Brian L. Day

Corticocortical inhibition in human motor cortex.

1. In ten normal volunteers, a transcranial magnetic or electric stimulus that was subthreshold for evoking an EMG response in relaxed muscles was used to condition responses evoked by a later, suprathreshold magnetic or electric test shock. In most experiments the test stimulus was given to the lateral part of the motor strip in order to evoke EMG responses in the first dorsal interosseous muscle (FDI). 2. A magnetic conditioning stimulus over the hand area of cortex could suppress responses produced in the relaxed FDI by a suprathreshold magnetic test stimulus at interstimulus intervals of 1‐6 ms. At interstimulus intervals of 10 and 15 ms, the test response was facilitated. 3. Using a focal magnetic stimulus we explored the effects of moving the conditioning stimulus to different scalp locations while maintaining the magnetic test coil at one site. If the conditioning coil was moved anterior or posterior to the motor strip there was less suppression of test responses in the FDI. In contrast, stimulation at the vertex could suppress FDI responses by an amount comparable to that seen with stimulation over the hand area. With the positions of the two coils reversed, conditioning stimuli over the hand area suppressed responses evoked in leg muscles by vertex test shocks. 4. The intensity of both conditioning and test shocks influenced the amount of suppression. Small test responses were more readily suppressed than large responses. The best suppression was seen with small conditioning stimuli (0.7‐0.9 times motor threshold in relaxed muscle); increasing the intensity to motor threshold or above resulted in less suppression or even facilitation. 5. Two experiments suggested that the suppression was produced by an action on cortical, rather than spinal excitability. First, a magnetic conditioning stimulus over the hand area failed to produce any suppression of responses evoked in active hand muscles by a small (approximately 200 V, 50 microsecond time constant) anodal electric test shock. Second, a vertex conditioning shock had no effect on forearm flexor H reflexes even though responses in the same muscles produced by magnetic cortical test shocks were readily suppressed at appropriate interstimulus intervals. 6. Small anodal electric conditioning stimuli were much less effective in suppressing magnetic test responses than either magnetic or cathodal electric conditioning shocks.(ABSTRACT TRUNCATED AT 400 WORDS)

INterhemispheric inhibition of the human motor cortex

1. Using two magnetic stimulators, we investigated the effect of a conditioning magnetic stimulus over the motor cortex of one hemisphere on the size of EMG responses evoked in the first dorsal interosseous (FDI) muscle by a magnetic test stimulus given over the opposite hemisphere. 2. A single conditioning shock to one hemisphere produced inhibition of the test response evoked from the opposite hemisphere when the conditioning‐test interval was 5‐6 ms or longer. We shall refer to this as interhemispheric inhibition. However, the minimum latency of inhibition observed using surface EMG responses may have underestimated the true interhemispheric conduction time. Single motor unit studies suggested values 4‐7 ms longer than the minimum interval observed with surface EMG. 3. Interhemispheric inhibition was seen when the test muscle was active or relaxed. Increasing the intensity of the conditioning stimulus increased the duration of inhibition: increasing the intensity of the test stimulus reduced the depth of inhibition. 4. The conditioning coil had to be placed on the appropriate area of scalp for inhibition to occur. The effect of the conditioning stimulus was maximal when it was applied over the hand area of motor cortex, and decreased when the stimulus was moved medial or lateral to that point. 5. The inhibitory effect on the test stimulus probably occurred at the level of the cerebral cortex. In contrast to the inhibition of test responses evoked by magnetic test stimuli, test responses evoked in active FDI by a small anodal electric shock were not significantly inhibited by a contralateral magnetic conditioning stimulus. Similarly, H reflexes in relaxed forearm flexor muscles were unaffected by conditioning stimuli to the ipsilateral hemisphere. However, inhibition was observed if the experiment was repeated with the muscles active.

Electric and magnetic stimulation of human motor cortex: surface EMG and single motor unit responses.

