Modulation of corticospinal excitability and intracortical inhibition during motor imagery is task-dependent

Previous studies have clearly shown that motor imagery modulates corticospinal excitability. However, there is no clear evidence for the modulation of intracortical inhibition (ICI) during imagined task performance. The aim of this study was to use transcranial magnetic stimulation (TMS) to assess changes in corticospinal excitability and ICI during the imagined performance of two types of task. In Experiment 1, eight subjects performed phasic depression of a computer mouse button using the dominant index finger in time with a 1 Hz auditory metronome. Single and paired pulse magnetic stimuli were delivered at rest, and during the on and off phases of actual and imagined task performance. Motor evoked potentials (MEPs) were recorded from FDI and APB. In Experiment 2, eight subjects performed phasic isometric abduction of the dominant thumb in time with a 1 Hz auditory metronome. As before, single and paired pulse magnetic stimuli were delivered at rest, and during the on and off phases of actual and imagined task performance. In both experiments, the conditioning stimulus intensity was set to produce 50% inhibition at rest, to enable both increases and decreases in ICI during task performance to be detected. No significant temporal or spatial modulation of MEP amplitude or ICI was observed in Experiment 1. In contrast, MEP amplitude was significantly greater, and ICI significantly lower during the on phase of imagined task performance in Experiment 2. These results are most likely related to the higher levels of target muscle activation required during actual task performance and the greater anatomical distance between target and control muscles in Experiment 2. These task characteristics may influence the observed degree of corticospinal excitability and ICI modulation.     

Imagined and actual arm movements have similar durations when performed under different conditions of direction and mass

Several experiments have suggested that similar physiological substrates are involved in movement execution and motor imagery, and that the same laws of movement control apply to both processes. Using a mental chronometry paradigm, we examined the effects of movement direction and added mass on the duration of actual and imagined movements. Six subjects executed or imagined arm movements in the sagittal and horizontal plane, in three different loading conditions: without added mass, and with an added mass of 1 and 1.5 kg. The duration of both actual and imagined movements was measured by an electronic stopwatch. The actual movements were significantly increased in duration as a function of mass, for both movement directions. However, direction per se had no effect on duration. The duration of imagined movements was very similar to that of actual movements whatever the subject and mass and direction condition. These results show that both inertial and gravitational constraints are accurately incorporated in the timing of the motor imagery process, which appears therefore to be functionally very close to the process of planning and performing the actual movement.     

Dynamic changes in corticospinal excitability during motor imagery

 We investigated temporal changes in the amplitudes of motor-evoked potentials (MEPs) induced by transcranial magnetic stimulation over the left motor cortex during motor imagery. Nine subjects were instructed to imagine repetitive wrist flexion and extension movements at 1 Hz, in which the flexion timing was cued by a tone signal. Electromyographs (EMGs) were recorded from the first dorsal interosseous, flexor carpi radialis and extensor carpi radialis muscles of the right hand, and magnetic stimulation was delivered at 0, 250, 500 and 750 ms after the auditory cue. On average, the evoked EMG responses were larger in the flexor muscle during the phase of imagined flexion than during extension, whilst the opposite was true for the extensor muscle. There were no consistent changes in the amplitudes of MEPs in the intrinsic hand muscle (first dorsal interosseous). The EMG remained relaxed in all muscles and did not show any significant temporal changes during the test. The H-reflex in the flexor muscle was obtained in four subjects. There was no change in its amplitude during motor imagery. These observations lead us to suggest that motor imagery can have dynamic effects on the excitability of motor cortex similar to those seen during actual motor performance. Received: 23 July 1998 / Accepted: 26 October 1998     

