting the right lumbar ES with a servo‐motor prodder. 3 The results from the voluntary task show that muscle pain decreased the modulation depth of ES EMG activity. This pattern was associated with a decreased range and velocity of motion of the painful body segment, which would normally serve to avoid further injury. Interestingly, when subjects overcame this guarding tendency and made exactly the same movements during pain as before pain, the EMG modulation depth was still reduced. The results seem to reconcile the controversy of previous studies, in which both hyper‐ and hypoactivity of back muscles in pain have been reported. 4 In the tapped muscle, the EMG response consisted of two peaks (latency 19.3 ± 2.1 and 44.6 ± 2.5 ms, respectively) followed by a trough. On the contralateral side the first response was a trough (26.2 ± 3.2 ms) while the second (46.4 ± 4.3 ms) was a peak, similar to the second peak on the tapped side. Cutaneous pain had no effect on the short‐latency response but significantly increased the second response on the tapped side. Surprisingly, deep muscle pain had no effect on the stretch reflex. A short‐latency reciprocal inhibition exists between the right and left human ES. 5 It is concluded that deep back pain does not influence the stretch reflexes in the back muscles but modulates the voluntary activation of these muscles.

Intraspinal micro stimulation generates locomotor-like and feedback-controlled movements

Intraspinal microstimulation (ISMS) may provide a means for improving motor function in people suffering from spinal cord injuries, head trauma, or stroke. The goal of this study was to determine whether microstimulation of the mammalian spinal cord could generate locomotor-like stepping and feedback-controlled movements of the hindlimbs. Under pentobarbital anesthesia, 24 insulated microwires were implanted in the lumbosacral cord of three adult cats. The cats were placed in a sling leaving all limbs pendent. Bilateral alternating stepping of the hindlimbs was achieved by stimulating through as few as two electrodes in each side of the spinal cord. Typical stride lengths were 23.5 cm, and ample foot clearance was achieved during swing. Mean ground reaction force during stance was 36.4 N, sufficient for load-bearing. Feedback-controlled movements of the cats foot were achieved by reciprocally modulating the amplitude of stimuli delivered through two intraspinal electrodes generating ankle flexion and extension such that the distance between a sensor on the cats foot and a free sensor moved back and forth by the investigators was minimized.. The foot tracked the displacements of the target sensor through its normal range of motion. Stimulation through electrodes with tips in or near lamina IX elicited movements most suitable for locomotion. In chronically implanted awake cats, stimulation through dorsally located electrodes generated paw shakes and flexion-withdrawals consistent with sensory perception but no weight-bearing extensor movements. These locations would not be suitable for ISMS in incomplete spinal cord injuries. Despite the complexity of the spinal neuronal networks, our results demonstrate that by stimulating through a few intraspinal microwires, near-normal bipedal locomotor-like stepping and feedback-controlled movements could be achieved.

Quantification of the UPDRS rigidity scale

In the clinical setting, Parkinsonian rigidity is assessed using subjective rating scales such as that of the Unified Parkinsons Disease Rating System (UPDRS). However, such scales are susceptible to problems of sensitivity and reliability. Here, we evaluate the reliability and validity of a device designed to quantify Parkinsonian rigidity at the elbow and the wrist. The method essentially quantifies the clinical examination and employs small sensors to monitor forces and angular displacements imposed by the clinician onto the limb segment distal to the joint being evaluated. Force and displacement data are used to calculate elastic and viscous stiffnesses and their vectorial sum, mechanical impedance. Interexaminer agreement of measures of mechanical impedance in subjects with Parkinsons disease was comparable to that of clinical UPDRS scores. Examiners tended to overrate rigidity on the UPDRS scale during reinforcement manoeuvres. Mechanical impedance was nonlinearly related to UPDRS ratings of rigidity at the elbow and wrist; characterization of such relationships allows interpretation of impedance measurements in terms of the clinical rating scales.

Tooth-Click Control of a Hands-Free Computer Interface

People with severe upper limb paralysis use devices that monitor head movements to control computer cursors. The three most common methods for producing mouse button clicks are dwell-time, sip-and-puff control, and voice-recognition. Here, we tested a new method in which small tooth-clicks were detected by an accelerometer contacting the side of the head. The resulting signals were paired with head tracking technology to provide combined cursor and button control. This system was compared with sip-and-puff control and dwell-time selection. A group of 17 people with disabilities and ten people without disabilities tested each system by producing mouse clicks as inputs to two software programs. Tooth-click/head-mouse control was much faster than dwell-time control and not quite as fast as sip-and-puff control, but it was more reliable and less cumbersome than the latter.

