Covariation of the amplitude and latency of motor evoked potentials elicited by transcranial magnetic stimulation in a resting hand muscle.

Transcranial magnetic stimulation (TMS) is a non-invasive brain stimulation technique used to study human neurophysiology. A single TMS pulse delivered to the primary motor cortex can elicit a motor evoked potential (MEP) in a target muscle. MEP amplitude is a measure of corticospinal excitability and MEP latency is a measure of the time taken for intracortical processing, corticofugal conduction, spinal processing, and neuromuscular transmission. Although MEP amplitude is known to vary across trials with constant stimulus intensity, little is known about MEP latency variation. To investigate MEP amplitude and latency variation at the individual level, we scored single-pulse MEP amplitude and latency in a resting hand muscle from two datasets. MEP latency varied from trial to trial in individual participants with a median range of 3.9 ms. Shorter MEP latencies were associated with larger MEP amplitudes for most individuals (median r = - 0.47), showing that latency and amplitude are jointly determined by the excitability of the corticospinal system when TMS is delivered. TMS delivered during heightened excitability could discharge a greater number of cortico-cortical and corticospinal cells, increasing the amplitude and, by recurrent activation of corticospinal cells, the number of descending indirect waves. An increase in the amplitude and number of indirect waves would progressively recruit larger spinal motor neurons with large-diameter fast-conducting fibers, which would shorten MEP onset latency and increase MEP amplitude. In addition to MEP amplitude variability, understanding MEP latency variability is important given that these parameters are used to help characterize pathophysiology of movement disorders.

Long-interval intracortical inhibition is asymmetric in young but not older adults.

Aging is typically accompanied by a decline in manual dexterity and handedness; the dominant hand executes tasks of manual dexterity more quickly and accurately than the nondominant hand in younger adults, but this advantage typically declines with age. Age-related changes in intracortical inhibitory processes might play a role in the age-related decline in manual dexterity. Long-interval intracortical inhibition (LICI) is asymmetric in young adults, with more sensitive and more powerful LICI circuits in the dominant hemisphere than in the nondominant hemisphere. Here we investigated whether the hemispheric asymmetry in LICI in younger adults persists in healthy older adults. Paired-pulse transcranial magnetic stimulation was used to measure LICI in the dominant and nondominant hemispheres of younger and older adults; LICI stimulus-response curves were obtained by varying conditioning stimulus intensity at two different interstimulus intervals [100 ms (LICI100) and 150 ms]. We have replicated the finding that LICI100 circuits are more sensitive and more powerful in the dominant than the nondominant hemisphere of young adults and extend this finding to show that the hemispheric asymmetry in LICI100 is lost with age. In the context of behavioral observations showing that dominant hand movements in younger adults are more fluent than nondominant hand movements in younger adults and dominant hand movements in older adults, we speculate a role of LICI100 in the age-related decline in manual dexterity.NEW & NOTEWORTHY In younger adults, more sensitive and more powerful long-interval intracortical inhibitory circuits are evident in the hemisphere controlling the more dexterous hand; this is not the case in older adults, for whom long-interval intracortical inhibitory circuits are symmetric and more variable than in younger adults. We speculate that the highly sensitive and powerful long-interval intracortical inhibition circuits in the dominant hemisphere play a role in manual dexterity.

Independence of force production by digits of the human hand.

The hypothesis that handedness stems from a greater ability to produce independent forces in the digits of the preferred than the non-preferred hand was investigated in 20 right-handed males who made a sustained isometric flexion of the distal phalanx of a single digit (the instructed digit). Instructed flexion forces were accompanied by non-instructed forces in all other digits. Mean non-instructed force was least when the thumb was the instructed digit, and increased progressively when the index, middle, ring, and little finger was the instructed digit. Both flexion and extension were recorded in non-instructed digits. There was no asymmetry in production of non-instructed force, and hence no evidence for greater independence of force production in the digits of the preferred than the non-preferred hand.

Reversal of fusimotor reflex responses during locomotion in the decerebrate cat.