Temporal features of imagined locomotion in normal aging

Motor imagery is the ability to mentally simulate a movement without executing it. Previous investigations have reported a deterioration of this ability during complex arm movements in aged adults. In the present study, we aimed to extend these findings by investigating the temporal features of imagined precision gait in healthy elderly adults. Locomotion is a unique example of imagined movement because it involves simulated full-body movement and the concurrent updating of environmental spatial information. Nine young and nine older adults actually or mentally walked (walking distance: 5 m) along three paths having different widths (15 cm, 25 cm, and 50 cm). The narrowest path required balance control and accurate foot placement. We used the mental chronometry paradigm, notably the temporal similarity between actual and imagined movements, as an indicator of the accuracy of the motor imagery process. Our findings indicated that while motor imagery ability was preserved in the young group whatever the width of the path, it was significantly deteriorated in the elderly group. Aged adults systematically overestimated the duration of imagined movements with respect to those of executed movements. Moreover, paths width negatively influenced the motor imagery performances in the elderly group. We assume that motor imagery decline may reflect functional changes in the aging brain, and could be a clinical tool to detect deteriorations in motor planning and prediction in aged adults.     

Corticospinal facilitation during first and third person imagery

Motor imagery can be defined as the covert rehearsal of movement. Previous research with transcranial magnetic stimulation (TMS) has demonstrated that motor imagery increases the corticospinal excitability of the primary motor cortex in the area corresponding to the representation of the muscle involved in the imagined movement. This research, however, has been limited to imagery of oneself in motion. We extend the TMS research by contrasting first person imagery and third person imagery of index finger abduction-adduction movements. Motor evoked potentials were recorded from first dorsal interosseous (FDI) and abductor digiti minimi (ADM) during single pulse TMS. Participants performed first and third person motor imagery, visual imagery, and static imagery. Visual imagery involved non biological motion while static imagery involved a first person perspective of the unmoving hand. Relative to static imagery, excitability during imagined movement increased in FDI but not ADM. The facilitation in first person imagery adds to previous findings. A greater facilitation of MEPs recorded from FDI was found in third person imagery where the action was clearly attributable to another person. We interpret this novel result in the context of observed action and imagined observation of self action, and attribute the result to activation of mirror systems for matching the imagined action with an inner visuo-motor template.     

Facilitation of cortically evoked potentials with motor imagery during post-exercise depression of corticospinal excitability

This study examined whether muscle fatigue alters the facilitatory effect of motor imagery on corticospinal excitability. We aimed to determine if post-exercise depression of potentials evoked magnetically from the motor cortex is associated with alterations in internally generated movement plans. In experiment 1, motor-evoked potentials (MEPs) were recorded from two right hand and two right forearm muscles, at rest and during motor imagery of a maximal handgrip contraction, in eight neurologically normal subjects, before and after a 2-min maximal voluntary handgrip contraction. Resting MEP amplitude was facilitated by motor imagery in three of the four muscles, but consistently only in two. Motor imagery also reduced the trial-to-trial variability of resting MEPs. Following the exercise, resting MEP amplitude was depressed reliably in only one muscle engaged in the task, although two other muscles exhibited some depression. Motor imagery MEPs were smaller after exercise, but the degree of facilitation compared to the rest MEP was unchanged. In experiment 2, TMS intensity was increased after exercise-induced MEP depression so that the MEP amplitude matched the pre-exercise baseline. The amplitude of the MEP facilitated with motor imagery was not altered by MEP depression, nor was it increased when the TMS intensity was increased. These results suggest, at least with a simple motor task, that while post-exercise depression reduces corticospinal excitability, it does not appear to significantly affect the strength of the input to the motor cortex from those areas of the brain responsible for the storage and generation of internal representations of movement.     

Motor imagery beyond the joint limits: A transcranial magnetic stimulation study

The processes and neural bases used for motor imagery are also used for the actual execution of correspondent movements. Humans, however, can imagine movements they cannot perform. Here we explored whether plausibility of movements is mapped on the corticospinal motor system and whether the process is influenced by visuomotor vs. kinesthetic-motor first person imagery strategy. Healthy subjects imagined performing possible or biomechanically impossible right index finger movements during single pulse TMS of the left motor cortex. We found an increase of corticospinal excitability during motor imagery which was higher for impossible than possible movements and specific for the muscle involved in the actual execution of the imagined movement. We expand our previous action observation studies, suggesting that the plausibility of a movement is computed in regions upstream the primary motor cortex, and that motor imagery is a higher-order process not fully constrained by the rules that govern motor execution.     