Tremor suppression using functional electrical stimulation: a comparison between digital and analog controllers

In this study, we compared digital and analog versions of a functional electrical stimulator designed to suppress tremor. The device was based on a closed-loop control system designed to attenuate movements in the tremor frequency range, without significantly affecting slower, voluntary movements. Testing of the digital filter was done on three patients with Parkinsonian tremor and the results compared to those of a functional electrical stimulation device based on an analog filter evaluated in a previous study. Additional testing of both the analog and digital filters was done on three subjects with no neurological impairment performing tremor-like movements and slow voluntary movements. We found that the digital controller provided a mean attenuation of 84%, compared to 65% for the analog controller.

The sense of movement elicited by transcranial magnetic stimulation in humans is due to sensory feedback

It has been claimed that transcranial magnetic stimulation (TMS) of the human motor cortex can produce a sense of movement of the contralateral hand, even when the hand is paralysed. The sense of movement was equated with a ‘corollary discharge’, a nulling mechanism originally posited for maintaining constancy of the visual field during eye movements. Our experiments were designed to test whether the sensation that accompanies TMS‐evoked finger movements is generated centrally or whether it arises as a result of sensory feedback. Matched twitches of the left and right fingers were elicited either by bilateral electrical stimulation of forearm extensor muscles, or by a combination of TMS of left motor cortex (eliciting twitches of the right forefinger), and electrical stimulation of the left forearm muscles (eliciting twitches of the left forefinger). The time interval between stimuli activating left and right twitches was varied randomly (range ± 90 ms) from trial to trial. Subjects reported whether they sensed that the left or the right movement occurred first, or if they could detect no difference. The left and right movements evoked by bilateral electrical stimulation of muscles were sensed as near simultaneous when there was zero delay between them. When TMS was applied in conjunction with unilateral muscle stimulation, the TMS‐evoked movement was felt, on average, 20 ms after the movement evoked by muscle stimulation. Similar results were obtained when the skin under the cathodal electrodes was anaesthetized. Since the TMS‐evoked movements were felt later rather than earlier than the electrically evoked movements, the results do not support the idea that the sensation of movement was elicited centrally by TMS. Rather, they favour sensory feedback as the source of the sense of movement. The earlier perception of electrically evoked versus TMS‐evoked movements was probably due to earlier sensory responses in the periphery rather than a suppression of the excitability of somatosensory cortex.

Evaluation of tooth-click triggering and speech recognition in assistive technology for computer access.

Background. Computer access can play an important role in employment and leisure activities following spinal cord injury. The authors’ prior work has shown that a tooth-click detecting device, when paired with an optical head mouse, may be used by people with tetraplegia for controlling cursor movement and mouse button clicks. Objective. To compare the efficacy of tooth clicks to speech recognition and that of an optical head mouse to a gyrometer head mouse for cursor and mouse button control of a computer. Methods. Six able-bodied and 3 tetraplegic subjects used the devices listed above to produce cursor movements and mouse clicks in response to a series of prompts displayed on a computer. The time taken to move to and click on each target was recorded. Results. The use of tooth clicks in combination with either an optical head mouse or a gyrometer head mouse can provide hands-free cursor movement and mouse button control at a speed of up to 22% of that of a standard mouse. Tooth clicks were significantly faster at generating mouse button clicks than speech recognition when paired with either type of head mouse device. Conclusions. Tooth-click detection performed better than speech recognition when paired with both the optical head mouse and the gyrometer head mouse. Such a system may improve computer access for people with tetraplegia.

First permanent human implant of the Stimulus Router System, a novel neuroprosthesis: Preliminary testing of a polarity reversing stimulation technique

Neuroprostheses (NPs) are electrical stimulators that help to restore sensory or motor functions lost as a result of neural damage. The Stimulus Router System (SRS) is a new type of NP developed in our laboratory. The system uses fully implanted, passive leads to “capture” and “route” some of the current flowing between pairs of surface electrodes to the vicinity of the target nerves, hence eliminating the need for an implanted stimulator. In June 2008, 3 SRS leads were implanted in a tetraplegic man for restoration of grasp and release. To reduce the size of the external wristlet and thereby optimize usability, we recently implemented a polarity reversing stimulation technique that allowed us to eliminate a reference electrode. Selective activation of three target muscles was achieved by switching the polarities of the stimulus current delivered between pairs of surface electrodes located over the pick-up terminals of the implanted leads and reducing the amplitude of the secondary phases of the stimulus pulses.