Stimulus timing by people with Parkinson's disease.

Previous literature suggests that Parkinson's disease is marked by deficits in timed behaviour. However, the majority of studies of central timing mechanisms in patients with Parkinson's disease have used timing tasks with a motor component. Since the motor abnormalities are a defining feature of the condition, the status of timing in Parkinson's disease remains uncertain. Data are reported from patients with mild to moderate Parkinson's disease (both on and off medication) and age- and IQ-matched controls on a range of stimulus timing tasks without counting. Tasks used were temporal generalization, bisection, threshold determination, verbal estimation, and a memory for duration task. Performance of patients was generally "normal" on all tasks, but significant differences from performance of controls were found on the memory for duration task. Among the "normal" effects noted were arithmetic mean bisection, asymmetric temporal generalization gradients, and subjective shortening on the memory for duration task. The results suggest (a) that some previous reports of timing "deficits" in Parkinson's patients were possibly due to the use of tasks requiring a timed manual response and (b) small differences between patients and controls may be found on tasks where two stimuli are presented on each trial, whether patients are on medication or off it.

Auditory cues can modify the gait of persons with early-stage Parkinson's disease: a method for enhancing parkinsonian walking performance?

OBJECTIVE: To investigate whether systematically adjusting the rate of auditory cues induces corresponding modulations of the temporal and spatial parameters of gait of patients with early-stage Parkinson's disease. DESIGN: Subjects performed a series of 15 9-metre walks along a level floor under uncued and four cued conditions; the order of cued conditions was randomized. SETTING: A physiotherapy gymnasium, Manchester, UK. SUBJECTS: Eleven subjects with early-stage idiopathic Parkinson's disease. INTERVENTIONS: Preferred pace was established from the initial three uncued walks. The rate of auditory cues delivered throughout subsequent walks was systematically adjusted for each subject, representing 85, 92.5, 107.5 and 115% of their mean cadence at preferred walking pace. MAIN OUTCOME MEASURES: Mean cadence, mean stride length and mean velocity. RESULTS: Repeated measures ANOVA indicated that the mean velocity and mean cadence of subjects' gait significantly (p < or = 0.01) increased relative to baseline values at cue rates of 115 and 107.5% of cadence at preferred pace and decreased at cue rate of 85%. Mean stride length was unaffected by variations in cue rate. CONCLUSION: The rate of auditory cues, within the range tested, can modulate cadence and thus velocity of gait of subjects with early-stage Parkinson's disease. The provision of auditory cues provides a potential strategy for enhancing walking performance in these patients.

Tactile inhibition of return: non-ocular response inhibition and mode of response.

Inhibition of return (IOR) refers to the slowing of responses to stimuli presented at the same location as a preceding cue. However, the cue-target paradigm used in most previous studies may have overestimated the contribution of eye movement programming to IOR, due to the existence of manual response inhibition effects. This confound can be circumvented by using a target-target paradigm in which participants respond to all stimuli. Here, we compared IOR magnitude from tactile cue-target and target-target tasks involving identical interstimulus intervals of 1,400 and 1,800 ms. Reaction times were measured using a foot pedal toe-lift response and a vocal response. Tactile IOR was observed using both modes of response, demonstrating IOR for the first time using a non-spatial, vocal response. Moreover, IOR effects were significantly smaller in target-target compared to cue-target conditions, thereby confirming the existence of the response inhibition confound.

Human locognosic acuity on the arm varies with explicit and implicit manipulations of attention: implications for interpreting elevated tactile acuity on an amputation stump.

In Experiment 1, normal subjects' ability to localize tactile stimuli (locognosia) delivered to the upper arm was significantly higher when they were instructed explicitly to direct their attention selectively to that segment than when they were instructed explicitly to distribute their attention across the whole arm. This elevation of acuity was eliminated when subjects' attentional resources were divided by superimposition of an effortful, secondary task during stimulation. In Experiment 2, in the absence of explicit attentional instruction, subjects' locognosic acuity on one of three arm segments was significantly higher when stimulation of that segment was 2.5 times more probable than that of stimulation of the other two segments. We surmise that the attentional mechanisms responsible for such modulations of locognosic acuity in normal subjects may contribute to the elevated sensory acuity observed on the stumps of amputees.

