Reaction time and spinal excitability in a simple reaction time task

Granger causality mapping during joint actions reveals evidence for forward models that could overcome sensory-motor delays

Studies investigating joint actions have suggested a central role for the putative mirror neuron system (pMNS) because of the close link between perception and action provided by these brain regions [1], [2], [3]. In contrast, our previous functional magnetic resonance imaging (fMRI) experiment demonstrated that the BOLD response of the pMNS does not suggest that it directly integrates observed and executed actions during joint actions [4]. To test whether the pMNS might contribute indirectly to the integration process by sending information to brain areas responsible for this integration (integration network), here we used Granger causality mapping (GCM) [5]. We explored the directional information flow between the anterior sites of the pMNS and previously identified integrative brain regions. We found that the left BA44 sent more information than it received to both the integration network (left thalamus, right middle occipital gyrus and cerebellum) and more posterior nodes of the pMNS (BA2). Thus, during joint actions, two anatomically separate networks therefore seem effectively connected and the information flow is predominantly from anterior to posterior areas of the brain. These findings suggest that the pMNS is involved indirectly in joint actions by transforming observed and executed actions into a common code and is part of a generative model that could predict the future somatosensory and visual consequences of observed and executed actions in order to overcome otherwise inevitable neural delays.

Startle-induced reaction time shortening is not modified by prepulse inhibition

A startling auditory stimulus delivered during preparation for execution of a ballistic movement in a simple reaction time task experiment induces two effects: a startle response and a reaction time shortening (the StartReact effect). We investigated whether prepulse inhibition of the startle response is effective in suppressing either one of these effects during motor preparation. Twelve healthy volunteers were presented with seven different experimental conditions in random order: while at rest, subjects received a low intensity electrical shock on the middle finger of the left hand (Prep), a loud auditory stimulus (Start), or a combination of these two (PrepStart). While engaged in preparation for a visual simple reaction time task, they were presented with the imperative signal for execution of the reaction (React), or with any of the combinations PrepReact, StartReact, or PrepStartReact. We recorded the EMG activity from the orbicularis oculi and the sternocleidomastoid muscles to assess the startle response, and from the wrist extensor muscles to assess reaction time. The startle response was markedly reduced when Prep was presented 100 ms before Start regardless of whether the subjects were at rest or preparing for the reaction. Reaction time shortened significantly in StartReact trials with respect to React trials, and the percentage shortening was not different in trials in which Prep preceded StartReact and inhibited the startle response. The fact that prepulse inhibition of the startle response is not accompanied by modification of the StartReact effect indicates that there are separate physiological mechanisms for the two effects, an observation that has implications for further understanding of the processes underlying motor preparation for a ballistic reaction.

Impaired antagonist inhibition may contribute to akinesia and bradykinesia in Huntington's disease

OBJECTIVE

To test the hypothesis that besides impaired agonist facilitation, impaired antagonist inhibition also contributes to delayed initiation (akinesia) and slow execution (bradykinesia) of voluntary movements in Huntington's disease.

METHODS

Fifteen patients with Huntington's disease and 11 age-matched controls participated in the study. The amount of agonist facilitation was measured as the increase in soleus H-reflex amplitude prior to ballistic voluntary plantar flexion (soleus contraction). Antagonist inhibition was measured as the decrease in soleus H-reflex prior to ballistic dorsiflexion (tibialis anterior (TA) contraction). The amount of agonist facilitation and antagonist inhibition was correlated with the time needed for motor initiation (reaction time) and movement execution (movement time).

RESULTS

Starting 50ms prior to soleus contraction, soleus H-reflex increased in control subjects but less so in patients. Soleus H-reflexes decreased in controls 25ms prior to TA contraction, while this antagonist inhibition was completely lacking in patients. Thus, patients with Huntington's disease not only displayed reduced agonist facilitation, but impaired antagonist inhibition as well. Moreover, more impairment of antagonist inhibition correlated significantly with more severe akinesia and bradykinesia.