The effect of brief trains of electrical stimulation, at 2, 3 and 20 x threshold (T), of cutaneous afferents in the medial plantar nerve on the discharges of single medial gastrocnemius static and dynamic gamma-efferents has been investigated at rest and during locomotion in a decerebrate cat preparation. The units were classified as dynamic (10 units) or static (10 units) indirectly on the basis of their resting and locomotor discharge characteristics. Responses were assessed by calculating the change in mean gamma-rate during the 100 ms after stimulus onset compared with a control period. At rest, most dynamic neurones were inhibited by stimulation at 2T (9 of 10 units) and above. In contrast, the resting responses of most static neurones were excitatory at 2T (9 of 10 units) and 3T, while 20T produced static gamma-effects that varied in sign. During locomotion the responses of both types of gamma-efferent were phase related. Two patterns were observed with dynamic units. For seven dynamic neurones, at stimulus levels of 2T (7 units) and above, responses during electromyogram (EMG) bursts were inhibitory while those between bursts were not significantly different from zero. However, for three other dynamic units, a phase-related reversal of reflex responses was observed at some stimulus intensities (always 2T, 3 units) comprising inhibition during, and excitation between, EMG bursts. For static neurones, inhibitory (never excitatory) responses occurred during walking at stimulus intensities of 2T (10 units) and above. The locomotor responses of static units were maximum during (3 units) or between (7 units) EMG bursts and were minimum in the opposite phase of EMG activity. A task-related reversal of reflex responses was thus generally apparent (9 of 10 units) to low intensity stimulation (2T) for static gamma-efferents during locomotion (inhibition) compared with rest (excitation). During locomotion there was a significant linear relation between the magnitude of response and the background gamma-rate for static units and those dynamic units that did not exhibit phase-related reflex reversal (total, 17 units). For dynamic gamma-efferents, inhibition at rest and during locomotion occurred at short (spinal) latencies which were not significantly different and are consistent with the involvement of the same interneuronal pathway. We conclude that pathways of opposite sign may dominate the responses of fusimotor neurones to low threshold cutaneous afferents from the plantar surface of the foot depending on behavioural context. Furthermore, the cutaneous reflex responses of both types of gamma-motoneurones during locomotion appear to vary with the source of the afferent input and do not constitute a general excitatory drive. The results are discussed in relation to the role and reflex control of the fusimotor system.

Parameters affecting gap detection in the rat.

The present research used a startle amplitude reduction paradigm to investigate the ability of the rat's auditory system to track rapidly changing acoustic transients. Specifically examined was the ability of brief gaps in otherwise continuous noise to reduce the amplitude of a subsequently elicited acoustic startle reflex. The duration of the gap, time between gap offset and startle elicitation (the interstimulus interval or ISI), and rise-fall characteristics of the gap were systematically varied. Consistent with previous research, gaps reliably reduced startle amplitude. Gaps 2 msec long were reliably detected, and a 50-msec ISI resulted in the greatest amplitude reduction. Gaps presented at short ISIs produced amplitude reduction that followed a different time course than did gaps presented at longer ISIs. These results may reflect differences in the length of time available for the processing of the stimulus and may involve two different processes.

The locomotor discharge characteristics of ankle flexor gamma-motoneurones in the decerebrate cat.

1. The discharge patterns of ankle flexor, tibialis anterior (TA), gamma-motoneurones were recorded during locomotion in the decerebrate cat. 2. At rest gamma-efferents had no background discharge. During locomotion two patterns of gamma activity could be distinguished. Most units (16) were phasically recruited with homonymous electroneurogram (ENG) activity, while the remainder (5) were tonically active throughout the step cycle. 3. The modulation of phasic units was greater (P < 0.01) than tonic neurones. Phasic units had lower (P < 0.02) mean, but higher (P < 0.01) peak, rates during the step cycle. 4. The discharge rate of both types of efferent increased around the onset of ENG activity and peaked during ENG activity, or shortly after its cessation. The conduction velocities of phasic and tonic units overlapped widely. 5. It is proposed, on the basis of muscle spindle afferent recordings during locomotion, that TA phasic and tonic units correspond to static and dynamic gamma-motoneurones, respectively. This correspondence is functionally advantageous for the role of ankle flexor muscles during locomotion. Thus phasic static gamma discharge during flexion would aid muscle contraction via increased Ia afferent activity, while tonic dynamic gamma firing would enhance Ia afferent stretch sensitivity throughout the step cycle. Such enhancement during flexion would oppose unexpected muscle lengthening while, during extension, it would contribute to reciprocal inhibition of ankle extensor muscles. 6. The results are discussed in relation to strategies of gamma usage during rhythmic movements. It is postulated that, for such behaviour, muscle contraction is accompanied by coactivity in static and dynamic gamma-motoneurones. A functional rationale is suggested for this strategy.