A saccular origin of frequency tuning in myogenic vestibular evoked potentials?: implications for human responses to loud sounds.

Previous research has indicated that an early component of click-evoked myogenic potentials in the sternocleidomastoid muscle is vestibularly mediated, since it can be obtained in subjects with loss of cochlear function, but is absent in subjects with loss of vestibular function (Colebatch et al., 1994). We report here the results of an experiment to investigate whether this response shows any tuning properties. In a sample of 11 subjects, we obtained acoustically evoked EMG from the sternocleidomastoid muscle in response to 110 dB SPL 10 ms tone pips with frequencies of 100 Hz, 200 Hz, 400 Hz, 800 Hz, 1600 Hz and 3200 Hz. The results of this experiment indicate that this response does indeed have a well-defined frequency tuning which may be modelled as a resonance with a maximum response at frequencies between 300-350 Hz. The possible saccular origin of the tuning response and the consequences that this may have in human responses to loud sounds is discussed. Also discussed are the consequences of particular electrode arrangements in relation to the innervation and anatomy of sternocleidomastoid.

Vestibular responses to loud dance music: a physiological basis of the "rock and roll threshold"?

In this paper new evidence is provided to indicate that vestibular responses may be obtained from loud dance music for intensities above 90 dB(A) SPL (Impulse-weighted). In a sample of ten subjects acoustically evoked EMG were obtained from the sternocleidomastoid muscle in response to a sample of techno music typical of that which may be experienced in a dance club. Previous research has shown that this response is vestibularly mediated since it can be obtained in subjects with loss of cochlear function, but is absent in subjects with loss of vestibular function (Colebatch et al. [J. Neurol. Neurosurg. Psychiatr. 57, 190-197 (1994)]. Given that pleasurable sensations of self-motion are widely sought after by more normal means of vestibular stimulation, it is suggested that acoustically evoked sensations of self-motion may account for the compulsion to exposure to loud music. Given further the similarity between the thresholds found, and the intensities and frequency distributions that are typical in rock concerts and dance clubs, it is also suggested that this response may be a physiological basis for the minimum loudness necessary for rock and dance music to work-the "rock and roll threshold".

Proprioceptive regulation of voluntary ankle movements, demonstrated using muscle vibration, is impaired by Parkinson's disease.

OBJECTIVE: To test the hypothesis that the proprioceptive regulation of voluntary movement is disturbed by Parkinson's disease, the effects of experimental stimulation of proprioceptors, using muscle vibration, on the trajectories of voluntary dorsiflexion movements of the ankle joint were compared between parkinsonian and control subjects. METHODS: Twenty one patients with Parkinson's disease, on routine medication (levodopa in all but one), and an equal number of age matched, neurologically intact controls, were trained initially to make reproducible ankle dorsiflexion movements (20 degrees amplitude with a velocity of 9.7 degrees /s) following a visual "go" cue while movement trajectories were recorded goniometrically. During 50% of the experimental trials, vibration (105 Hz; 0.7 mm peak to peak) was applied to the Achilles tendon during the ankle movement to stimulate antagonist muscle spindles; vibrated and non-vibrated trials were interspersed randomly. Subjects' performance was assessed by measuring end point position-that is, the ankle angle attained 2 seconds after the visual "go" cue, from averaged (20 trials) trajectories. RESULTS: Statistical analysis of the end point amplitudes of movement showed that, whereas the amplitudes of non-vibrated movements did not differ significantly between patients with Parkinson's disease and controls, antagonist muscle vibration produced a highly significant reduction in the amplitudes of ankle dorsiflexion movements in both the patient and control groups. However, the extent of vibration induced undershooting produced in the patients with Parkinson's disease was significantly less than that in the controls; the mean vibrated/non-vibrated ratios were 0.86 and 0.54 for, respectively, the patient and control groups. CONCLUSIONS: The present finding of a reduction of vibration induced ankle movement errors in parkinsonian patients resembles qualitatively previous observations of wrist movements, and suggests that Parkinson's disease may produce a general impairment of proprioceptive guidance.

Phasic and tonic reflexes evoked in human antagonistic wrist muscles by tendon vibration.