Single fiber electromyography. A method for the evaluation of motor axonopathy during toxicity studies in dogs

In man, single fiber electromyography is used as a very sensitive indicator for the assessment of functional changes in motor nerves. The purpose of the present study was to adapt the above testing procedure to allow repeated investigations of dogs used in subchronic toxicity tests.Experiments were performed on anesthetized purebred beagle dogs. Action potentials from single muscle fibers in response to electrical stimulation of motor nerves were recorded with Medelec microelectrodes, amplified with a Medelec system and monitored on a Tektronix oscilloscope. Repeated clectrical stimulation with pulses of 0.03 msec and 1 p.p.s. produced characteristic action potentials of single muscle fibers consisting of a positive, followed by a negative, deflection having a duration of from 500 to 800 sec altogether. Successive potentials occurred with a variable latency (the jitter) ranging from±5 to 15 sec.Quantitative evaluation of the jitter will allow the clinical evaluation of motor axonopathies in dogs.     

Modulation of corticospinal excitability dependent upon imagined force level

Motor imagery is defined as the mental execution of a movement without any muscle activity. In the present study, corticospinal excitability was assessed by motor evoked potentials (MEPs) when the subjects imagined isometric elbow flexion at various force levels. Electromyography was recorded from the right brachioradialis, the biceps brachii and the triceps brachii muscles. First, the maximum voluntary contraction (MVC) of elbow flexion was recorded in each subject. Subjects practiced performing 10, 30 and 60 % MVC using visual feedback. After the practice, MEPs were recorded during the imagery of elbow flexion with the forces of 10, 30 and 60 % MVC without any feedback. After the MEPs recording, we assigned subjects to reproduce the actual elbow flexion force at 10, 30 and 60 % MVC. The MEPs amplitudes in the brachioradialis and biceps brachii in the 60 % MVC condition were significantly greater than those in the 10 % MVC condition (p < 0.05). These findings suggest that the enhancement of corticospinal excitability during motor imagery is associated with an increase in imagined force level.     

Sprouting of fusimotor neurones after partial denervation of the cat soleus muscle

Summary Normally, motoneurones innervate only the intrafusal fibres of muscle spindles. This is a report of sprouting of motoneurones to innervate extrafusal muscle fibres following partial denervation of the soleus muscle of kittens. In eight newborn animals, the L7 ventral root was cut on one side under anaesthesia and the animals were then allowed to recover. At approximately 100 days of age animals were reanaesthetised and a study made of mechanical properties of motor units whose axons ran in the S1 ventral root and supplied the partially denervated soleus muscle. Evidence was obtained for sprouting of all surviving motoneurones. In addition, in four experiments axons conducting within the range, on stimulation, produced measurable tension. In one experiment, stimulation of one such axon also produced specific fusimotor effects on four afferents identified as coming from primary endings of muscle spindles. The axon was therefore a fusimotor axon. The effect observed on stimulation of the axon suggested a largely dynamic action. Other examples of axons were encountered that on stimulation produced tension, but which could not be specifically associated with spindles. In addition, a number of axons that did not develop tension were shown, on stimulation, to have fusimotor effects that were static in action. It is concluded that in extensively denervated muscles motoneurones may sometimes sprout to innervate extrafusal fibres. The mechanical properties of the extrafusal fibres innervated by such axons were similar to those of ordinary motor units.     