Short latency cutaneous reflex responses of gamma-efferents in the decerebrate cat.

The effect of single shock stimulation, up to 20 x threshold (T), of the sural nerve on the discharges of triceps surae gamma-efferents was investigated in decerebrate cats. Units were classified as static (12) or dynamic (7) on the basis of their resting discharge rates (Murphy et al. 1984). All neurones were excited at short latency by sural nerve stimulation and response size was graded with stimulus intensity. Short latency mixed or inhibitory responses were not evident. Although reflex effects first occurred at low stimulus strengths (less than or equal to 1.5T) in both types of efferent, most responses appeared at higher intensities (greater than 1.5T). The estimated central delays of the responses of static (3.0 +/- 1.1 ms, mean +/- SD) and dynamic (3.4 +/- 1.0 ms) gamma-motoneurones were not significantly different and are consistent with spinal oligosynaptic pathways. The present results differ from those of the only previous study (Johansson and Sojka 1985) of the short latency responses of triceps surae static and dynamic gamma-motoneurones to sural nerve stimulation, in which mixed and inhibitory effects were common in anaesthetised cats. Although differences in recording techniques and gamma sampling may account for the apparent disparity between these studies, it is also feasible that a difference in the setting of interneuronal pathways in the two types of preparation is responsible. The results are discussed in relation to the control of gamma-motoneurones with particular reference to the "final common input" hypothesis (Johansson 1981; Appelberg et al. 1983).

The role of cutaneous afferents in the control of gamma-motoneurones during locomotion in the decerebrate cat.

1. The effect of electrical stimulation, up to 20x threshold (T), of the sural nerve on the discharges of single medial gastrocnemius static and dynamic gamma-motoneurones has been investigated at rest and during locomotion in the decerebrate cat. 2. A total of twenty-three gamma-motoneurones were recorded. The neurones were identified as static (15) or dynamic (8) on the basis of their discharge characteristics (Murphy, Stein & Taylor, 1984). 3. Low intensity stimulation (less than or equal to 1.5T) had no effect on the discharges of most (22 of 23) gamma-efferents at rest or during locomotion. Hence the largest afferents in the sural nerve had little influence on the discharges of static or dynamic gamma-motoneurones in either condition. 4. Higher intensity stimulation (greater than 1.5T) excited both types of gamma-efferent in the resting state and response size was graded with stimulus intensity. For most neurones (20 of 23) excitatory effects appeared in the range 1.5-2T. 5. Stimulation at intensities greater than 1.5T also excited dynamic and some static gamma-motoneurones during locomotion. The responses of dynamic gamma-motoneurones were unchanged during locomotion compared to the resting state. In contrast, the responses of static neurones were significantly reduced, or even abolished, during locomotion and stimuli less than or equal to 3T generally (12 of 13) had no effect. Thus the responses of static, but not dynamic, gamma-efferents were task dependent. Further, the thresholds of responses indicate that activation of low threshold mechanoreceptors in the sural receptive field excites both types of gamma-efferent at rest, and dynamic neurones during locomotion. In contrast, it is proposed that the same peripheral input does not affect static gamma-efferents during locomotion. 6. The responses of static and dynamic gamma-motoneurones during locomotion were not obviously related to step cycle phase, or gamma rate, and responses occurring during or between homonymous electromyogram (EMG) bursts were not significantly different. Thus gamma responses during locomotion were not phase dependent. 7. Stimulation at intensities greater than 3T excited dynamic and some static gamma-motoneurones during locomotion but simultaneously inhibited on-going EMG activity. Peripheral inputs are therefore capable of influencing alpha- and gamma-motoneurones independently during locomotion. 8. The significance of the results is discussed in relation to the control and function of gamma-motoneurones.

Augmentation of the rat's acoustic startle reflex by nonreflexogenic stimuli.

The size of the rat's acoustic startle reflex was augmented by brief acoustic clicks (which did not themselves elicit startle) presented several milliseconds before the reflex-eliciting stimulus (RS). The same clicks presented after the RS gave relatively weak augmentation that was present in the 1st, but not the 2nd, testing session. Brief footshocks set to 75% of each animal's flinch threshold augmented startle when presented both before and after the RS in both testing sessions. Augmentation by a leading footshock increased with shock intensity and was unaffected by the intensity of the RS. Augmentation by a trailing footshock increased with shock intensity and also with the intensity of the RS. Reflex size is not fixed at the time of reflex elicitation but can be augmented by a later nonreflexogenic stimulus. Reflex augmentation may be caused by the 2nd member of a stimulus pair discharging elements of the reflex pathway that were partially activated by the 1st.