1. The effects of different forms of brain stimulation on the discharge pattern of single motor units were examined using the post‐stimulus time histogram (PSTH) technique and by recording the compound surface electromyographic (EMG) responses in the first dorsal interosseous (FDI) muscle. Electrical and magnetic methods were used to stimulate the brain through the intact scalp of seven normal subjects. Electrical stimuli were applied either with the anode over the lateral central scalp and the cathode at the vertex (anodal stimulation) or with the anode at the vertex and the cathode lateral (cathodal stimulation). Magnetic stimulation used a 9 cm diameter coil centred at the vertex; current in the coil flowed either clockwise or anticlockwise when viewed from above. 2. Supramotor threshold stimuli produced one or more narrow (less than 2 ms) peaks of increased firing in the PSTH of all thirty‐two units studied. Anodal stimulation always produced an early peak. The latencies of the peaks produced by other forms of stimulation, or by high intensities of anodal stimulation, were grouped into four time bands relative to this early peak, at intervals of ‐0.5 to 0.5, 1‐2, 2.5‐3.5 and 4‐5.5 ms later. Peaks occurring within these intervals are referred to as P0 (the earliest anodal), P1, P2 and P3 respectively. 3. At threshold, anodal stimulation evoked only the P0 peak; at higher intensities, the P2 or more commonly the P3 peak also was recruited. The size of the P0 peak appeared to saturate at high intensities. 4. In five of six subjects, cathodal stimulation behaved like anodal stimulation, except that there was a lower threshold for recruitment of the P2 or P3 peak relative to that of the P0 peak. In the other subject, the P3 peak was recruited before the P0 peak. 5. Anticlockwise magnetic [corrected] stimulation, at threshold, often produced several peaks. These always included a P1 peak, and usually a P3 peak. A P0 peak in the PSTH was never produced by an anticlockwise stimulation [corrected] at intensities which we could explore with the technique. 6. Clockwise magnetic [corrected] stimulation never recruited a P1 peak; in most subjects a P3 peak was recruited first and at higher intensities was accompanied by P0 or P2 peaks. 7. On most occasions when more than one peak was observed in a PSTH, the unit fired in only one of the preferred intervals after each shock. However, double firing was seen in five units when high intensities of stimulation were used.(ABSTRACT TRUNCATED AT 400 WORDS)

STIMULATION OF THE HUMAN MOTOR CORTEX THROUGH THE SCALP

Historical introduction. Physiology of electrical stimulation of the exposed motor cortex in primates. Transcranial electrical and magnetical stimulation. The effects of multiple descending volleys on the characteristics of surface EMG responses in hand muscles to magnetic and electrical cortical stimulation (CS). Somatotopy. Calculation of central motor conductaion time. Recruitment order of motoneurones. Inhibitory effects from motor CS. Effects of inputs to sensorimotor cortex on the size of muscle responses evoked by CS. Interruption of brain activity by CS. Application to studies of the motor system in man.

Effect of vision and stance width on human body motion when standing: implications for afferent control of lateral sway.

1. Measurements of human upright body movements in three dimensions have been made on thirty‐five male subjects attempting to stand still with various stance widths and with eyes closed or open. Body motion was inferred from movements of eight markers fixed to specific sites on the body from the shoulders to the ankles. Motion of these markers was recorded together with motion of the point of application of the resultant of the ground reaction forces (centre of pressure). 2. The speed of the body (average from eight sites) was increased by closing the eyes or narrowing the stance width and there was an interaction between these two factors such that vision reduced body speed more effectively when the feet were closer together. Similar relationships were found for components of velocity both in the frontal and sagittal planes although stance width exerted a much greater influence on the lateral velocity component. 3. Fluctuations in position of the body were also increased by eye closure or narrowing of stance width. Again, the effect of stance width was more potent for lateral than for anteroposterior movements. In contrast to the velocity measurements, there was no interaction between vision and stance width. 4. There was a progressive increase in the amplitude of position and velocity fluctuations from markers placed higher on the body. The fluctuations in the position of the centre of pressure were similar in magnitude to those of the markers placed near the hip. The fluctuations in velocity of centre of pressure, however, were greater than of any site on the body. 5. Analysis of the amplitude of angular motion between adjacent straight line segments joining the markers suggests that the inverted pendulum model of body sway is incomplete. Motion about the ankle joint was dominant only for lateral movement in the frontal plane with narrow stance widths (< 8 cm). For all other conditions most angular motion occurred between the trunk and leg. 6. The large reduction in lateral body motion with increasing stance width was mainly due to a disproportionate reduction in the angular motion about the ankles and feet. A mathematical model of the skeletal structure has been constructed which offers some explanation for this specific reduction in joint motion.(ABSTRACT TRUNCATED AT 400 WORDS)