CONCLUSIONS

Antagonist inhibition prior to and during agonist contractions is markedly impaired in Huntington's disease. This impairment might contribute to motor slowness in these patients.

Soleus H-reflex dynamics during fast plantarflexion in humans

The relationship between the size of the soleus (Sol) Hoffmann (H-) reflex and the level of background (BG) electromyographic (EMG) activity was examined during plantarflexing at different force levels. The experiments were carried out on seven healthy male subjects aged 20-37 years. The subjects were asked to perform fast plantarflexion under a reaction-time condition. The amounts of contraction force were 10, 20, 50 and 80% of maximum voluntary contraction (MVC). Since the maximum size of the M-wave (Mmax) changed systematically during the plantarflexion, we tried to maintain the size of the reference M-wave, an indicator of the efficiency of the electrical stimulation, at a constant value (20% of Mmax) throughout the experiment. The size of the H-reflex was rapidly increased at the very beginning of the movement, and then it tended to decrease in the later phase of the movement. Consequently, even with the same level of BG EMG, the size of the H-reflex was always larger in the early rising phase of the EMG activity than in the later falling phase. The maximum size of the H-reflex was poorly correlated with the force exerted. In contrast, the size of the F-response was proportional to the force exerted. The non-linear relationship between the size of the H-reflex and the BG EMG suggests that the level of the presynaptic inhibition onto Ia terminals was modified depending on the required force level and during the course of the movement.

Spinal cord modulation associated with isometric contractions

OBJECTIVE

The dual-strategy hypothesis explains single-joint voluntary movement by dividing movements into two different strategies and suggesting that different excitation pulses modulate these movements. The existence of this excitation pulse was evaluated by quantifying magnitude and timing changes in the H-reflex (changes in spinal excitability) prior to a voluntary contraction. These changes in spinal excitability were assessed during a ballistic plantar flexion isometric contraction, where both the target size and force level was manipulated.

METHODS AND MATERIALS

Subjects were seated in a modified chair with a force transducer placed under the metatarsal heads to measure ankle force output. Following a visual stimulus subjects were trained to produce a plantar flexion force of 25% and 50% of a maximum voluntary contraction, within target sizes of 5% and 15% of the selected force level. Soleus motor neuron reflex excitability was analyzed by measuring changes in the H/M ratio. The H-reflex was randomly elicited by tibial nerve stimulations at 15, 30, 45, 60, 75 and 90 ms prior to the recorded average soleus premotor time for each of the force and target size conditions.

RESULTS

A two-way repeated-measures analysis of variance indicated a significant effect among target sizes for the time of change in spinal excitability, slope of facilitation (rate of rise of spinal excitability), and peak facilitation. A significant difference was also established between force levels for the slope and peak facilitation, but there was no difference with time of facilitation.

CONCLUSIONS

These results indicate that changes in both target size and force level can influence slope and peak of facilitation. However, only target size appears to affect the time of facilitation. Results clearly support the existence of an excitation pulse that is regulated by the type of movement.

Neurophysiological correlate of clinical signs in Parkinson's disease

Clinical diagnosis of Parkinson's disease (PD) is not always coincident with pathological findings. A better characterization of the disease from the results of studies in various areas of neuroscience can help in improving the rate of diagnostic certainty. Neurophysiology is among the techniques with better chances to furnish specific diagnostic cues on motor aspects of the disease. Neurophysiology provides quantifiable data using non-invasive, relatively inexpensive, methods. Neurophysiological tests can be applied with no previous preparation, and repeated many times without dangerous consequences. To be rewarding, however, neurophysiological examination should be done in close cooperation between the clinician who detects relevant specific signs, and the neurophysiologist who devises the most demonstrative methods to document those signs. In this review, we describe the neurophysiological correlate of symptoms and signs in patients with PD, and particularly their pathophysiological meaning, with special focus on those that could be more helpful to the neurologists in establishing differences with respect to other diseases presenting with parkinsonism.