Temporal regularity of tapping by the left and right hands in timed and untimed finger tapping.

The temporal characteristics of repetitive finger tapping by the left and right hands were examined in two experiments. In the first experiment, interresponse intervals (IRIs) were recorded while right-handed male subjects tapped in synchrony with an auditory timing pulse (the synchronization phase) and then attempted to maintain the same tapping rate without the timing pulses (the continuation phase). The left and right hands performed separately, at four different rates (interpulse intervals of 250, 500, 750, and 1500 ms). There was no asymmetry of the asynchronies of the timing pulses and the associated responses in the synchronization phase or of the IRIs in either phase, but there was an asymmetry of chronization phase or of the IRIs in either phase, but there was an asymmetry in the temporal dispersion of the responses in both phases. in the second experiment, right-handed males tapped separately with each hand at three different speeds: as quickly as possible, at a fast but steady rate, and at a slow rhythmical rate. The speed asymmetry present when tapping as quickly as possible (with the preferred hand tapping more quickly ) was reduced when tapping at the fast steady rate and was absent when tapping at the slow rhythmical rate. The temporal dispersion of the IRIs produced by the nonpreferred hand was greater than the temporal dispersion of those produced by the preferred hand in all speed conditions. These results show smaller temporal dispersion of tapping by the preferred hand in right-handed males under different conditions, including submaximal speeds at which both hands respond at the same rate. This suggests that the motor system controlling the preferred hand in right-handers had more precise timing of response output than that controlling the nonpreferred hand.

The effect of motor preparation on changes in h reflex amplitude during the response latency of a warned reaction time task.

Monosynaptic Hoffman reflexes (H reflexes) were recorded from the soleus muscle during the response latency of a warned reaction time (RT) task that required plantarflexion of the foot. The task was done under four conditions of predictability of the response signal (RS), created by the factorial combination of foreperiod duration (1 and 4 s) and variability (fixed and variable). RT varied systematically with RS predictability and was facilitated in conditions that favored prediction of the RS. The response latency was divided into two successive phases by the onset of reflex augmentation: a premotor phase of constant reflex amplitude and a succeeding motor phase marked by progressively increasing reflex amplitude. Reflex augmentation during the motor phase was coupled more closely to the imminent movement than to the preceding signal to respond. The duration of the premotor phase was unaffected by RS predictability, but the duration of the motor phase (like RT) was shorter when the RS was more predictable. The maximum H reflex amplitude reached during the motor phase was greater when the RS was more predictable. The tonic level of H reflex amplitude during the premotor phase was greater in conditions that made prediction of the RS difficult. A second experiment showed that this difference was present throughout the foreperiod. These results suggest that conditions that favor prediction of the RS enhance motor preparation. changes in motor preparation (which affect RT) affect the processes underlying reflex amplitudes in the premotor phase and throughout the preceding foreperiod, in conditions that make prediction of the RS difficult, appear to reflect heightened general arousal.

Gamma-motoneurone discharge patterns during fictive locomotion in the decerebrate cat.

Triceps surae gamma-motoneurones were recorded during fictive locomotion in the paralysed high decerebrate cat. Two distinctive patterns of discharge were observed which were similar to those reported for static and dynamic gamma-motoneurones during locomotion in the same preparation, but without paralysis (Murphy, Stein & Taylor, 1984). These results suggest that movement-related afferent feedback is not essential for the generation of the basic patterns of static and dynamic gamma-motoneurone activity during locomotion. The results are discussed in relation to the generation of alpha and gamma locomotor rhythms.

Lateral asymmetry of the scalp distribution of somatosensory evoked potential amplitude.

Somatosensory potentials evoked by brief low-intensity electrical pulses delivered separately to the left and right index fingers were recorded from the scalp over the posterior half of the contralateral hemisphere in normal human subjects. In two experiments it was found that the scalp areas enclosed by 75 and 90% of maximum-amplitude isopotential contour lines of the early cortically generated complexes were more restricted over the left hemisphere. This asymmetrical evoked potential distribution supports the proposal by Semmes from her observations of brain-damaged patients that elementary somatosensory representation is focal in the left hemisphere and diffuse in the right.