70 μM caffeine treatment enhances in vitro force and power output during cyclic activities in mouse extensor digitorum longus muscle

Caffeine ingestion by human athletes has been found to improve endurance performance primarily acting via the central nervous system as an adenosine receptor antagonist. However, a few studies have implied that the resultant micromolar levels of caffeine in blood plasma (70 M maximum for humans) may directly affect skeletal muscle causing enhanced force production. In the present study, the effects of 70 M caffeine on force and power output in isolated mouse extensor digitorum longus muscle were investigated in vitro at 35°C. Muscle preparations were subjected to cyclical sinusoidal length changes with electrical stimulation conditions optimised to produce maximal work. 70 M caffeine caused a small but significant increase (2–3%) in peak force and net work produced during work loops (where net work represents the work input required to lengthen the muscle subtracted from the work produced during shortening). However, these micromolar caffeine levels did not affect the overall pattern of fatigue or the pattern of recovery from fatigue. Our results suggest that the plasma concentrations found when caffeine is used to enhance athletic performance in human athletes might directly enhance force and power during brief but not prolonged activities. These findings potentially confirm previous in vivo studies, using humans, which implied caffeine ingestion may cause acute improvements in muscle force and power output but would not enhance endurance.     

Distribution of skeletofusimotor axons in lumbrical muscles of the monkey

Summary The nerve supply to 25 poles of muscle spindles in the monkey was reconstructed by light microscopy of serial 1-m thick transverse sections of lumbrical muscles. Twenty of 60 motor axons that supplied the spindle poles were identified as skeletofusimotor (). Twenty-eight percent of the spindle poles were innervated by axons, in addition to axons. Every -innervated spindle pole transected an endplate zone of extrafusal muscle. Most axons coinnervated extrafusal fibers rich in mitochondria and the nuclear bag₁ or nuclear chain intrafusal fibers. All but two axons innervated one type of intrafusal fiber only. The intramuscular organization of motor system in lumbrical muscles of the monkey was similar to that of the cat tenuissimus muscle. The function of -innervated spindles may be preferentially to monitor mechanical disturbances arising from the activity of extrafusal muscle units with which they share motor innervation.     

Motor evoked potential depression following repetitive central motor initiation

Prior reports have described a transient and focal decline in transcranial magnetic stimulation (TMS)-induced motor evoked potential (MEP) amplitude following fatiguing motor tasks. However, the neurophysiological causes of this change in MEP amplitude are unknown. The aim of this study was to determine whether post-task depression of MEPs is associated with repetitive central motor initiation. We hypothesized that MEP depression is related to repeated central initiation of motor commands in task-related cortex independent of motor fatigue. Twenty healthy adults had MEPs measured from the dominant first dorsal interosseous (FDI) muscle before and after six different tasks: rest (no activity), contralateral fatiguing hand-grip, ipsilateral fatiguing hand-grip, contralateral finger tapping, ipsilateral finger tapping, and imagined hand-grip (motor imagery). Changes in MEPs from baseline were assessed for each task immediately following the task and at 2-min intervals until MEPs returned to a stable baseline. Measures of subjective effort and FDI maximum voluntary contractions (MVC) were also recorded following each task. A statistically significant drop in MEP amplitude was noted only with contralateral finger tapping and imagined grip. Changes in MEP amplitude did not correlate with subjective fatigue or effort. There was no significant change in FDI MVCs following hand-grip or finger-tapping tasks. This study extends our knowledge of the observed decline in MEP amplitude following certain tasks. Our results suggest that central initiation of motor programs may induce a change in MEP amplitude, even in the absence of objective fatigue.     