The electromyographic reflex responses of the voluntarily contracting wrist flexor and extensor muscles to periods of vibration-evoked enhanced, Ia-dominated afferent discharge from flexor carpi radialis (FCR) were studied in normal human subjects. Three main response phases were characterised, namely, (i) phasic 'on' responses elicited at the commencement of stimulation, (ii) tonic response levels occurring during prolonged stimulation and (iii) phasic 'off' responses elicited at the termination of stimulation. The phasic 'on' reflex responses of FCR and extensor carpi radialis (ECR) comprised, respectively, a peak of autogenetic excitation of group mean latency 18.8 ms and a trough of reciprocal inhibition of group mean latency 38.0 ms. Prolonged (2 s) trains of FCR (agonist) vibration evoked a phase of tonic reflex excitation in FCR whose mean level was significantly increased, by 20%, above pre-stimulus activity and which did not change over the 0.5-2.0 s vibration period. Progressive reduction of the duration (from 2000 ms to 100 ms) of vibration trains demonstrated that phasic disfacilitatory 'off' troughs regularly occurred, with a consistent latency (mean 24.2 ms), on withdrawal of each period of enhanced Ia-input. This indicates that the responsible excitatory reflex mechanism was operational for the entire duration of each of the vibration periods tested. The extra latency (on average 5.4 ms) of phasic 'off' relative to 'on' responses may be attributed to factors (e.g. 5-10 ms duration of unitary muscle action potentials and afterdischarge in reflex pathways) which inevitably delay the appearance of overt disfacilitatory reductions in EMG rather than the involvement of different reflex pathways. Thus, short-latency, possibly monosynaptic, reflex excitation contributed throughout the entire tonic excitatory response. Sustained FCR (antagonist) vibration produced a significant tonic reciprocal inhibitory reflex depression, by 7% pre-stimulus EMG, of ECR activity which remained steady during the 0.5-2.0 s vibration period. The absence of well-defined phasic disinhibitory 'off' responses in ECR suggests that the contribution of oligosynaptic reflex inhibitory mechanisms to the tonic suppression of activity occurring during continuing vibration is relatively small.

Proprioceptive control of wrist movements in Parkinson's disease. Reduced muscle vibration-induced errors.

The effects upon the trajectories of practised slow (approximately 9 degrees/s) voluntary wrist-extension movements of applying vibration to the tendon of an antagonist muscle (flexor carpi radialis) during the course of the movement have been studied in patients with idiopathic Parkinson's disease and age-matched healthy individuals. In both patient and control groups, flexor vibration elicited undershooting of wrist-extension movements. Wrist extensor and flexor surface EMG recordings indicated that, in patients and controls, such undershooting resulted principally from sustained reductions in extensor (prime mover) activity. Small vibration reflexes were commonly elicited in the wrist flexors which, in both Parkinson's disease and healthy subjects, were usually otherwise virtually quiescent during these slow extension movements. The amplitudes of such vibration reflexes did not differ systematically between patient and control groups and appeared inadequate to have exerted an appreciable braking action upon the extension trajectories. However, the extent of vibration-induced undershooting was, on average, significantly less in the Parkinson's disease group. In a subgroup of patients with asymmetrical parkinsonism the effects of antagonist vibration upon wrist movements of the more and less affected limb were compared. The degree of vibration-induced undershooting was significantly smaller on the more affected side. This finding suggests that disturbed proprioceptive guidance of voluntary movements in Parkinson's disease is related to the severity of clinical motor deficits. A small number Parkinson's disease patients were studied 'ON' and 'OFF' their routine anti-parkinsonian medication. A non-significant tendency was found for vibration-induced errors to be less marked in the 'OFF' state. In a separate series of experiments, under isometric conditions, vibration-induced EMG changes were recorded whilst subjects attempted to maintain a steady (15% maximum) voluntary wrist extensor effort. Results in control subjects suggested that prolonged flexor vibration produced significant tonic reflex reciprocal inhibition of the extensor muscles. However, the strength of reflex inhibition appeared sufficient to account for only a small fraction of the undershooting observed during the movement tasks. Thus, our results are consistent with the existence of an abnormality of higher-level proprioceptive integration in Parkinson's disease in which there is a mismatch of sensory (proprioceptive) and motor (corollary discharge) information.

Abnormalities of motor timing in Huntington's disease.

