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Trunk stability is essential to maintain upright posture and support functional movements. In this study, we aimed to characterize the muscle activity and movement patterns of trunk flexion during an arm reaching task in sitting healthy subjects and investigate whether trunk stability is affected by a startling acoustic stimulus (SAS). For these purposes, we calculated the electromyographic (EMG) onset latencies and amplitude parameters in 8 trunk, neck, and shoulder muscles, and the tilt angle and movement features from smartphone accelerometer signals recorded during trunk bending in 33 healthy volunteers. Two-way repeated measures ANOVAs were applied to examine the effects of SAS and target distance (15 cm vs 30 cm). We found that SAS markedly reduced the response time and EMG onset latencies of all muscles, without changing neither movement duration nor muscle recruitment pattern. Longer durations, higher tilt angles, and higher EMG amplitudes were observed at 30 cm compared to 15 cm. The accelerometer signals had a higher frequency content in SAS trials, suggesting reduced movement control. The proposed measures have helped to establish the trunk flexion pattern in arm reaching in healthy subjects, which could be useful for future objective assessment of trunk stability in patients with neurological affections. [ABSTRACT FROM AUTHOR]

The excitability of brainstem circuitries mediating defensive blinking in response to abrupt sensory inputs is continuously modulated by cortical areas, e.g., the hand-blink reflex (HBR), elicited by intense electrical median nerve stimulation, is enhanced when the stimulated hand is close to the face, with the behavioural purpose to optimize self-protection from increased threat. Here we investigated whether such cortically mediated HBR facilitation can be influenced by prepulse inhibition (PPI), which is known to occur entirely at the subcortical level. Twenty healthy volunteers underwent HBR recordings in five experimental conditions. In conditions 1 and 2, the stimulated hand was held either near (1) or far (2) from the face, respectively. In conditions 3 and 4, stimulation of the hand near the face was preceded by a peri-liminal prepulse to the index finger of the contralateral hand held either near (3) or far from the face (4). In condition 5, participants self-triggered the stimulus eliciting the HBR. We observed a reproducible HBR in 14 out of 20 participants and measured onset latency and area of the HBR in orbicularis oculi muscles bilaterally. HBR area decreased and latency increased in condition 2 relative to condition 1; HBR area decreased and latency increased markedly in condition 3, and somewhat less in condition 4, relative to conditions 1 and 2; self-stimulation (condition 5) also suppressed HBRs, but less than prepulses. These findings indicate that PPI of the HBR is more robust than the cognitive modulation exerted by top-down cortical projections. Possibly, an attentional shift to a prepulse may serve to reduce blinking in response to perturbation when it is convenient, in a given situation, not to interrupt ongoing visual processing. [ABSTRACT FROM AUTHOR]

Modulation of spinal reflexes depends largely on the integrity of the corticospinal tract. A useful method to document the influence of descending tracts on reflexes is to examine the effects of transcranial magnetic stimulation (TMS) on the soleus H reflex elicited by posterior tibial nerve electrical stimuli (PTS). In 12 healthy volunteers, we investigated how postural or voluntary muscle contraction modified such descending modulation. We first characterized the effects of TMS at 95 % of motor threshold for leg responses on the H reflex elicited by a preceding PTS at inter-stimuli intervals (ISIs) between 0 and 120 ms at rest and, then, during voluntary plantar flexion (pf), dorsal flexion (df), and standing still (ss). During pf, there was an increase in the facilitation of the H reflex at ISIs 0-20 ms. During df, there were no effects of TMS on the H reflex. During ss, there was inhibition at ISIs 40-60 ms. Our observations suggest that muscle contraction prevails over the baseline effects of TMS on the soleus H reflex. While contraction of the antagonist (df) suppressed most of the effects, contraction of the agonist had different effects depending on the type of activity (pf or ss). The characterization of the interaction between descending corticospinal volleys and segmental peripheral inputs provides useful information on motor control for physiological research and further understanding of the effects of spinal cord lesions. [ABSTRACT FROM AUTHOR]