The effect of transcranial magnetic stimulation on reaction time in progressive supranuclear palsy

OBJECTIVE

Reaction time is shortened when a startling acoustic stimulus (SAS) is delivered together with the 'go' signal in normal subjects and patients with Parkinson's disease (IPD), but not in patients with progressive supranuclear palsy (PSP). Similar shortening of reaction time has been reported in normal subjects and patients with IPD with transcranial magnetic stimulation (TMS). In this paper, we analyzed the effect of TMS on reaction time in patients with PSP.

METHODS

Six patients with PSP received the instruction to extend the wrist to a visual cue. In test trials, the visual signal was accompanied by either a SAS or a subthreshold TMS applied to the motor area. The same experimental paradigm was applied to 7 patients with IPD, and 10 normal subjects. We measured both reaction time and the slope of the initial accelerometric displacement (SAD).

RESULTS

Neither TMS nor SAS changed significantly reaction time in PSP patients. This observation was in contrast with the marked reaction time shortening induced by both stimuli in a similar amount in normal subjects and patients with IPD. Furthermore, SAS and TMS did not modify the SAD in PSP, but shortened it significantly in IPD.

CONCLUSION

The absence of an effect of TMS and SAS on reaction time in PSP patients suggests that these patients have a dysfunction in the mechanisms of facilitation of reaction time. The fact that TMS and SAS induced similar effects on reaction time in normal subjects, IPD and PSP patients indicate the possibility of common mechanisms of action for both types of stimuli.

Relationship between motoneuron pool excitability and parameters of timing and force in an isometric ankle-extension movement

The changes in peak facilitation and slope of facilitation of the H-reflex were compared to the parameters of an isometric plantarflexion task. The amplitude of the reflex was significantly greater for reflexes elicited within 50 msec. of the onset of soleus muscle activity. Neither peak amplitude of the reflex nor slope correlated with rate of rise of isometric force; however, they did show a modest relationship between premotor time and reaction time.

Effect of circumferential pressure on response parameters during ballistic ankle plantar flexion in healthy adults

This study investigated the effect of circumferential pressure on timing and muscle-response parameters during ballistic ankle plantar flexion in 15 young healthy subjects. Circumferential pressure was provided by an air-splint applied around the lower calf. Data were collected at 1, 3, and 5 min. prior to, during, and following application of pressure. A 3 x 3 repeated-measures analysis of variance indicated significant increases in premotor and reaction times during application of pressure. There was no main effect for magnitude of muscle activity or motor time. Results support previous studies showing decreases in the excitability of the motor-neuron pool with circumferential pressure and an inverse relationship between excitability and premotor time.

Facilitation of agonist motoneurons upon initiation of rapid and slow voluntary movements in man

The time course of facilitation of the agonist motoneurons upon initiation of voluntary ankle dorsiflexion was investigated in eight healthy subjects. The H-reflex and visually guided tracking methods were used for testing the excitability of the motoneuron pool and for controlling the initiation of movement as well as speed and force. Since the onset of voluntary EMG activity (EMG reaction time: EMGvRT) was delayed and/or obscured by test H-reflexes which were evoked very close to the behavioral responses, the subject was instructed to make response movements bilaterally, and EMGvRT was measured on the side without stimulation. In every subject, the EMGvRT was invariably longer in the ramp movement than in the step movement. The onset of H-reflex facilitation prior to EMG onset, which was regarded as indicating the arrival time of the descending motor command to the motoneuron pool, always started earlier in the ramp movement than in the step movement. The difference in facilitation onset between the two tasks was smaller than that in EMGvRT. Since the amplitude of the H-reflex at the onset of the voluntary EMG was equivalent in both movements, the development of H-reflex towards the behavioral EMG onset was more gradual in the ramp movement than in the step movement. The present results demonstrate that the longer reaction time in the slow ramp task depends on 2 factors: delay in the arrival of descending facilitatory impulses to the agonist motoneuron pool, and its slow recruitment thereafter.

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.