Actual and mental motor preparation and execution: a spatiotemporal ERP study

Studies evaluating the role of the executive motor system in motor imagery came to a general agreement in favour of the activation of the primary motor area (M1) during imagery, although in reduced proportion as compared to motor execution. It is still unclear whether this difference occurs within the preparation period or the execution period of the movement, or both. In the present study, EEG was used to investigate separately the preparation and the execution periods of overt and covert movements in adults. We designed a paradigm that randomly mixed actual and kinaesthetic imagined trials of an externally paced sequence of finger key presses. Sixty channel event-related potentials were recorded to capture the cerebral activations underlying the preparation for motor execution and motor imagery, as well as cerebral activations implied in motor execution and motor imagery. Classical waveform analysis was combined with data-driven spatiotemporal segmentation analysis. In addition, a LAURA source localization algorithm was applied to functionally define brain related motor areas. Our results showed first that the difference between actual and mental motor acts takes place at the late stage of the preparation period and consists of a quantitative modulation of the activity of common structures in M1. Second, they showed that primary motor structures are involved to the same extent in the actual or imagined execution of a motor act. These findings reinforce and refine the functional equivalence hypothesis between actual and imagined motor acts.This study was supported by a grant (1114–56777.99) from the Swiss National Science Foundation and by the Programme Commun de Recherche en Génie Biomédical 1999–2002     

Influence of somatosensory input on corticospinal excitability during motor imagery

Our previous studies showed that corticospinal excitability during imagery of squeezing a foam ball was enhanced by somatosensory input generated by passively holding the ball. In the present study, using the same experimental model, we investigated whether corticospinal excitability was influenced by holding the object with the hand opposite to the imagined hand. Corticospinal excitability was assessed by monitoring motor evoked potentials (MEPs) in the first dorsal interosseous muscle following transcranial magnetic stimulation over the motor cortex during motor imagery. Subjects were asked to imagine squeezing a foam ball with the right hand (experiment 1) or the left hand (experiment 2), while either holding nothing (Null condition), a ball in the right hand (Right condition) or a ball in the left hand (Left condition). The MEPs amplitude during motor imagery was increased, only when the holding hand and the imagined hand were on the same side. These results suggest that performance improvement and rehabilitation exercises will be more effective when somatosensory stimulation and motor imagery are done on the same side.     

Movement-specific enhancement of corticospinal excitability at subthreshold levels during motor imagery

This study examined modulation of corticospinal excitability during both actual and imagined movements. Seven young healthy subjects performed actual (3–50% maximal voluntary contractions) and imagined index finger force production, and rest. Individual responses to focal transcranial magnetic stimulation (TMS) in four fingers (index, middle, ring, and little) were recorded for all three tested conditions. The force increments at the threshold of activation were predicted from regression analysis, representing the TMS-induced response at the threshold activation of the corticospinal pathways. The measured increment in the index finger during motor imagery was larger than that at rest, but smaller than the predicted increment at the threshold of activation. On the other hand, the measured increment in the uninstructed (middle, ring, and little), slave fingers during motor imagery was larger than that at rest, but not different from the predicted increment at the threshold of activation. These contrasting results suggest that the degree of imagery-induced enhancement in corticospinal excitability was significantly less than what could be predicted for threshold levels from regression analysis, but only for the index finger, and not the adjacent slave fingers. It is concluded that corticospinal excitability for the explicitly instructed index finger is specifically enhanced at subthreshold levels during motor imagery.     

The force-frequency relationship: A comparative study between warm- and cold-blooded animals

Summary In a comparative study, the mechanical and electrical responses of the guinea pig's papillary muscles and strips of the turtle's and frog's ventricles to various stimulation patterns were investigated. Typical forcefrequency relationships were found to be present in all preparations. It is, however, much more pronounced in the guinea pig's heart than in the other preparations. Striking differences exist between the warm-blooded and the cold-blooded animals, as far as pure frequency potentiation is concerned, i.e., the frequency dependence of the maximal actively developed force after a certain resting period (test-interval) following a series of conditioning rhythmical stimuli. Whereas in the guinea pig's papillary muscle the amplitude of optimal test contraction increases with the frequency of foregoing stimuli, the amplitude is depressed in the cold-blooded preparations by a rise of frequency. This effect is found to be due to the shortening of the action potential. Thus the mechanical response of cold-blooded preparations seems to depend primarily on the duration of depolarization under different conditions of stimulation. In the guinea pig's papillary muscle, the same changes in the time course of depolarization can be observed, but their effect on the contractile force cannot be revealed in such experiments. A much more predominant role in the force development of a papillary muscle may be attributed to the immediate influence of frequency on the contractile mechanism, i.e. to the pure frequency potentiation which does not exist in the myocardium of cold-blooded animals. These differences may be explained by the different development of Ca⁺⁺ stores of the sarcoplasmic reticulum in heart muscle of cold- and warm-blooded animals.     

