Stanley C. Flavel

Effect of human grip strategy on force control in precision tasks.

Alternate grip strategies are often used for object manipulation in individuals with sensorimotor deficits. To determine the effect of grip type on force control, ten healthy adult subjects were asked to grip and lift a small manipulandum using a traditional precision grip (lateral pinch), a pinch grip with the fingers oriented downwards (downward pinch) and a “key grip” between the thumb and the side of the index finger. The sequence of grip type and hand used was varied randomly after every ten lifts. Each of the three grips resulted in different levels of force, with the key grip strategy resulting in the greatest grip force and the downward pinch grip using the least amount of grip force to lift the device. Cross-correlation analysis revealed that the ability to scale accurately the rate of grip force and load force changes was lowest in the downward pinch grip. This was also associated with a more variable time-shift between the two forces, indicating that the precise anticipatory control when lifting an object is diminished in this grip strategy. There was a difference between hands across all grips, with the left non-dominant hand using greater grip force during the lift but not the hold phase. Further, in contrast with the right hand, the left hand did not reduce grip force during the lift or the hold phase over the ten lifts, suggesting that the non-dominant hand did not quickly learn to optimise grip force. These findings suggest that the alternate grip strategies used by patients with limited fine motor control, such as following stroke, may partly explain the disruption of force control during object manipulation.

Control of human mandibular posture during locomotion

Mandibular movements and masseter muscle activity were measured in humans during hopping, walking and running to determine whether reflexes contribute to the maintenance of jaw position during locomotion. In initial experiments, subjects hopped so that they landed either on their toes or on their heel. Landing on the toes provoked only small mandibular movements and no reflex responses in the masseter electromyogram (EMG). Landing on the heels with the jaw muscles relaxed caused the mandible to move vertically downwards relative to the maxilla, and evoked a brisk reflex response in the masseter at monosynaptic latency. Neither this relative movement of the mandible nor the reflex was seen when the teeth were clenched: hence the reflex is not the result of vestibular activation during head movement. The same variables were measured in a second series of experiments while subjects stood, walked and ran at various speeds and at various inclinations on a treadmill. During walking, the vertical movements of the head and therefore the mandible were slow and small, and there was no tonic masseter EMG or gait‐related activity in the jaw‐closing muscles. When subjects ran, the vertical head and jaw movement depended on the running speed and the inclination of the treadmill. Landing on the heels induced larger movements than landing on the toes. About 10 ms after each foot‐strike, the mandible moved downwards relative to the maxilla, thereby stretching the jaw‐closing muscles and activating them at segmental reflex latency. This caused the mandible to move back upwards. The strength of the reflex response was related to the speed and amplitude of the vertical jaw movement following landing. It is concluded that, during walking, the small, slow movements of the mandible relative to the maxilla are subthreshold for stretch reflexes in the jaw muscles: i.e. the mandible is supported by visco‐elasticity of the soft tissues in the masticatory system. However, the brisker downward movements of the mandible after heel‐landing during hopping and running evoke segmental reflex responses which contribute to the active maintenance of the posture of the mandible. This is a unique demonstration of how a stretch reflex operates to maintain posture under entirely natural conditions.

An apparatus for controlled stretch of human jaw-closing muscles

The design of a special-purpose muscle stretcher for reflex studies of the human jaw-closing muscles is described. The device is based on a servo-controlled electromagnetic vibrator which imposes controlled displacements on the lower jaw. The mechanics of the device keep jaw movements coaxial with the temporomandibular joint during the stretches. The design incorporates important safety features including mechanical stops and electronic cut-outs to protect the jaws from excessive stretches.

Induction of plasticity in the dominant and non-dominant motor cortices of humans

There are clear hemispheric differences in the human motor system. Studies using magnetic resonance morphometry have shown that representation of hand muscles is larger in the dominant hemisphere than the non-dominant hemisphere. There is some limited evidence of electrophysiological differences between hemispheres. For example, it has been reported recently that there is less intracortical inhibition in the dominant hemisphere than the non-dominant hemisphere, and it has been hypothesised that this reduction in inhibition may facilitate use-dependent plasticity in the dominant motor cortex. In the present study we examined this hypothesis in human subjects by examining plasticity induction in both dominant and non-dominant hemispheres using an experimental paradigm known to induce motor cortical plasticity, namely paired associative stimulation (PAS). Additionally, we investigated changes in dominant and non-dominant hand performance on a simple ballistic training task. Short-interval intracortical inhibition (SICI) was also measured for both dominant and non-dominant hands at a range of conditioning intensities. There was significantly less SICI in the dominant motor cortical hand area than in the non-dominant hand area. PAS induced a significant, and similar, increase in motor cortical excitability in both the dominant and non-dominant hemispheres. Motor training resulted in significant performance improvement in both dominant and non-dominant hands. However, there was significantly more improvement in the non-dominant hand. The results from these studies provide some further evidence of electrophysiological differences between the motor cortices of the two hemispheres. Additionally, these findings offer no support for the hypothesis that the dominant hemisphere is positioned more favourably, due to decreased inhibitory tone, than the non-dominant hemisphere for use-dependent plasticity.