H-reflexes during a motor skill acquisition task

H-reflex amplitudes were studied during the acquisition of a motor skill involving coordinated isometric plantarflexion at the ankle joints as subjects learned to trace a triangular pattern on an oscilloscope screen by controlling plantarflexion torque applied against load cells. Torque feedback was presented on a subject oscilloscope with the right foot controlling a vertical cursor and the left foot a horizontal cursor. Eleven subjects reached criterion performance. H-reflexes were recorded from the right soleus and timed to the initiation of soleus muscle activity to plantarflex the foot. Average time to complete one trial decreased from 9 sec. in the prelearning block to 4 sec. postlearning. No single learning strategy was evident as subjects varied widely in their levels of H-reflex amplitude, but during postlearning, H-reflex amplitude became consistent within subjects as tracing performance became faster and accurate.

Changes in short and long latency stretch reflexes prior to movement initiation

The amplitudes of both short (M1) and long (M2) latency stretch reflexes mechanically elicited in the triceps brachii muscle were investigated prior to its voluntary activitation. The results obtained from 14 healthy subjects indicated that the amplitude of the M2 response increased steadily as the perturbation was delivered closer to the movement suggesting that the M2 changes are associated with the preparation to respond to the visual stimulus. In contrast, the amplitude of the M1 response showed no systematic variations over the same time period. This dissociation of the M1 and M2 reflex responses prior to movement may be attributable to selective gain modulation in supraspinal stretch reflex pathways or to descending projections to polysynaptic spinal pathways.

Interaction Between Visually and Kinesthetically Triggered Voluntary Responses

A voluntary motor response that is prepared in advance of a stimulus may be triggered by any sensory input. This study investigated the combination of visual and kinesthetic inputs in triggering voluntary torque responses. When a visual stimulus was presented alone, subjects produced a fast and accurate increase in elbow flexion torque. When a kinesthetic stimulus was presented instead of the visual stimulus, subjects produced a similar response with a reduced response latency. When a visual stimulus was presented in combination with a kinesthetic stimulus, subjects initiated their responses after either a visual or a kinesthetic response latency, depending on the relative timing of the two stimuli. An analysis of response amplitude suggested that when visual and kinesthetic stimuli were combined, both stimuli triggered a response. The results are more consistent with a simple behavioral model of addition of visual and kinesthetic responses (which predicts that the response to combined stimuli should be the sum of individual responses) than with a model of exclusion of one response (which predicts that the response to combined stimuli should be identical to either the visual or the kinesthetic response). Because addition of visually and kinesthetically triggered responses produced a response with an erroneously large amplitude, it is suggested that visual and kinesthetic inputs are not always efficiently integrated.

Could parkinsonian akinesia be attributable to a disturbance in the motor preparatory process?

To test the hypothesis that parkinsonian akinesia could be due to a disturbance in motor preparatory process, we measured the extent to which the normal pattern of H-reflex excitability prior to a ballistic movement, in a simple reaction time (RT) paradigm, is modifiable by akinesia. Nine age-matched normals and 11 parkinsonians were examined. They were instructed to plantarflex their ankle rapidly in response to a visual signal (RS) following a bleep (WS), while EMGs were recorded from soleus and tibialis anterior (TA) simultaneously with ankle position. Under this 'control' condition: mean soleus EMG and movement RTs were significantly longer in parkinsonians than normals. Given no evidence of fatigue or other effects, this finding indicated that our patients were akinetic. Furthermore, the normal pattern of agonist-antagonist bursts was often delayed, reduced or prolonged in these patients. Next, we measured changes in soleus motoneuronal pool excitability at 4 predetermined intervals relative to the RS but prior to EMG onset, by means of H-reflex testing. Three findings emerged: mean soleus EMG and movement RTs were again significantly delayed in parkinsonians. However, no intersensory facilitation/inhibition of the RTs occurred between H-reflex and visual stimuli, in that these values remained unchanged within the group, despite the addition of H-test stimulation. More importantly, facilitation of H-reflex was similarly time-locked to EMG onset for the two groups, with increases in H-reflex amplitude commencing some 60 ms prior to agonist discharge. Bearing in mind the prolonged RTs in parkinsonians, these findings pointed to a delay in the facilitation of the H-reflex relative to the RS as a cause of akinesia. Our proposition that parkinsonian akinesia could be attributable to an impairment in the motor preparatory process therefore remains a tantalizing possibility.