We analyzed the effects of an anesthetic sciatic nerve block on the cutaneomuscular reflex (cMR) and the cutaneous silent period (cSP) of foot muscles, in order to investigate further the type of fibers involved in their generation. In 14 neurologically normal patients with indication for surgical treatment of hallux valgus, we recorded from the extensor digitorum brevis muscle the reflex responses elicited by high-intensity electrical stimulation of the big toe at various time periods, ranging from 0 to 20 min, after ultrasound-guided sciatic nerve popliteal anesthetic block. The first effect was a delay in cSP onset latency, with no changes in end latency. The cMR remained unaltered up to when subjects were no longer able to maintain the contraction. The effects of local anesthetics on peripheral nerves allow for recognition of the different types of fibers contributing to the cMR and the cSP in muscles of the lower limb. [ABSTRACT FROM AUTHOR]

Numerous studies have detailed involvement of the peripheral autonomic nervous system (PANS) in Parkinson's disease (PD). We assessed autonomic innervation of dermal annexes through quantitative fluorescence measurement from skin obtained via punch biopsies at distal leg region in PD and control subjects. We defined a ratio between the area corresponding to protein gen product (PGP) immunoreactivity and the area corresponding to blood vessel or sweat gland as a quantitative measure of autonomic innervation. Presence of alpha-synuclein (AS) deposits in dermis and hypodermis was also assessed by immunohistochemistry. Skin biopsies form six PD patients and six healthy controls were studied. Autonomic innervation scores were lower in PD than in controls in both blood vessels and sweat glands. No AS or phosphorylated AS (pAS) immunoreactivity was detected in dermis or hypodermis in any of the studied subjects. The results of this investigation suggest that autonomic innervation of dermal annexes in living patients with PD is reduced compared to controls. AS or pAS deposits were not found in dermis or hypodermis suggesting that distal leg skin study is not useful for in vivo detection of AS in PD. [ABSTRACT FROM AUTHOR]

The study of body representation and ownership has been a very active research area in recent years. Synchronous multisensory stimulation has been used for the induction of the illusion of ownership over virtual body parts and even full bodies, and it has provided experimental paradigms for the understanding of the brain processing of body representation. However, the illusion of ownership of a virtual body has rarely been used for patient evaluation and diagnosis. Here we propose a method that exploits ownership of a virtual body in combination with a simple brain computer interface (BCI) and basic physiological measures to complement neurological assessment. A male patient presenting a fixed posture dystonia featuring a permanently closed left fist participated in this case study. The patient saw a virtual body that substituted his own after donning a head-mounted display and thereby entering the virtual reality. The left virtual hand had the same posture as his corresponding real hand. After inducing virtual hand ownership by correlated visuo-tactile stimulation and dynamic reflections in a virtual mirror, the virtual hand would open either automatically or through a cognitive task assessed through a BCI that required him to focus attention on the virtual hand. The results reveal that body ownership induced changes on electromyography and BCI performance in the patient that were different from those in five healthy controls. Overall, the case study shows that the induction of virtual body ownership combined with simple electrophysiological measures could be useful for the diagnosis of patients with neurological conditions. [ABSTRACT FROM AUTHOR]

A single-pulse transcranial magnetic stimulus (TMS) may induce contraction in many muscles of the body at the same time. This is specially the case when using the double-cone coil to obtain the motor evoked potentials in leg muscles. Even if intensity is kept below threshold for the soleus muscle, TMS induces facilitation of the soleus H reflex that is separated into two phases: the first, peaking at 10-20 ms and the second, peaking at 70-90 ms. We investigated the possibility that TMS-induced facilitation of the H reflex was related, at least in part, to the reafferentation volley reaching the alpha motoneuron after synchronized contraction of other muscles in the body. To test this hypothesis, we examined the effects of vibration on the TMS-induced facilitation of the soleus H reflex. As expected, vibration applied over the triceps tendon caused a significant reduction in H reflex amplitude: 42.4 ± 6.4 % of control values. When conditioned by TMS at intervals corresponding to the first phase, the H reflex was facilitated to the same extent in both conditions: with and without vibration. However, at intervals corresponding to the second facilitation phase, there was a significantly reduced facilitation with vibration. These differential effects of vibration on the two phases of the TMS-induced facilitation of the H reflex indicate a different mechanism for each facilitation phase. The first phase could result from direct corticospinal excitatory input, while the second phase might depend on inputs via Ia afferents from heteronymous muscles. [ABSTRACT FROM AUTHOR]

