Effects of motor preparation and spatial attention on corticospinal excitability in a delayed-response paradigm

The preparation of motor responses during the delay period of an instructed delay task is associated with sustained neural firing in the primate premotor cortex. It remains unclear how and when such preparation-related premotor activity influences the motor output system. In this study, we tested modulation of corticospinal excitability using single-pulse transcranial magnetic stimulation (TMS) during a delayed-response task. At the beginning of the delay interval participants were either provided with no information, spatial attentional information concerning location but not identity of an upcoming imperative stimulus, or information regarding the upcoming response. Behavioral data indicate that participants used all information available to them. Only when information concerning the upcoming response was provided did corticospinal excitability show differential modulation for the effector muscle compared to other task-unrelated muscles. We conclude that modulation of corticospinal excitability reflects specific response preparation, rather than non-specific event preparation

The effects of expectancy on corticospinal excitability: passively preparing to observe a movement

[Abstract] The corticospinal tract excitability is modulated when preparing movements. Earlier to movement execution, the excitability of the spinal cord increases waiting for supraspinal commands to release the movement. Movement execution and movement observation share processes within the motor system, although movement observation research has focused on processes later to movement onset. We used single and paired pulse transcranial magnetic stimulation on M1 (n = 12), and electrical cervicomedullary stimulation (n = 7), to understand the modulation of the corticospinal system during the “preparation” to observe a third person's movement. Subjects passively observed a hand that would remain still or make an index finger extension. The observer's corticospinal excitability rose when “expecting to see a movement” vs. when “expecting to see a still hand.” The modulation took origin at a spinal level and not at the corticocortical networks explored. We conclude that expectancy of seeing movements increases the excitability of the spinal cord.Galicia. Consellería de Educación; 2007/000140-

Changes in corticospinal excitability and the direction of evoked movements during motor preparation: A TMS study

BACKGROUND: Preparation of the direction of a forthcoming movement has a particularly strong influence on both reaction times and neuronal activity in the primate motor cortex. Here, we aimed to find direct neurophysiologic evidence for the preparation of movement direction in humans. We used single-pulse transcranial magnetic stimulation (TMS) to evoke isolated thumb-movements, of which the direction can be modulated experimentally, for example by training or by motor tasks. Sixteen healthy subjects performed brisk concentric voluntary thumb movements during a reaction time task in which the required movement direction was precued. We assessed whether preparation for the thumb movement lead to changes in the direction of TMS-evoked movements and to changes in amplitudes of motor-evoked potentials (MEPs) from the hand muscles. RESULTS: When the required movement direction was precued early in the preparatory interval, reaction times were 50 ms faster than when precued at the end of the preparatory interval. Over time, the direction of the TMS-evoked thumb movements became increasingly variable, but it did not turn towards the precued direction. MEPs from the thumb muscle (agonist) were differentially modulated by the direction of the precue, but only in the late phase of the preparatory interval and thereafter. MEPs from the index finger muscle did not depend on the precued direction and progressively decreased during the preparatory interval. CONCLUSION: Our data show that the human corticospinal movement representation undergoes progressive changes during motor preparation. These changes are accompanied by inhibitory changes in corticospinal excitability, which are muscle specific and depend on the prepared movement direction. This inhibition might indicate a corticospinal braking mechanism that counteracts any preparatory motor activation

The role of stimulus-driven versus goal-directed processes in fight and flight tendencies measured with motor evoked potentials induced by Transcranial Magnetic Stimulation

This study examines two contrasting explanations for early tendencies to fight and flee. According to a stimulus-driven explanation, goal-incompatible stimuli that are easy/difficult to control lead to the tendency to fight/flee. According to a goal-directed explanation, on the other hand, the tendency to fight/flee occurs when the expected utility of fighting/fleeing is the highest. Participants did a computer task in which they were confronted with goal-incompatible stimuli that were (a) easy to control and fighting had the highest expected utility, (b) easy to control and fleeing had the highest expected utility, and (c) difficult to control and fleeing and fighting had zero expected utility. After participants were trained to use one hand to fight and another hand to flee, they either had to choose a response or merely observe the stimuli. During the observation trials, single-pulse Transcranial Magnetic Stimulation (TMS) was applied to the primary motor cortex 450 ms post-stimulus onset and motor evoked potentials (MEPs) were measured from the hand muscles. Results showed that participants chose to fight/flee when the expected utility of fighting/fleeing was the highest, and that they responded late when the expected utility of both responses was low. They also showed larger MEPs for the right/left hand when the expected utility of fighting/fleeing was the highest. This result can be interpreted as support for the goal-directed account, but only if it is assumed that we were unable to override the presumed natural mapping between hand (right/left) and response (fight/flight)

From rubber hands to neuroprosthetics: Neural correlates of embodiment

© 2023 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/)Our interaction with the world rests on the knowledge that we are a body in space and time, which can interact with the environment. This awareness is usually referred to as sense of embodiment. For the good part of the past 30 years, the rubber hand illusion (RHI) has been a prime tool to study embodiment in healthy and people with a variety of clinical conditions. In this paper, we provide a critical overview of this research with a focus on the RHI paradigm as a tool to study prothesis embodiment in individuals with amputation. The RHI relies on well-documented multisensory integration mechanisms based on sensory precision, where parietal areas are involved in resolving the visuo-tactile conflict, and premotor areas in updating the conscious bodily representation. This mechanism may be transferable to prosthesis ownership in amputees. We discuss how these results might transfer to technological development of sensorised prostheses, which in turn might progress the acceptability by users.Peer reviewe