Histochemical evidence for the existence of skeletofusimotor (β) innervation in the primate

Summary A total of ten a motor axons which innervated the peroneus brevis muscle were isolated in two cynomolgus monkeys. In each experiment, the isolated axons were stimulated collectively to deplete glycogen from their muscle units. The muscle was then frozen quickly, cut serially, and stained for glycogen. Of the 52 muscle spindles that were examined, zones of glycogen depletion were found in the intrafusal fibres of 32 spindles. The glycogen-depleted motor units included both fast-twitch and slow-twitch types. Depleted zones were observed in all three types of intrafusal muscle fibres. It was concluded that skeletofusimotor () efferents were among the stimulated motor axons. This finding constitutes the first anatomical evidence for the existence of innervation in the primate.Supported in part by funds from NINCDS grants NS-14702, NS-14546, and NS-11949     

Muscle substrate utilization from alveolar gas exchange in trained cyclists

The respiratory exchange ratio (R) during steady-state exercise is equivalent to whole-body respiratory quotient (RQ), but does not represent muscle metabolism alone. If steady-state values of carbon dioxide production ( ) and oxygen uptake ( ) are plotted for different work rates, the slope of the line fitting these points should estimate muscle RQ. Twelve cyclists randomly performed five 8-min, constant work rate tests of 40, 80,120,160 and 200 W. Whole-bodyR, averaged over the final 2 min of each exercise bout, increased with increasing work rate. When was plotted as a function of , the regression lines through the five points displayed excellent linearity, had negative -intercepts, and a slope of 0.915 (0.043) [mean (SD)], which was greater than the whole-bodyR at any individual work rate [range 0.793 (0.027) at 40 W to 0.875 (0.037) at 200 W]. This slope was comparable to the lower slope of the versus plot of an increasing work rate (ramp) protocol [0.908 (0.054)]. We conclude that, during mild and moderate exercise of relatively short duration, contracting muscle has a high and constant RQ, indicating that carbohydrate is the predominant metabolic substrate. WholebodyR does not accurately reflect muscle substrate utilization and probably underestimates muscle RQ at a given work rate.     

Exercise starts and ends in the brain

Classically the limit to endurance of exercise is explained in terms of metabolic capacity. Cardio-respiratory capacity and muscle fatigue are thought to set the limit and the majority of studies on factors limiting endurance exercise discuss issues such as maximal oxygen uptake (O₂max), aerobic enzyme capacity, cardiac output, glycogen stores, etc. However, this paradigm does not explain the limitation to endurance exercise with large muscle groups at altitude, when at exhaustion exercise is ended without limb locomotor muscle fatigue and with sub-maximal cardiac output. A simple fact provides a basis for an explanation. Voluntary exercise starts and ends in the brain. It starts with spatial and temporal recruitment of motor units and ends with their de-recruitment. A conscious decision precedes a voluntary effort. The end of effort is again volitional and a forced conscious decision to stop precedes it, but it is unknown what forces the off-switch of recruitment at exhaustion although sensation of exertion certainly plays a role. An alternative model explaining the limitation of exercise endurance thus proposes that the central nervous system integrates input from various sources all related to the exercise and limits the intensity and duration of recruitment of limb skeletal muscle to prevent jeopardizing the integrity of the organism. This model acknowledges the cardio-respiratory and muscle metabolic capacities as prime actors on the performance scene, while crediting the central nervous system for its pivotal role as the ultimate site where exercise starts and ends.