The vestibular system.

Supported by grants from the MRC of Great Britain and the NHMRC of Australia. We thank P.B.C. Matthews for his description of the silent sense.

The effect of magnetic coil orientation on the latency of surface EMG and single motor unit responses in the first dorsal interosseous muscle

We examined the effect of the orientation of a figure-of-eight coil on the latency of surface electromyographic (EMG) responses and the firing pattern of single motor units evoked in the first dorsal interosseous muscle by transcranial magnetic brain stimulation. Two coil positions were used: the coil held on a parasagittal line either with the induced current in the brain flowing in a postero-anterior direction (PA) or with the current flowing latero-medially (LM). The results were compared with those observed after anodal electrical stimulation. LM stimulation produced surface and single unit responses which occurred 0-3 msec earlier than PA stimulation. In many cases responses to LM stimulation had the same latency as those produced by anodal electrical stimulation. Responses evoked by LM stimulation were less affected by changes in motor cortical excitability (cortico-cortical inhibition and transcallosal inhibition) than those to PA stimulation. We suggest that LM stimulation can sometimes stimulate corticospinal fibres directly, at or near the same site as anodal stimulation. In contrast, PA stimulation tends to activate corticospinal fibres trans-synaptically. The difference in stimulation sites may make a comparison of PA and LM stimulation a useful method of localising changes in corticospinal excitability to a cortical level.

Postural electromyographic responses in the arm and leg following galvanic vestibular stimulation in man.

Application of a small (around 1 mA), constant electric current between the mastoid processes (galvanic stimulation) of a standing subject produces enhanced body sway in the approximate direction of the ear behind which the anode is placed. We examined the electromyographic (EMG) responses evoked by such stimulation in the soleus and in the triceps brachii muscles. For soleus, subjects stood erect, with their eyes closed, leaning slightly forward. The head was turned approximately 90° to the right or left relative to the feet. In averaged records (n=40), current pulses of 25 ms or longer modulated the EMG in a biphasic manner: a small early component (latency 62±2.4 ms, mean ± SEM) was followed by a larger late component (latency 115±5.2ms) of opposite sign, which was appropriate to produce the observed body sway. The early component produced no measurable body movement. Lengthening the duration of the stimulus pulse from 25 to 400 ms prolonged the late component of the response but had little effect on the early component. Short- and long-latency EMG responses were also evoked in the triceps brachii muscle if subjects stood on a transversely pivoted platform and had to use the muscle to maintain their balance in the anteroposterior plane by holding a fixed handle placed by the side of their hip. The latency of the early component was 41±2.6 ms; the latency of the late component was 138±4.3 ms and was again of appropriate sign for producing the observed body sway. Galvanic stimulation evoked no comparable responses in either triceps brachii or soleus muscles if these muscles were not being used posturally. The responses were most prominent if vestibular input provided the dominant source of information about postural stability, and were much smaller if subjects lightly touched a fixed support or opened their eyes. The difference in latency between the onset of the early component of the response in arm and leg muscles suggests that this part of the response uses a descending pathway which conducts impulses down the spinal cord with a velocity comparable with that of the fast conducting component of the corticospinal tract. The late component of the EMG response occurs earlier in the leg than the arm. We suggest that it forms part of a patterned, functional response which is computed independently of the early component.