Over the past two decades, the H reflex has been used as a neural tool to assess the effect on the motoneuronal pool of conditioning volleys in supraspinal descending tracts elicited by transcranial magnetic stimulation (TMS) or auditory stimuli. However, mechanisms mediating such modulation are unclear. These hypothesized neural pathways are likely to be affected by single electrical stimulus applied through the electrodes implanted in the subthalamic nucleus for deep brain stimulation (sSTNDBS). To improve our knowledge on such mechanisms, we examined in 11 Parkinson's disease patients the effects of conditioning sSTNDBS applied contralateral and ipsilateral to the H reflex recording on the amplitude of the soleus H reflex, at interstimulus intervals (ISIs) between 0 and 110 ms. There was a significant main effect of the ISI ( P < 0.001) and of the sSTNDBS stimulation side ( P = 0.019) on the percentage change in the soleus H-reflex amplitude. Contralateral sSTNDBS modulation of the soleus H reflex resembles that of TMS in healthy subjects with two facilitation phases (at 5-20 ms and at 60 ms), while after ipsilateral sSTNDBS, there is only a single facilitation phase peaking up at 5 ms later than the first facilitation period observed with contralateral stimulation. These findings contribute to the discussion of the mechanisms underlying the excitability of the spinal alpha motoneuron pool and the modulation of the H reflex by supraspinal stimuli. [ABSTRACT FROM AUTHOR]

Unexpected sensory inputs can generate a patterned startle reaction, aimed at protection and defense. Experimentally, it is usually triggered by auditory stimuli while the startle reaction to somatosensory inputs (SSS) has not received much attention so far. This may be in part due to the fact that somatosensory inputs inevitably cause local reactions, such as short and long latency reflexes and withdrawal reactions, which could interfere with recognition of the startle-related activity. Therefore, we have undertaken a study aimed at separating the SSS from other responses by exploring the responses that are common to somatosensory stimuli applied to different sites and examining the inhibitory effects of prepulse stimuli. In 13 healthy naive subjects, we applied electrical stimuli to the median nerve at the wrist (MW) or the posterior tibial nerve at the ankle (PT) and recorded from orbicularis oculi (OOC), masseter (MAS), sternocleidomastoid (SCM) and representative muscles of the limbs being stimulated (flexor carpi radialis for MW and tibialis anterior for PT). In random trials, we also applied prepulse stimuli, either a low-intensity auditory stimulus or low-intensity electrical stimuli, 100 ms before the SSS-eliciting stimulus. The pattern of SSS was different for upper and lower limb stimuli. While stimuli applied to MW induced a prominent reaction of the OOC, at a mean latency of 61.1 ms (SD = 16.3 ms), followed by the SCM at a mean latency of 83.3 ms (SD = 28.6 ms), those applied to the PT caused a small or absent response in the OOC and a consistent response of the SCM at a mean latency of 89.7 ms (SD = 30.1 ms). Prepulse stimuli effectively inhibited the responses of facial and neck muscles but only partially those of the wrist flexors to MW or the tibialis anterior to PT. Our results indicate that, although there are common neck and facial muscle reactions to somatosensory stimuli applied to upper and lower limbs, the pattern of the SSS differs according to the source of the input. Prepulse inhibition is more effective on the responses of neck and facial muscles than on the responses of limb muscles to somatosensory stimuli. These results could help in distinguishing between withdrawal and SSS reactions. [ABSTRACT FROM AUTHOR]