A simple and inexpensive system for monitoring jaw movements in ambulatory humans

A simple and inexpensive method for recording vertical movements of the human mandible relative to the maxilla is presented. Measurements are made from accelerometers and a Hall-effect device temporarily glued to the upper and lower anterior teeth. The accelerometer signals are integrated once to give velocity and a second time to give position. Movements of the mandible relative to the maxilla are obtained by integrating the difference between the two accelerometer signals. The (relative) velocity and position records derived in this way are linear, but subject to drift when the jaw is stationary. Steady mandibular position is obtained from the Hall-effect system, but this signal must be corrected for its inherent non-linearity. This device can record rapid movements of the mandible even when the head is unrestrained, and interferes minimally with normal jaw movements.

Voluntary movement and repetitive transcranial magnetic stimulation over human motor cortex

Repetitive transcranial magnetic stimulation (rTMS) can induce short-term reorganization of human motor cortex. Here, we investigated the effect of rTMS during relaxation and weak voluntary muscle contraction on motor cortex excitability and hand function. Subjects (n = 60) participated in one of four studies. Single transcranial magnetic stimuli were delivered over the motor area of the first dorsal interosseus for measurement of motor evoked potential (MEP) size before and after real or sham rTMS delivered at an intensity of 80% of active motor threshold. rTMS involved trains of stimuli applied at 6 Hz for 5 s and repeated every 30 s for 10 min. Resting MEP size was suppressed for 15 min after rTMS during relaxation. However, MEP suppression was abolished when additional brief voluntary contractions were performed before and after rTMS (study 1). Resting MEP size was suppressed for 30 min after rTMS during weak voluntary contraction. MEP suppression was present even though voluntary contractions were performed before and after rTMS (study 2). The MEP suppression most likely reflects a decrease in motor cortical excitability. Surprisingly, rTMS during voluntary contraction did not alter maximal finger tapping speed or performance on a grooved pegboard test, object grip and lift task (study 3), and visuomotor tracking task (study 4). These studies document the complex relationship between voluntary movement and rTMS-induced plasticity in motor cortex. This work has implications for the optimization of rTMS parameters for improved efficacy and potential therapeutic applications.

Modulation of intracortical excitability in human hand motor areas. The effect of cutaneous stimulation and its topographical arrangement

Changes in afferent input can alter the excitability of intracortical inhibitory systems. For example, using paired transcranial magnetic stimulation (TMS), both electrical digital stimulation and muscle vibration have been shown to reduce short-interval intracortical inhibition (SICI). The effects following muscle vibration are confined to the corticospinal projection to the vibrated muscles. The results following digital stimulation are less clear and the relative timing of the cutaneous stimulation and TMS is critical. Here we investigated further whether changes in SICI following digit stimulation exhibit topographic specificity. Eleven normal subjects were investigated (age 28.2±7.5 years, mean±SD). Electromyographic recordings were made from the right first dorsal interosseous (FDI), abductor digiti minimi (ADM) and abductor pollicis brevis (APB) muscles. SICI was measured, with and without preceding electrical digit II or digit V cutaneous stimulation. The interval between the digital nerve stimulus and test magnetic stimulus was independently set for each subject and established by subtracting the onset latency of the motor evoked potential (MEP) from the latency of the E2 component of the cutaneomuscular reflex. Therefore, measures of intracortical excitability were made at a time at which it is known that cutaneous input is capable of modulating cortical excitability. Single digital nerve stimuli applied to digit II significantly reduced SICI in FDI but not in ADM. Single digital nerve stimuli applied to digit V significantly reduced SICI in ADM but not in FDI or APB. There was a more generalised effect on intracortical facilitation (ICF) with both digit II and digit V stimulation significantly increasing ICF in FDI and ADM. Digital stimulation (either DII or DV) did not significantly affect SICI/ICF in APB. These findings show that appropriately timed cutaneous stimuli are capable of modulating SICI in a topographically specific manner. We suggest that the selective decrease in SICI seen with cutaneous stimulation may be important for focusing of muscle activation during motor tasks.

Illicit stimulant use is associated with abnormal substantia nigra morphology in humans.