Initial agonist burst is modified by perturbations preceding movement

Experiments were conducted to investigate the role of peripheral feedback in modulating the initial agonist burst. Subjects performed visually guided step-tracking movements about the elbow. Brief (50 ms) torque pulse perturbations were randomly applied immediately prior to movement and either opposed (load) or assisted (unload) the forthcoming movement. Load perturbations caused both components of the initial agonist burst to increase in magnitude. In contrast, unloading increased the magnitude of the first component but decreased the second component. These effects were graded with the size of the perturbation, with large perturbations causing a greater change in component magnitude than small perturbations. These results support recent evidence that the initial agonist burst is comprised of individual subunits of activity which can be independently modulated by the central nervous system prior to movement.

Spinal motor preparation in humans

This study examined the influence of preparatory 'set' on the modulation of monosynaptic reflex excitability immediately prior to the initiation of a voluntary movement. Six subjects performed 3 tasks which varied the level of preparatory set according to the availability of temporal and event anticipation. A choice reaction-time task restricted both temporal and event anticipation; a simple reaction-time task restricted temporal anticipation while allowing event anticipation; and a coincidence-timing task permitted both temporal and event anticipation. The response for all tasks performed was ankle plantarflexion. The excitability of the monosynaptic reflex was examined by eliciting H reflexes to the right soleus muscle at 50 msec intervals over the final 400 msec preceding the initiation of voluntary muscle contraction. All 3 tasks showed an initial depression and later increase in the H reflex amplitude. The onset of reflex facilitation was the same for the choice and simple reaction-time tasks, commencing 74 msec prior to voluntary muscle contraction. However, the coincidence-timing task showed an earlier onset of reflex facilitation. Reflex facilitation preceded voluntary muscle contraction by 130-337 msec during the performance of this task. Thus, it appears that the modulation of monosynaptic reflex excitability is not time-locked to the onset of voluntary muscle contraction, but rather is controlled by the preparatory set of the subject. The modulation of reflex excitability represents a separate, though necessary, stage in the organization and initiation of voluntary movement.

Acoustic facilitation of the Hoffmann reflex

The facilitative effect of a pure tone on the amplitude of the Hoffmann reflex was tested in 11 subjects under four conditions: (i) trying not to respond to the tone, (ii) voluntary abduction of the right index finger in response to the tone, (iii) voluntary plantarflexion of the left leg, and (iv) right plantarflexion in response to the tone. The acoustic stimulus was a 1000-Hz pure tone (80 dB, re 0.0002 dyne/cm2), presented on a random set of trials. Tone onset occurred 100 ms prior to the electrical stimulus that evoked the Hoffmann reflex. The Hoffmann reflex was larger for "tone" trials than for "silent" trials. The facilitative effect was equal in four cases: (i) making no response (NR), (ii) finger abduction (FA), (iii) noninvolved muscle (NIM), and (iv) involved muscle during treadle pressing (IM). The effect of the first tone presented was larger than that of subsequent tones, indicating some habituation of the acoustic effect on the H-reflex. Tests for spontaneous recovery and stimulus generalization were not significant. The results supported the existence of a purely auditory facilitation of spinal excitability.