Nociceptive suppression of tonic voluntary electromyographic (EMG) activity in human hand muscles (cutaneous silent period, CSP) is in its functional organization consistent with a spinal protective reflex. Motoneuronal excitability and its modulation may also be investigated by conditioned motor evoked potentials (MEPs). To date, effects of exteroceptive stimuli on tonic EMG and on MEPs have been compared mainly using innocuous stimuli, while noxious stimuli have not been studied in great detail. In ten subjects, we recorded CSPs induced in volitionally activated flexor pollicis brevis muscle (FPB) by noxious digit II (D2) stimulation, and in first dorsal interosseous muscle (FDI) following noxious D2 and digit V (D5) stimulation. Then, transcranial magnetic stimulation (TMS) was used to evoke MEPs in the same hand muscles at rest—conditioned by equal noxious D2 or D5 stimulation and individually delayed—so that the MEPs occurred at times corresponding to immediately before, during, and immediately after the CSP in each subject. Immediately before the CSP, there was no significant difference between nociceptive MEP modulation and tonic EMG modulation in any muscle–finger-combination. In the middle of the CSP, noxious finger stimulation exerted suppression of TMS-induced MEPs in all the three muscle–finger-combinations, but less so as compared to corresponding tonic EMG levels. After the CSP, MEPs remained suppressed when corresponding tonic EMG levels were significantly enhanced. Notably, MEPs were also suppressed in cases when occurring at times corresponding to the excitatory long-loop reflex. Incomplete MEP suppression during the CSP may allow for an “emergency grip” even during noxious stimulation. MEP suppression outlasting the CSP is compatible with a “passive” re-synchronization of volitionally activated motor units rather than an “active” reflex involving recruitment of corticospinal motoneurons. The differences in tonic EMG and MEP modulation favors an effect of noxious digital nerve stimulation on interneurons responsible for presynaptic inhibition rather than a postsynaptic inhibitory effect on the motoneuron pool. The present findings caution against the use of nociceptive MEP modulation at rest to substitute for tonic EMG modulation as tested in CSP studies. [ABSTRACT FROM AUTHOR]

The startle reaction (SR) is usually understood as an involuntary reaction to an unexpected sensory input. However, there is evidence that the mechanisms involved in the SR contribute also to the execution of actions that we consider voluntary. We need to exert a fine control of the motor output, which usually involves inhibition of the reflex reaction but it may also imply letting the SR manifest, shaped in accordance with our willed actions. The latter occurs mainly with relatively simple ballistic movements but it has also been shown to occur in complex movements. In this review, we describe arguments published in the literature in favour of the contribution of subcortical motor tracts involved in the generation of the SR to the execution of voluntary movements. Some of these studies have brought insight on the mechanisms accounting for motor preparation and execution of voluntary movements. [ABSTRACT FROM AUTHOR]

Central nervous system reorganization following spinal cord injury (SCI) may cause functional changes in the motor tracts in patients in whom increased auditory startle responses (ASRs) have been previously reported. We hypothesized that if increased ASRs in patients with incomplete SCI were due to compensatory mechanisms, these changes would be related to severity and/or localization of the lesion. We examined ASR characteristics in 29 SCI patients and 14 age-matched healthy volunteers. Fourteen patients had incomplete and 15 complete SCI; 10 patients had cervical and 19 thoracolumbar SCI. Five auditory stimuli were applied binaurally to subjects in a sitting position, with a 5-min interstimulus interval. Surface electromyographic recordings were obtained from orbicularis oculi (OOc), sternocleidomastoid (SCM), biceps brachii (BB), and tibialis anterior (TA) muscles. ASR probability was significantly higher and area-under-the-curve was significantly larger in SCM and BB in patients than in controls. ASR latency was significantly shorter in SCM and BB in patients with cervical than in those with thoracolumbar SCI (p < 0.02), but there were no statistically significant differences between complete and incomplete SCI (p > 0.1). Time span since onset correlated significantly with ASR area in OOc, SCM and BB (p < 0.05). The capability of the adult central nervous system to reorganize its circuits over time for improved functionality following injury is probably the key to understanding the increased ASRs in patients with SCI. The exaggeration of the startle reflex is potentially important since it may be useful for augmenting voluntary movement in the clinical rehabilitation of patients with SCI. [ABSTRACT FROM AUTHOR]