Use of illicit stimulants such as methamphetamine, cocaine, and ecstasy is an increasing health problem. Chronic use can cause neurotoxicity in animals and humans but the long-term consequences are not well understood. The aim of the current study was to investigate the long-term effect of stimulant use on the morphology of the human substantia nigra. We hypothesised that history of illicit stimulant use is associated with an abnormally bright and enlarged substantia nigra (termed ‘hyperechogenicity’) when viewed with transcranial sonography. Substantia nigra morphology was assessed in abstinent stimulant users (n = 36; 31±9 yrs) and in two groups of control subjects: non-drug users (n = 29; 24±5 yrs) and cannabis users (n = 12; 25±7 yrs). Substantia nigra morphology was viewed with transcranial sonography and the area of echogenicity at the anatomical site of the substantia nigra was measured at its greatest extent. The area of substantia nigra echogenicity was significantly larger in the stimulant group (0.273±0.078 cm2) than in the control (0.201±0.054 cm2; P<0.001) and cannabis (0.202±0.045 cm2; P<0.007) groups. 53% of stimulant users exhibited echogenicity that exceeded the 90th percentile for the control group. The results of the current study suggest that individuals with a history of illicit stimulant use exhibit abnormal substantia nigra morphology. Substantia nigra hyperechogenicity is a strong risk factor for developing Parkinsons disease later in life and further research is required to determine if the observed abnormality in stimulant users is associated with a functional deficit of the nigro-striatal system.

Pulsatile control of the human masticatory muscles

Spectral analysis of jaw acceleration confirmed that the human mandible ‘trembles’ at a peak frequency around 6 Hz when held in its rest position and at other stationary jaw openings. The 6 Hz tremor increased during very slow movements of the mandible, but other lower‐frequency peaks became prominent during more rapid jaw movements. These lower‐frequency peaks are likely to be the result of asymmetries in the underlying, voluntarily produced, ‘saw‐tooth’ movements. In comparison, finger tremor at rest and during slow voluntary movements had a mean peak frequency of about 8 Hz: this frequency did not change during rhythmical finger flexion and extension movements, but the power of the tremor increased non‐linearly with the speed of the movement. The resting jaw tremor was weakly coherent with the activity of the masseter and digastric muscles at the tremor frequency in about half the subjects, but was more strongly coherent during voluntary movements in all subjects. The masseter activity was at least 150 deg out of phase with the digastric activity at the tremor frequency (and at all frequencies from 2.5–15 Hz). The alternating pattern of activity in antagonistic muscles at rest and during slow voluntary movements supports the idea that the masticatory system is subject to pulsatile control in a manner analogous to that seen in the finger.

Transcranial magnetic stimulation reduces masseter motoneuron pool excitability throughout the cortical silent period

OBJECTIVE To evaluate the time-course of changes in masseter motoneuron pool excitability following transcranial magnetic stimulation of motor cortex, and relate this to the duration of the masseter cortical silent period (CSP). METHODS Surface EMG was recorded bilaterally from masseter and digastric muscles in 13 subjects. Focal TMS was applied at 1.3x active motor threshold (AMT) to motor cortex of one hemisphere to elicit a muscle evoked potential (MEP) and silent period bilaterally in masseter as subjects maintained an isometric bite at approximately 10% maximum. With jaw muscles relaxed, a servo-controlled stretcher evoked a stretch reflex in masseter which was conditioned by TMS (1.3x AMT) at 14 different conditioning-testing intervals. There were 20 trials at each interval, in random order. TMS evoked no MEP in resting masseter, but often produced a small MEP in digastric. RESULTS Mean (+/-SE) masseter CSP was 67+/-3ms. The masseter stretch reflex was facilitated when stretch preceded TMS by 8 and 10ms, which we attribute to spatial summation of corticobulbar and Ia-afferent excitatory inputs to masseter. Masseter stretch reflex amplitude was reduced when TMS was given up to 75ms before stretch, and for up to 2ms afterwards. CONCLUSIONS We conclude that descending corticobulbar activity evoked by TMS acts bilaterally on brainstem interneurons that either inhibit masseter motoneurons or increase pre-synaptic inhibition of Ia-afferent terminals for up to 75ms after TMS. The reduction of masseter motoneuron pool excitability following TMS has a similar time-course to the CSP. SIGNIFICANCE In contrast to the situation for spinal and facial (CN VII) muscles, the masseter CSP appears to have no component that can be attributed exclusively to cortical mechanisms. Abnormalities in the masseter cortical silent period observed in neurological conditions may be due to pathophysiological changes at cortical and/or sub-cortical levels.