The ability of patients with Huntington's disease (HD) and control subjects to produce rhythmic finger tapping movements at target frequencies (1-5 Hz) signalled by auditory cues, and to sustain the tapping tempo following sudden withdrawal of cues, was investigated. HD performance, in both the presence and absence of cues, was characterised by, (i) marked irregularity of instantaneous tapping rates and (ii) a tendency to tap too slowly at higher (3-5 Hz) frequency targets and too rapidly at low target frequencies. Analysis of the variability of inter-tap intervals, during uncued tapping at a target rate of 1.8 Hz, using Wing and Kristofferson's model of motor timing (Wing AM, Kristofferson AB. Percept. Psychophys. 1973; 14: 5-12), indicated disturbances of both hypothetical 'clock' and 'motor implementation' systems in HD.

The accuracy and precision of timing of self-paced, repetitive movements in subjects with Parkinson's disease.

In separate experiments, we studied the temporal accuracy and precision of self-paced, repetitive finger-tapping in two groups of 12 patients with Parkinson's disease and a group of 12 controls matched to the patients with respect to age and general cognitive state. One group (I) of patients was studied initially following 12-15 h abstinence from normal levodopa medication ('off') and again, subsequently, approximately 1 h after ingestion of a single normal dose ('on'). A second group (II) of patients, each of whom had bilaterally asymmetrical neurological signs, was tested using 'worse' and 'better' hands separately. Within each session, subjects were tested repeatedly on a tapping task during which they were required to produce a regular series of self-timed inter-tap intervals, the target duration (550 ms) of which had been established previously during an initial period of tapping in synchrony with the beats of a regular metronome. We employed Wing and Kristofferson's (1973) model of control of motor timing to partition the total variance (TV) about the mean inter-response interval (IRI) produced during the self-paced phase of each run into separate components ['clock' variance (CV) and 'motor-delay' variance (MDV)] attributable to hypothetical 'clock' and 'motor-implementation' processes. Although the mean self-paced IRI of parkinsonian patients was generally shorter than that of controls, only during the 'on' medication condition (Group I) was it significantly so. By comparison with control values, and those observed during the 'on' medication condition, values of TV, CV and MDV in Group I were all significantly higher when subjects were 'off' medication. During the 'on' medication condition, only CV was significantly higher than the control value. In Group II, values of TV, CV and MDV associated with use of the 'worse' hand were all significantly higher than both control values and those associated with use of the 'better' hand. Values of these variables when subjects used the 'better' hand did not, however, differ significantly from control values. The theoretical import of these results is discussed in the light of several important procedural, statistical and computational issues and we conclude that TV, CV, and MDV may all vary significantly as a function of the efficacy of dopaminergic transmission in the basal ganglia.

Increased use of target cues during visuo-motor tracking in Parkinson's disease.

The effect of temporarily suppressing the visual display of either the target or actual movement trajectory upon the accuracy of visuo-motor tracking was studied in patients with Parkinson's disease (PD) and healthy subjects. Subjects made wrist movements to superimpose a movement cursor upon a target cursor on a VDU screen. The tracking of slow ramp and sinewave target waveforms was investigated. Trials involving the three conditions of visual suppression, namely, target suppressed (TS), movement suppressed (MS) and non-suppressed (NS) were ordered randomly. In TS and MS trials, respectively, the target or movement cursor disappeared from the subject's view for a 4 s period whilst in NS trials both the target and movement cursors were continuously present. Prior to experimental trials, subjects initially practised a series of NS movements. Tracking errors were analysed by ANOVA for group, suppression condition and waveform effects. The tracking performance of the PD patients, during each form of suppression condition, was worse than that of healthy subjects. Both TS and MS elicited significant reductions in accuracy across groups and waveforms. TS induced a more pronounced impairment of tracking accuracy in the PD group than in the control group suggesting that parkinsonians exhibit an abnormally increased reliance upon visual information of the required trajectory during the present visuo-motor tracking tasks. By contrast, there was no between-group effect of MS in these tasks, suggesting that PD patients show a comparable dependence upon visual feedback of their own movements to that shown by controls.

Cortical control of human soleus muscle during volitional and postural activities studied using focal magnetic stimulation.