Achilles Tendon Reflexes and Surface EMG Activity during Anticipation of a Significant Event and Preparation for a Voluntary Movement

Achilles tendon reflexes were evoked bilaterally during and shortly after an interstimulus-interval (ISI) of 4 s and expressed as percentages of an averaged control reflex. Surface EMG of the soleus muscles were recorded continuously during the ISI, and expressed as percentages of a control EMG level. Three types of tasks were introduced, according to a between subjects design. Condition I consisted of a guessing task, involving anticipation of the second stimulus (S2) and not requiring a motor response. Conditions II and III were a warned choice and simple RT task respectively, the motor response to S2 being a plantar flexion of either the left or right foot in Condition II, and a plantar flexion of the right foot in Condition III. The results can be stated as follows: 1. Anticipation of a stimulus is not sufficient for a reflex increase to occur during an ISI. Preparation for a movement seems to be a necessary condition. 2. The reflex increase during preparation is rather independent of the amount of selectivity in the preparatory process; simple and choice RT tasks yield similar results, although the mean RTs do differ. 3. A difference between the involved and non-involved legs in the simple RT task is not found in the present experiment, as opposed to other studies. The particular instruction given to the subjects could be of importance in producing this difference. 4. Reflex changes cannot simply be accounted for by changes in background EMG of the agonist, as the EMG time course shows no changes over time in either condition.

Computer feedback reveals quantal aspects of the Hoffmann reflex

The Hoffmann reflex was studied in 13 normal volunteers, using computer feedback to control stimulus intensity. The intensity of the electrical pulse that evoked the H-reflex was maintained at near threshold values by a computer program, which increased or decreased the intensity on the basis of the amplitude of the recorded response. Stimulus intensity was increased for H-responses less than, and decreased for H-responses greater than, a criterion value of 50 microV peak-to-peak. More than 500 responses were recorded from each subject during a 40-min period. The feedback procedure tended to hold the H-reflexes near the criterion value. Slow drifts in the required current indicated waxing and waning of threshold. Examination of response amplitude distributions showed two different types. For the nonquantal type, all possible values were represented in the range zero to 200 microV. Responses in the upper half of that range were less frequent than those in the lower half. For the quantal type, the majority of the responses were clustered near zero and at an amplitude that was characteristic for a given recording configuration; very few responses of intermediate amplitude were observed. The quantal pattern of response was ascribed to the firing of low-threshold single motor units.

Effects of electrical stimulation of low-threshold muscle afferents on visual reaction time

Human subjects performed plantar flexions of the foot in visual reaction time (RT) situations. In the simple RT task, the same foot had to be moved during the whole session, in the choice RT task, the right or left foot was moved in a random sequence. Low-threshold muscle afferents which evoked a monosynaptic H reflex were electrically activated on both sides at various intervals after the onset of the light stimulus. In both RT situations. RT was shortened equally by the electrical stimulation. The intersensory facilitation in the choice RT task provided support for the preparation enhancement model, since the subjects could not react to the electrical stimulus alone.

Relation between the specific H reflex facilitation preceding a voluntary movement and movement parameters in man

In a reaction-time situation, the monosynaptic spinal reflex (H reflex) is facilitated before the onset of an electromyographic (e.m.g.) response. The aim of the present investigation was to study aspects of this facilitation. Human subjects were required to perform isometric plantarflexions of the foot in response to a visual stimulus. The movement was always on the same side in the simple reaction-time situation, and randomly with the right or left foot in the choice reaction-time situation. Stimuli to evoke H reflexes were applied bilaterally 40-400 ms after the onset of the visual stimulus. Pre-motor time, i.e. the interval between the onset of the visual stimulus and the e.m.g. response, and reaction time, i.e. the interval between the onset of the visual stimulus and the response on the torque recording, were computed. In both reaction-time situations, there was a significant facilitation of the ipsilateral H reflex 100-160 ms before e.m.g. onset and, in some subjects, a small facilitation of the contralateral H reflex. The specific facilitation, i.e. the difference between the facilitation on the ipsi- and contralateral side relative to the movement, was not significantly different on the right and left side. Pre-motor time was divided into the interval from the light onset until the onset of the specific facilitation, and the interval from the onset of the facilitation until the onset of the voluntary response. Both intervals increased, and the slope and the amplitude of the facilitation decreased with increasing pre-motor time and reaction time. The specificity of the H reflex facilitation in a choice reaction-time situation implies that the interval from light onset until the onset of the facilitation includes stimulus identification and response selection, and the interval from the onset of the facilitation until the e.m.g. response preparation of the motor system for the required movement. The present results suggest that the specific facilitation of the H reflex before a movement is caused by removal of presynaptic inhibition at I a terminals or by activation of interneurones intercalated in polysynaptic components of the H reflex rather than by a subthreshold activation of motoneurones.

