SUMMARY1. 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 * Present address: Third Department of Internal Medicine, Division of Neurology, Yamagata University, School of Medicine, 2-2-2 Iida-Nishi, Yamagata City 990-23, Yamagata, Japan.

SUMMARY1. 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. A. FERBERT AND OTHERS 6. When the test muscle was relaxed, the amount of interhemispheric inhibition could be increased slightly by voluntary contraction of the muscles in the hand contralateral to the conditioning hemisphere. This effect disappeared if the test muscle was held active throughout the experiment.7. Magnetic conditioning stimuli over one hemisphere were also capable of affecting on-going voluntary EMG activity in the ipsilateral FDI. Inhibition began 10-15 ms after the minimum corticospinal conduction time to the muscle, and lasted for about 30 ms. The depth of inhibition was approximately proportional to the level of on-going EMG. A similar period of inhibition was also observed in the forearm flexor muscles, but in biceps it was less clear and sometimes preceded by excitation.8. The interhemispheric inhibition described in these experiments is probably produced via a transcallosal pathway.

A multiple case study was conducted in order to assess three leading theories of developmental dyslexia: (i) the phonological theory, (ii) the magnocellular (auditory and visual) theory and (iii) the cerebellar theory. Sixteen dyslexic and 16 control university students were administered a full battery of psychometric, phonological, auditory, visual and cerebellar tests. Individual data reveal that all 16 dyslexics suffer from a phonological deficit, 10 from an auditory deficit, four from a motor deficit and two from a visual magnocellular deficit. Results suggest that a phonological deficit can appear in the absence of any other sensory or motor disorder, and is sufficient to cause a literacy impairment, as demonstrated by five of the dyslexics. Auditory disorders, when present, aggravate the phonological deficit, hence the literacy impairment. However, auditory deficits cannot be characterized simply as rapid auditory processing problems, as would be predicted by the magnocellular theory. Nor are they restricted to speech. Contrary to the cerebellar theory, we find little support for the notion that motor impairments, when found, have a cerebellar origin or reflect an automaticity deficit. Overall, the present data support the phonological theory of dyslexia, while acknowledging the presence of additional sensory and motor disorders in certain individuals.

Galvanic vestibular stimulation (GVS) is a simple, safe, and specific way to elicit vestibular reflexes. Yet, despite a long history, it has only recently found popularity as a research tool and is rarely used clinically. The obstacle to advancing and exploiting GVS is that we cannot interpret the evoked responses with certainty because we do not understand how the stimulus acts as an input to the system. This paper examines the electrophysiology and anatomy of the vestibular organs and the effects of GVS on human balance control and develops a model that explains the observed balance responses. These responses are large and highly organized over all body segments and adapt to postural and balance requirements. To achieve this, neurons in the vestibular nuclei receive convergent signals from all vestibular receptors and somatosensory and cortical inputs. GVS sway responses are affected by other sources of information about balance but can appear as the sum of otolithic and semicircular canal responses. Electrophysiological studies showing similar activation of primary afferents from the otolith organs and canals and their convergence in the vestibular nuclei support this. On the basis of the morphology of the cristae and the alignment of the semicircular canals in the skull, rotational vectors calculated for every mode of GVS agree with the observed sway. However, vector summation of signals from all utricular afferents does not explain the observed sway. Thus we propose the hypothesis that the otolithic component of the balance response originates from only the pars medialis of the utricular macula.

SUMMARY1. 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 clectromyographic (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 stimnulation). Magnetic stiinulation 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 (< 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 0U5, 1-2, 2-5-3-5 and 4-5-5 ms later. Peaks occurring within these intervals are referred to as P0 (the earliest an6dal), 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 PO peak.5. Clockwise magnetic stimulation, at threshold, often produced several peaks. These always included a PI peak, and usually a P3 peak. A P0 peak in the PSTH was never produced by a clockwise stimulation at intensities which we could explore with the technique.6. Anticlockwise magnetic 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. 15PHY 412 B. L. DA ' AND) OTHERS 7. On most occasions when more than one peak was observed in a PSTH, the uniit 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. The intervals between the two discharges was the same as the intervals between peaks in the PSTH.8. Surface EMG responses in the FDI muscle behaved in a way predietable from the behaviour of the single motor units which had been studied.9. These results are discussed in terms of the D and I wave hypothesis proposed for responses of pyramidal tract neurones to surface anodal stimulation of the exposed motor cortex in primates.

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 degrees 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 +/- 5.2 ms) 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.(ABSTRACT TRUNCATED AT 250 WORDS)

SUMMARY1. 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.

We have investigated whether the processes underlying the visually evoked, automatic adjustments to a reach are: (1) modifiable by the subject's intention, and (2) available to initiate movement of a stationary arm. Unpredictable movement of a target (80 m/s through 10 cm, left or right in a third of trials) either evoked a mid-flight adjustment of a reaching movement or else acted as a trigger to start an arm movement. Subjects were instructed to respond as rapidly as possible by moving their finger either in the same or in the opposite direction to the target. The target shift evoked an early (125-160 ms) and/or a later (> 160 ms) class of response in the reaching arm. The early response was highly automatic in that it could not be reversed (move opposite) by the subjects' intention. However, the subjects' intention did influence the frequency of occurrence and the size of this early response. The later response was totally modifiable in that it changed direction according to the subjects' intention. Similar classes of response were observed in stationary limbs, but the early, more automatic response was substantially weaker than that elicited during a reach. Two possible mechanisms are proposed to explain these results. The first is a dual-pathway model, which assumes that the two response classes are each generated by separate visuo-motor processes with different properties. The second model assumes both responses are generated by a single visuo-motor mechanism that is under the control of a higher, attentional process.