The surface-recorded electromyographic (EMG) responses evoked in the ankle musculature by focal, transcranial, magnetic stimulation of the motor cortex were studied in healthy human subjects. Such soleus evoked motor responses (EMRs) were characterised over a wide range of background levels of motor activity and using different stimulus intensities. EMRs were recorded during predominantly (1) volitional and (2) postural tasks. In the former task subjects were seated and voluntarily produced prescribed levels of soleus activation by reference to a visual monitor of EMG. In the latter task subjects assumed standing postures without EMG feedback. Comparison of the EMRs of soleus, traditionally considered a slow anti-gravity extensor muscle, during these tasks was used to evaluate its cortical control in primarily volitional versus primarily postural activities. The form of soleus EMRs produced by single magnetic cortical stimuli comprised an initial (approx. 30 ms) increase and subsequent (approx. 50 ms) depression of EMG. Cortical stimulation could elicit substantial excitatory soleus EMG responses; for example, responses evoked by mild, magnetic stimuli (125% threshold for inducing a response in the relaxed muscle) as subjects exerted full voluntary plantarflexor effort averaged almost 20% of the maximum M-wave which could be elicited by an electrical stimulus to the posterior tibial nerve. Excitatory EMRs could be elicited in the voluntarily relaxed soleus muscle of the majority of subjects during sitting. The amplitude of soleus responses, induced by threshold stimuli for the relaxed state or approximately 125% threshold intensity, increased approximately linearly with background EMG over a wide range of volitional contraction levels. By contrast, there was no systematic change in the latency of excitatory soleus EMRs with increasing voluntary effort. The excitatory responses evoked in the voluntarily relaxed soleus of seated subjects by magnetic stimulation were regularly facilitated by incremental, voluntary contraction of the contralateral ankle extensors in a graded manner. However, such facilitation of responses was not observed when subjects voluntarily activated the muscle in which EMRs were elicited. The pattern of the responses elicited in soleus by magnetic stimulation during the postural task generally resembled that found during the volitional task. The amplitudes of excitatory soleus EMRs at a given stimulus intensity, obtained when subjects stood quietly, leaned forwards or stood on their toes to produce differing levels of ankle extensor contraction, increased with background EMG.(ABSTRACT TRUNCATED AT 400 WORDS)

Increased dependence upon visual information of movement performance during visuo-motor tracking in cerebellar disorders.

The effect of temporarily suppressing the visual display of either the target (desired) trajectory or the actual movement trajectory upon the accuracy of visuo-motor tracking was studied in 6 patients with cerebellar syndromes and 6 healthy subjects. Subjects made extension and flexion movements of the wrist to superimpose a cursor displaying their actual movement (movement cursor) upon one indicating the target (target cursor) on a VDU screen. The target trajectory consisted of a sawtooth pattern of slow (4 deg/sec) ramp extension and instantaneous flexion return phases. Following practice, the tracking of cerebellar patients was significantly less accurate than that of healthy subjects for each phase (P = 0.02). Temporary suppression of the movement cursor during both the mid-section of the ramp phase (P = 0.05) and around the reversal phase (P = 0.04) caused a significant increase in tracking errors in the patients whereas suppression of the target cursor did not alter their performance. Suppression of neither cursor altered the tracking accuracy of healthy subjects during the ramp extensions whilst suppression of either caused reduced (P = 0.02) performance for the reversal phase. We interpret the increased dependence of patients upon visual information of their movements during slow trajectories as indicating an impairment of proprioceptive guidance.

The influence of external timing cues upon the rhythm of voluntary movements in Parkinson's disease.

The ability of patients with Parkinson's disease (PD) and healthy subjects to synchronise finger tapping, produced by rhythmic wrist movements, with auditory signals of target frequencies (range 1-5 Hz) and to sustain such rhythms following sudden withdrawal of auditory cues was studied. Healthy subjects were able, in the presence of auditory cues, to duplicate target frequencies accurately over the range investigated both in terms of mean tapping rate and in regularity of tapping. PD patients were less accurate under these conditions and on average tended to tap too rapidly at the lower (1-3 Hz) target frequencies and too slowly at the highest (5 Hz) target frequency. In addition, the variability of their tapping rhythms was generally greater. Healthy subjects were able to sustain tapping rhythms well following suppression of auditory signals. By contrast, withdrawal of external timing cues resulted in marked impairment of the patients' rhythm generation. At lower frequency targets (1-3 Hz) patients' tapping rates increased over rates which were already elevated in the presence of external cues. Conversely, at higher target frequencies (4-5 Hz), the average tapping rate tended to decline further from previously depressed levels. The accuracy of almost all patients fell outside the normal range. Two patterns of tapping errors were found. The first was hastening of tapping which was most evident at intermediate target frequencies. The second was faltering which occurred mainly at the higher target frequencies. These forms of behaviour may result from inherent abnormalities of internal rhythm generation since they occurred both in the presence and absence of external timing signals. Overall, our findings are consistent with the view that the basal ganglia have a role in the internal cueing of repetitive voluntary movements.