Surface EMG and Achilles tendon reflexes during a foot movement in a reaction time task

A forewarned simple visual reaction time (RT) experiment was conducted to study the relation between surface EMG of the soleus muscle and Achilles tendon reflex amplitudes during the initiation of a voluntary plantar flexion of the right foot. In two conditions the tension of the response-involved soleus muscle was manipulated: (1) gradual isometric contraction towards the end of the foreperiod (FP) and (2) complete relaxation during the FP. Gradual contraction of the involved soleus muscle produced faster RTs. It was found in both conditions that reflex amplitudes and EMG of the involved soleus muscle were equally informative about the growing motor excitation during the execution of the response. In the tension condition reflex amplitudes in the involved leg were smaller than the reflex amplitudes in the non-involved leg prior to the initiation of the movement, 0-50 msec after the expiration of the FP. The EMG, however, showed a reversed involved/non-involved difference in this condition. In the relaxation condition, neither the EMG nor the reflex amplitudes showed a significant involved/non-involved difference. It was concluded from these results that changes in Achilles tendon reflex amplitudes should only cautiously be interpreted as an indication of motor preparation.

Interaction between the long-latency stretch reflex and voluntary electromyographic activity prior to a rapid voluntary motor reaction

The size of the long-latency component of the stretch reflex has been examined in the time interval between a signal to move and the required rapid voluntary contraction of triceps and flexor pollicis longus in 8 normal subjects. Bilateral movements of the elbow and thumb were made following an auditory signal. In 50% of the trials a torque pulse was applied unilaterally in order to elicit a stretch reflex response in one arm. The voluntary response in the contralateral arm was uncorrupted by a stretch reflex response, so was used as an indicator of voluntary reaction time. Control experiments, using an electrical stimulus to the fingers rather than muscle stretch, verified that both arms reacted almost simultaneously to the auditory cue, even when the reaction time was shortened by the presence of a unilateral electrical stimulus. Similarly, an interposed muscle stretch stimulus considerably reduced the reaction time to the audio signal. Because of this, the start of the voluntary EMG response frequently 'over-lapped' the end of the long-latency stretch reflex. Failure to take this shortening of voluntary reaction time into consideration can lead to the erroneous conclusion that reflex gain is increased prior to a rapid movement. If the 'overlap' of EMG responses is accounted for, very little change in the size of the long-latency stretch reflex is evident prior to activation of the muscles responsible for the movement of either the elbow or the thumb.

Early selective-attention effects on the evoked potential: a critical review and reinterpretation

Recent research on the effect of selective attention on the N1, component of the evoked potential is reviewed. These studies are based on the finding of Hillyard Hink, Schwent and Picton (1973) that this component is selectively enhanced in response to attended stimuli when a very rapid rate of stimulus delivery is used. On the basis of the subsequent set of experiments, the conditions and limits of the existence of the auditory 'N1 effect' are now quite clear. Moreover, this finding has been extended to somatosensory and visual modalities. In the present review a detailed examination of these studies has suggested a re-interpretation of the N1 effect. According to this reinterpretation, it is not a 'true' N1 component which is enhanced but the effect is produced by a summation of a negative shift with the evoked-potential wave form. Under some conditions such as those involving a very fast rate of stimulus delivery, this effect commences very early, making the N1 component appear larger. It is suggested that this shift reflects orienting to, and further processing of, an input found relevant in a preliminary sensory analysis. Topographical evidence for this kind of interpretation is provided by several studies. This negative shift is, hence, associated with voluntary attention. Some of the reviewed studies have described a rather similar negative shift as a correlate of involuntary attention to rare stimuli among the much more frequent, 'standard', stimuli.