Temporal integration of acoustic and cutaneous stimuli shown in the blink reflex.

Temporal integration of pairs of brief blink-eliciting acoustic and cutaneous stimuli was investigated to determine if there was integration of stimuli from different modalities. Reflexes elicited by a tone burst or by a brief electrical shock to the supraorbital nerve followed by a second tone burst or shock at short stimulus onset asynchronies (SOAs) were larger and faster than control reflexes elicited by a single stimulus identical to the lead stimulus of the stimulus pairs. Reflex amplitude was augmented at longer SOAs where there was no effect on latency. Temporal integration was evident for all stimulus pairs, showing that it is due, at least in part, to processes that occur outside specific sensory pathways. Heterogeneous stimulus pairs produced greater reflex enhancement than did homogeneous stimulus pairs. This finding was examined further in Experiment 2, which showed that reflex enhancement with pairs of acoustic pulses was unaffected by the frequency of the second stimulus, suggesting that sensory masking was not acting to suppress reflex expression with acoustic pulse pairs. Integration of reflexogenic acoustic stimuli shown in the blink reflex is restricted to shorter intervals than is integration of acoustic stimuli shown by psychophysical procedures, suggesting that the two methods reflect different aspects of stimulus processing. Integration of reflexogenic stimuli may result from summation of activity associated more directly with reflex expression than with perceptual awareness.

Mutual interactions between speech and finger movements.

Speech output and finger movements were recorded as right-handed males repeated a syllable while making cyclical finger movements in three experimental conditions: (2) maintaining constant amplitude in both response systems; (b) alternating speech amplitude while attempting to maintain constant finger movement amplitude; and (c) alternating finger movement amplitude while attempting to maintain constant speech amplitude. Observations showed that output of the two response systems was coupled (one syllable was uttered with each finger movement) and entrained in amplitude (the amplitude pattern of the response that the subject attempted to keep constant followed that of the concurrently-active amplitude-modulated response). These interactions were bidirectional and were present with both left-handed and right-handed finger movements. The interactions are more extensive and subtle than mere interference wtih one response system by the other, and apparently do not depend on anatomical overlap of the responding neural systems.

Hemispheric differences in temporal resolution.

A review of the relevant clinical and experimental literature gives the conclusion that the cerebral hemispheres differ in temporal resolution of input, with the language-dominant hemisphere showing finer acuity. This conclusion is supported by evidence from performance of patients with unilateral brain damage on tests of temporal resolution, performance of developmental dyslexics on similar tasks, and left-right sensory field differences in temporal acuity in normal human subjects. While it is unlikely that a hemispheric difference in temporal resolution is sufficient to give a complete account of lateralized functions, such attempts to show more primitive physiological differences between the hemispheres are more likely to be fruitful than attempts which differentiate the hemispheres in terms of higher-order psychological functions.

Properties of twitch motor units in snake costocutaneous muscle.

1. Single motor units of twitch muscle fibres were studied in isolated nerve-muscle preparations of m. costocutanei inferiores from grass and garter snakes. Preparations were superfused with Ringer solution at a controlled temperature of 22 degrees C.2. The peak and time-to-peak tension (contraction time) were measured for isometric twitches of forty-seven whole muscles and 83 motor units. The sample of motor units was drawn from an estimated total population of 213-355 twitch units. Peak tetanic tensions were also measured. The measurements were made at muscle lengths at which the twitch tension was maximal, and this length was not always the same for whole muscle and unit twitches. In fifty-nine cases 1 and in twelve cases 2 motor units were isolated from each muscle.3. Whole muscle contraction times ranged from 22-61 msec (mean +/- S.D. = 40.3 +/- 9.8 msec) and those for units from 18-92 msec (mean +/- S.D. = 46.9 +/- 15.9 msec). The wide range for whole muscles is discussed.4. The percentage of the whole muscle tetanic tension contributed by each unit (unit size) was calculated. Contraction time was inversely related to unit size.5. Twitch-tetanus ratios were calculated and found not to be related to unit contraction time.6. The conduction velocities of axons innervating 23 motor units were calculated from latency measurements at two points along the length of the nerve. They ranged from 1.9 to 10.4 m/sec. Axon conduction velocity was inversely related to unit contraction time, and directly related to unit size.