The placebo effect in the motor domain: a neural and behavioral approach

The placebo effect is a fascinating psychobiological phenomenon that allows to investigate the mind-body interaction. It is typically induced by the application of an inert treatment along with verbal suggestion of beneficial outcomes. The placebo effect has been deeply investigated in the field of pain, although different lines of evidence suggest that it is also present in other domains, like the motor domain. Extending our knowledge of the placebo effect in the motor domain can have important future translational impacts in sports and pathology. The aim of my PhD project was to study the placebo effect in the motor domain at two different levels: the neural and the behavioral level. Regarding the neural level, knowledge on the brain regions related to placebo effect in the motor domain is limited. We aimed at filling in this knowledge gap by investigating the role of the dlPFC, a brain region also involved in placebo analgesia. The dlPFC elaborates expectation, a cognitive function at the basis of the placebo effect and shares some connections with other brain regions involved in motor control. Hence, there are many clues to hypothesize a role of the dlPFC in the motor placebo effect. To tackle this issue, three different experiments were conducted in which the dlPFC was stimulated by means of transcranial direct current stimulation (tDCS) together with a placebo procedure on force production. We found that the left dlPFC is involved in the expectation-induced enhancement of force, specifically in those subjects who respond to the placebo effect (placebo-responders). Regarding the behavioral level, it should be noticed that many behavioral studies have shown that the placebo effect can enhance different aspects of motor performance associated to sports, such as force, speed or endurance. It is still unknown, however, whether the placebo effect can also improve other motor functions, important for many daily life activities, like balance or motor sequence learning. Thus, another objective of my PhD was to investigate the potential influence of the placebo effect on two motor functions that are closer to daily life activities. To this aim, a first study was conducted to understand whether balance control, a motor function needed for many daily life activities and for preventing falls, could be enhanced in healthy participants by a placebo procedure consisting 9 of verbal suggestion. We found that different parameters of balance (in the three-dimensional space and in the medial-lateral direction) and the subjective perception of stability were improved by the placebo procedure. A second behavioural study was run to investigate whether the application of a placebo treatment consisting of verbal suggestion could help in improving motor sequence learning. In this case, we also aimed to tackle a differential role of two types of placebo treatments: one motor and one cognitive. The motor placebo procedure consisted of transcutaneous electrical nerve stimulation (TENS) applied the hand muscles involved in the task together with verbal information on the beneficial effects on muscle activity. The cognitive placebo procedure consisted of sham transcranial direct current stimulation (tDCS) applied over the frontal region together with verbal information on the beneficial effects on attention. Our findings did not show a clear improvement of performance following the placebo procedures, but a significant effect on the subjective perception of fatigue. More precisely, while the placebo procedure directed to the motor function (TENS) could reduce the perception of physical fatigue, the placebo procedure focused on cognitive functions (sham tDCS) could decrease the perception of both mental and physical fatigue. Altogether these investigations represent an attempt to deepen our understanding of the neural correlates of the motor placebo effect and to enlarge the potential behavioural influence of placebos on different motor functions

Neurophysiological Adaptations to Resistance Training and Repetitive Grasping

Perhaps the most prominent feature of the central nervous system is its ability to respond to experience and its environment. Understanding the processes and mechanisms that govern adaptive behavior provides insights into its plastic nature. Capitalizing on this plasticity is of critical importance in response to injury and recovery: 35, 106), and the importance of its promotion is increasingly recognized by rehabilitation scientists. Neurophysiological techniques permitting study of cortical function in vivo may play a significant role in validating exercise interventions and disease management approaches: 14). It may be possible that with these advances we may better understand the relationship between brain function and therapeutic approaches. For this purpose, we present data on both cumulative and acute effects of motor training to better understand adaptive processes. Neural adaptations accompany resistance training, but current evidence regarding the nature of these adaptations is best characterized as indirect, particularly with respect to adaptation within central or supraspinal centers: 56). To this end, we recorded movement-related cortical potentials: MRCP), i.e. electroencephalography: EEG)-derived event-related potentials, in healthy adults prior to and following a program of lower body resistance training. The cumulative effects of nine progressive training sessions resulted in attenuation of relative MRCP amplitudes. We interpreted these findings in terms of neural efficiency such that for the same pre-training load, central effort is diminished post-training. These data demonstrate the impact of cumulative motor training sessions in fostering a reduction in the level of cortical motor activation. Such a program may be of a particular utility for individuals with limited motor reserves such as those with Parkinson disease: PD). Although cumulative effects may foster a more efficient cortical network, the acute demands of a training session have received less attention. It is reasonable to assume that the reverse might be expected: i.e. augmented amplitude) during a motor training session, much like the muscular system is taxed during resistance training exercise. At the level of the cortex, neural activity was studied by recording the MRCP during 150 repetitive handgrip contractions at a high intensity. The goal of this work was to examine whether central adaptive processes used to maintain task performance vary as a function of age or PD. We found that for healthy young adults, augmented activation of motor cortical centers is responsible for maintaining performance. However, this was not observed for older adults with and without PD, where minimal changes in cortical activity were observed over the duration of the protocol. Our findings suggest that older adults and those with PD may rely on alternative mechanisms: i.e. mobilization of additional cortical and subcortical structures) to maintain task performance as compared to increasing activity locally as seen with younger adults. Taken together, our work further supports the adaptable nature of the central nervous system. We note in passing the utility of the MRCP paradigm for observing such effects