Simple ballistic movements are executed faster in reaction time task paradigms when the imperative signal (IS) is accompanied by a startling auditory stimulus (SAS). We examined whether this effect also occurs in complex movements such as the sit-to-stand manoeuvre (STS). Nine healthy volunteers performed STS to visual IS, either presented alone (control trials) or together with SAS (test trials). Reaction time, measured as the time between IS and take-off (TO), was significantly shortened in test trials when SAS was applied at an interval of 0 ms with respect to IS. The onset latency of EMG bursts recorded from tibialis anterior, lumbar paraspinal, quadriceps and biceps femoris muscles reduced proportionally to the shortening of TO. However, these effects were not observed if SAS was delivered 150 ms after IS, when the manoeuvre had already started. Our results suggest that stimuli acting on subcortical motor structures speed-up but do not otherwise interfere with the execution of the motor programs underlying the STS manoeuvre. [ABSTRACT FROM AUTHOR]

Quantitative sensory testing (QST) is a subjective method of assessing thermal sensation, but it does not provide an objective measure of physiological changes. Our aim was to establish whether fluctuations in sudomotor autonomic activity correlate with warmth and heat pain (thermoalgesic) perception. We conducted various thermal stimuli protocols in 22 healthy volunteers and recorded the visual analog scale (VAS) and sudomotor activity using surface electrodes attached to the hand. Sympathetic skin responses (SSR) and the mean level of electrodermal activity (EDA) were measured. EDA was analyzed by looking at the four VAS-based phases, which were divided into segments defined by the relevant psychophysical markers. In all stimuli paradigms, the mean delay time of the SSR was 1.6 s after the warmth or pain sensation. There was an association between the low amplitude (predominantly negative) SSR and warm stimuli, and the large amplitude (predominantly positive) SSR and heat pain stimuli (chi-square; P < 0.05). Mean EDA was significantly higher during the pain phase in comparison with pre-perception, warmth and post-perception phases. Thermoalgesic stimuli induce reflex changes in sudomotor activity that correlate with subjective perception of warmth and heat pain sensations. This association may be useful in clinical practice. [ABSTRACT FROM AUTHOR]

The control of eye movements depends in part on subcortical motor centres. Gaze is often directed towards salient visual stimuli of our environment with no conscious voluntary commands. To further understand to what extent preprogrammed eye movements can be triggered subcortically, we carried out a study in normal volunteers to examine the effects of a startling auditory stimulus (SAS) on externally guided saccades. A peripheral visual cue was presented in the horizontal plane at a site distant 15° from the fixation point, and subjects were instructed to make a saccade to it. SAS was presented together with the peripheral visual cue in 20% of trials. To force rapid visual fixation at the end of the saccade, targets were loaded with a second cue, a small arrow pointing towards the right or the left (or a neutral sign), not distinguishable with peripheral vision. Subjects were requested to perform a flexion/extension wrist movement, according to the direction of the arrow (or not to move if the second cue was the neutral sign). SAS presented together with the visual target caused a significant shortening of the latency of saccadic movements. The wrist movements performed as a response to the second cue had similar reaction times regardless of whether the trial contained a SAS or not. Our results show that voluntary saccades to peripheral targets are speeded up by activation of the startle circuit, and that this effect does not cause a significant disturbance in the execution of simple in-target cues. These results suggest that subcortical structures play a main role in preparation of externally guided saccades. [ABSTRACT FROM AUTHOR]