Stretch reflexes of individual parkinsonian patients studied during changes in clinical rigidity following medication.

Stretch reflexes were elicited in flexor carpi radialis (FCR) of healthy subjects and patients with Parkinson's disease by forcible ramp and hold extensions of the wrist joint. Individual patients were studied off treatment when rigidity was detected clinically at the joint and throughout the clinical response to anti-parkinsonian medication that abolished or reduced their rigidity. In this way the possible effects of inter-subject variability upon the relationship between reflex behaviour and rigidity were eliminated. The long-latency (M2) stretch reflexes of the patient group were increased on average compared to those of healthy subjects. However, in the large majority of individual patients there were no significant correlations between the amplitudes of their M2 or total (short-latency (M1) + M2) reflex activities, recorded off and on treatment, and the accompanying changes in clinically assessed rigidity. These results suggest that parkinsonian rigidity cannot be uniquely attributed to the increased reflex responsiveness measured by the present laboratory techniques. However, the techniques used to test reflex function in our study differed in several respects (e.g., background activity, stretching wave form) from those employed during clinical assessment of rigidity so that the balance of reflex mechanisms may have varied in the two situations. Therefore, these results cannot be taken as definitive evidence against a reflex origin of rigidity.

Relationship between electromyographic activity and clinically assessed rigidity studied at the wrist joint in Parkinson's disease.

The electromyographic (EMG) patterns recorded from wrist muscles during manually applied, repetitive flexion and extension movements of the wrist joint, used for simultaneous clinical assessment of rigidity, were studied in patients with Parkinson's disease and healthy subjects. Recordings were made whilst patients/subjects attempted voluntarily to relax the muscle of the arm whose wrist joint was manipulated. Individual patients were investigated before and at varying times after their routine daily medication as their clinical rigidity underwent associated modulations. It was often possible to induce additional alterations in clinical rigidity by instructing patients to perform an activation or Jendrassik-like manoeuvre (clenching the contralateral fist). In rigid patients, the approximately sinusoidal wrist displacements (60 deg, 1-1.5 Hz) typically elicited pronounced, cyclic modulations of EMG activities in wrist flexors and extensors; increases in EMG activity were phase-locked to the respective periods of muscle stretch. Stretch-related EMG activity reduced or disappeared as rigidity was abolished by drug therapy. The EMG patterns of patients showing cogwheel rigidity featured discrete, phasic bursts superimposed upon more generalized stretch-related increases in activity. In healthy subjects, showing no clinical rigidity, the pronounced cyclic modulations of EMG activity characteristic of rigid patients were absent during similar manually applied wrist displacements. Quantitative EMG measurements for individual patients, made 'on' and 'off' medication and as their rigidity fluctuated, indicated that mild (grade 1) and moderate (grade 2) rigidity was consistently associated with increased stretch-related activity compared with non-rigid conditions. Pair-wise statistical analysis indicated such increases in EMG to be significant. Similarly, the ratios of EMG activities in the stretched versus released muscles were significantly greater for grades 1 and 2 rigidity than in the absence of rigidity. Overall, the present findings support the view that enhancement of stretch reflex activity has a major role in the genesis of parkinsonian rigidity.

Electromyographic reflexes evoked in human flexor carpi radialis by tendon vibration.