Human body‐segment tilts induced by galvanic stimulation: a vestibularly driven balance protection mechanism.

1. We have studied the effects of changes in posture on the motor response to galvanic vestibular stimulation (GVS). The purpose of the experiments was to investigate whether the function of the GVS‐evoked response is to stabilize the body or the head in space. Subjects faced forwards with eyes closed standing with various stance widths and sitting. In all cases the GVS‐evoked response consisted of a sway of the body towards the anodal ear. 2. In the first set of experiments the response was measured from changes in (i) electromyographic activity of hip and ankle muscles, (ii) the lateral ground reaction force, and (iii) lateral motion of the body at the level of the neck (C7). For all measurements the response became smaller as the feet were placed further apart. 3. In the second set of experiments we measured the GVS‐evoked tilts of the head, torso and pelvis. The basic response consisted of a tilt in space (anodal ear down) of all three segments. The head tilted more than the trunk and the trunk tilted more than the pelvis producing a leaning and bending of the body towards the anodal ear. This change in posture was sustained for the duration of the stimulus. 4. The tilt of all three segments was reduced by increasing the stance width. This was due to a reduction in evoked tilt of the pelvis, the bending of the upper body remaining relatively unchanged. Changing from a standing to a sitting posture produced additional reductions in tilt by reducing the degree of upper body bending. 5. The results indicate that the response is organized to stabilize the body rather than the head in space. We suggest that GVS produces a vestibular input akin to that experienced on an inclined support surface and that the function of the response is to counter any threat to balance by keeping the centre of mass of the body within safe limits.

Reciprocal inhibition between the muscles of the human forearm.

Peripheral and central mechanisms of reciprocal inhibition between antagonist muscles in the forearm have been studied in ten human subjects. H reflexes were evoked in flexor muscles by stimulating the median nerve with single shocks at around motor threshold intensity. Peripheral inhibition of the flexor H reflex was produced by motor threshold stimulation with a single shock of the radial nerve supplying the extensor muscles. The conditioning radial nerve stimulus produced inhibition of the flexor H reflex consisting of three phases. In some individuals, an H reflex could be evoked in extensor muscles of the forearm. Stimulation of the median nerve produced inhibition of the extensor H reflex with a similar time course to that from extensors to flexors. The first phase of inhibition was apparent when the test median nerve shock was given from 1 ms before to 3 ms after the conditioning radial nerve shock. It was abrupt in onset and short in duration and could be evoked with a conditioning stimulus intensity as low as 0.75 X motor threshold. The second and third phases of inhibition were evident when the conditioning radial nerve stimulus preceded the median nerve test shock by 5 to 50, and 50 to 500 ms respectively. The characteristics of these later phases of inhibition are to be the subject of a separate report. The difference in timing of the peak initial short‐latency inhibition from extensor to flexor and from flexor to extensor muscles enabled an estimate to be made of the central synaptic delay of the inhibitory process. This method yielded a central delay of 0.95 ms in excess of that of the H reflex. We conclude that the first phase of inhibition is mediated via large group I afferents acting through a single inhibitory interneurone . Central inhibition of the flexor H reflex was demonstrated with the radial nerve anaesthetized by injection of local anaesthetic at the elbow. Subjects were asked to try to contract the paralysed extensor muscles. Under this condition, attempted voluntary wrist extension inhibited the flexor H reflex even though no movement occurred. A shock was delivered to the radial nerve at a site proximal to the anaesthetic block. When the shock was applied in conjunction with an attempted voluntary contraction of the paralysed extensor muscles, the depth of inhibition was greater than that predicted from the effect of either a shock or a willed contraction acting independently.(ABSTRACT TRUNCATED AT 400 WORDS)