Studies with transcranial electrical or magnetic stimulation have shown a progressive increase of motor cortex excitability beginning at about 80 ms before the onset of electromyographic (EMG) activity in a voluntary movement. We studied whether a similar increase in excitability occurs in subcortical motor tracts before execution of a ballistic movement. In ten healthy volunteers, we examined the effects of a startling auditory stimulus (SAS) applied at various intervals following the imperative signal (IS) in a reaction time task experiment. We hypothesized that, if the excitability of the reticulospinal tract increases before onset of muscle activity, there would be a corresponding change in the size of the startle response elicited either in muscles responding to the SAS, the orbicularis oculi (OOc) and sternocleidomastoid (SCM), or in the agonist muscle for the reaction. A SAS was applied at intervals of 0, 20, 40, 60, 80, and 100 ms after IS in a reaction time task paradigm in which subjects had to perform a ballistic movement of wrist flexion. We measured the size of the EMG bursts recorded in the OOc and the SCM, as well as the size of the first EMG burst in the wrist flexors (WF). Comparisons were done with data obtained in baseline trials, in which unexpected SAS of the same intensity were delivered without preparation, and control trials, in which subjects performed the same motor task with no SAS. The size of the averaged OOc, SCM, and WF were larger in trials with SAS than in baseline or control trials, with significant differences in the SCM and in the WF ( p<0.05). However, there were no effects of time interval on the size of OOc, SCM, or WF ( p>0.05). These results indicate that, in the execution of a ballistic movement, the excitability of the subcortical motor tracts activated by a SAS is similarly enhanced at the time of the IS and 100 ms afterwards, just before the onset of EMG activity. We conclude that, in contrast with the reported progressive increase of excitability in the corticospinal tract, the excitability of the tracts activated by a SAS do not change between the IS and the onset of EMG activity. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Muscle hyperactivity can be a clinical feature on its own or, more commonly, an observation on electromyography (EMG) examinations. Whatever manifestation it takes,muscle hyperactivity always means enhanced excitability of muscle, axons or neurons. Clinical findings may be variable, ranging from fasciculations to muscle cramps. Even though clinical examination may lead in most instances to suggest the diagnosis of the underlying disease, EMG studies are necessary to identify the type of abnormal discharges and suggest the site of their suspected origin. Although in clinical studies, the action potential showing abnormal muscle hyperactivity is practically always recorded from muscle fibers, the site in which the impulse has arisen will determine its shape and firing patterns. In this review, we describe the EMG characteristics observed in syndromes featuring muscle hyperactivity and the pathophysiology underlying the abnormal firing of muscle fibers. [ABSTRACT FROM AUTHOR]

The excitability of spinal motoneurons is modified by central preparatory commands before muscle activation. In relatively complex long duration motor tasks such as the sit-to-stand (STS) movement, the central nervous system commands have to take into account the inputs from muscle, skin, and joint afferents during muscle contraction. We have investigated the changes occurring in tibialis anterior (TA) and soleus (SOL) motoneuronal excitability prior to and during the STS movement in normal subjects. Twelve healthy volunteers received the instruction to rise from a chair at the perception of an acoustic 'go' signal. Cortical transcranial magnetic stimuli (TMS) or peripheral nerve electrical stimuli (PNS) were used as test stimuli to elicit, respectively, the motor evoked potential (MEP) and the H reflex, at intervals of 50–1500 ms after the 'go' signal. Both the MEP and the H reflex were enhanced in the TA between 100 and 900 ms after the 'go' signal. At the same time there was inhibition of the H reflex but not of the MEP in the SOL. At the end of the STS movement, during quiet standing, the size of both the H reflex and the MEP of the TA were not different from those obtained in the sitting position. However, in SOL, the H reflex was smaller, and the MEP was larger, than at rest. Our observations suggest the participation of several mechanisms of control of motoneuronal excitability during the STS, ultimately leading to a dominant role of presynaptic inhibitory mechanisms in SOL during standing. [ABSTRACT FROM AUTHOR]

Fatigue of voluntary muscular effort is a complex and multifaceted phenomenon. Fatigue of peripheral nervous system components, including the contractile apparatus and the neuromuscular junction, has been well studied. Central nervous system components also fatigue, but studies have lagged for want of objective methods. Transcranial magnetic stimulation is a relatively new technique that can be used to assess central nervous system excitability from the motor cortex to the alpha-motoneuron. In six normal volunteers, including four of the investigators, the amplitudes of motor evoked potentials elicited by transcranial magnetic stimulation were transiently decreased after exercise, indicating fatigue of motor pathways in the central nervous system. The decrease in amplitude was associated with a feeling of fatigue. The mechanism of this phenomenon is apparently decreased efficiency in the generation of the motor command in the motor cortex. [ABSTRACT FROM AUTHOR]