The rectified, electromyographic (EMG) reflexes evoked in the voluntarily contracting flexor carpi radialis (FCR) muscle by vibration of its tendon were studied in healthy human subjects. Responses comprised a prominent, transient, short-latency (SL, 20-25 ms) increase in EMG, attributed to Ia mono- and/or oligo-synaptic action, followed by a series of less pronounced troughs and peaks of activity. Evidence of continuing Ia mono- or oligo-synaptic action was indicated by (i) the presence of small subpeaks, at vibration frequency, superimposed upon the excitatory components and (ii) the occurrence of a separate reduction in EMG, of consistent latency (ca. 30 ms), after cessation of stimulation. Progressively shortening the train of vibration from 29 cycles (at 145 Hz) to a single cycle significantly reduced net, excitatory reflex activity. Gradually increasing the level (10-50% maximum) of pre-existing voluntary contraction on top of which reflexes were elicited, by moderately prolonged (29 cycles) trains of vibration, resulted in small increases, in absolute terms, in SL peaks and in later, excitatory EMG activity. Excitatory reflexes, when normalised for pre-stimulus EMG, however, declined in an approximately hyperbolic manner with increasing background activity over this range. Thus, effective "automatic gain compensation" does not operate for vibration reflexes in FCR.

Proprioceptive guidance of human voluntary wrist movements studied using muscle vibration.

1. The alterations in voluntary wrist extension and flexion movement trajectories induced by application of vibration to the tendon of flexor carpi radialis throughout the course of the movement, together with the associated EMG patterns, have been studied in normal human subjects. Both extension and flexion movements were routinely of a target amplitude of 30 deg and made against a torque load of 0.32 N m. Flexor tendon vibration consistently produced undershooting of voluntary extension movements. In contrast, voluntary flexion movements were relatively unaffected. 2. The degree of vibration-induced undershooting of 1 s voluntary extension movements was graded according to the amplitude (0.75, 1.0 and 1.5 mm) of flexor tendon vibration. 3. As flexor vibration was initiated progressively later (at greater angular thresholds) during the course of 1 s voluntary extension movements, and the period of vibration was proportionately reduced, so the degree of vibration-induced undershooting showed a corresponding decline. 4. Varying the torque loads (0.32, 0.65 and 0.97 N m) against which 1 s extension movements were made, and thereby the strength of voluntary extensor contraction, produced no systematic changes in the degree of flexor vibration-induced undershooting. 5. Analysis of EMG patterns recorded from wrist flexor and extensor muscles indicated that vibration-induced undershooting of extension movements resulted largely from a reduction in activity in the prime-mover rather than increased antagonist activity. The earliest reductions in extensor EMG commenced some 40 ms after the onset of vibration, i.e. well before voluntary reaction time; these initial responses were considered to be 'automatic' in nature. 6. These results support the view that the central nervous system utilizes proprioceptive information in the continuous regulation of moderately slow voluntary wrist movements. Proprioceptive sensory input from the passively lengthening antagonist muscle, presumably arising mainly from muscle spindle I a afferents, appears to be particularly important and to act mainly in the reciprocal control of the prime-mover.

Reciprocal inhibition of soleus motor output in humans during walking and voluntary tonic activity.

1. The extent to which an active, human motoneuron pool can be inhibited via short-latency inhibitory pathways was studied by stimulating the common peroneal nerve and recording the inhibition of on-going soleus electromyographic (EMG) activity. The responses were compared at the same EMG level during walking and tonic voluntary activity to determine whether the inhibition was task dependent. 2. In both tasks the amount of inhibition (measured as the depression in rectified, filtered, and averaged EMG activity) increased approximately linearly with the amount of motor activity, as determined from the mean EMG level before stimulation (correlation coefficient greater than or equal to 0.9). No difference in the amount of inhibition was found between the two tasks at the same stimulus and EMG levels. 3. Previously published studies based on the H-reflex method have reported that the amount of inhibition decreases with the amount of motor activity. On the contrary, single-unit studies and the present results suggest that segmental inhibitory reflexes retain their capacity to mediate a rapid reduction of motoneuronal discharge during voluntary activity. This inhibition may be important in regulating the amount of activity early in the stance phase of walking and during the transition from stance to the swing phase. 4. Analytic results are derived in an APPENDIX that should be of general interest in interpreting the inhibition of motor units from a peristimulus time histogram (PSTH). The linear correlation between inhibition and level of voluntary activity can be explained if newly recruited units are strongly inhibited by the stimulus, whereas previously active motor units are inhibited relatively less, as their firing rate increases with increasing background activity.