Corticospinal excitability modulation during action observation

Corticospinal excitability and conductivity are related to the anatomy of the corticospinal tract

Probing the brain structure-function relationship is at the heart of modern neuroscientific explorations, enabled by recent advances in brain mapping techniques. This study aimed to explore the anatomical blueprint of corticospinal excitability and shed light on the structure-function relationship within the human motor system. Using diffusion magnetic resonance imaging tractography, based on the spherical deconvolution approach, and transcranial magnetic stimulation (TMS), we show that anatomical inter-individual variability of the corticospinal tract (CST) modulates the corticospinal excitability and conductivity. Our findings show for the first time the relationship between increased corticospinal excitability and conductivity in individuals with a bigger CST (i.e., number of streamlines), as well as increased corticospinal microstructural organization (i.e., fractional anisotropy). These findings can have important implications for the understanding of the neuroanatomical basis of TMS as well as the study of the human motor system in both health and disease.

Muscle-specific modulation of indirect inputs to primary motor cortex during action observation

Single-pulse transcranial magnetic stimulation (spTMS) studies report that movement observation facilitates corticospinal excitability in primary motor cortex (M1) in a muscle-specific manner. However, motor evoked potentials (MEPs) elicited by spTMS are known to reflect the summation of several descending volleys in corticospinal neurons which are evoked via mono- and polysynaptic inputs (so-called indirect waves or I-waves). It is unclear which of these components contribute to the muscle-specific modulation of M1 during action observation. The interactions between different I-waves are reflected in the facilitatory peaks elicited with a short-intracortical facilitation (SICF) protocol when two pulses are sent to M1 at precise intervals (i.e., 1.3, 2.5 or 4.1 ms). Here, we explored the modulation of early and late SICF peaks during action observation by measuring highly specific MEP amplitude changes measured in two muscles (index, FDI and little finger, ADM) while participants observed two different actions (precision and whole-hand grip). Our results demonstrate that both early (1.3 ms) and late (2.5 and 4.1 ms) SICF peaks are modulated in the context of movement observation. However, only the second peak (ISI 2.5 ms) was significantly associated with the muscle-specific modulation of corticospinal excitability as measured with spTMS. This late SICF peak is believed to reflect the activity cortico-cortical pathways involved in the facilitation of muscle-specific representations in M1. Thus, our findings suggest that movement observation leads to widespread activation of different neural circuits within M1, including those mediating cortico-cortical communication.

Primary motor cortex and phonological recoding: A TMS-EMG study

Since the 1960s, evidence from healthy participants and brain-damaged patients, neuroimaging and non-invasive brain stimulation studies has specified the neurofunctional architecture of the short-term memory (STM) system, supporting the temporary retention of a limited amount of verbal material. Auditory-verbal, later termed Phonological (Ph) STM or Phonological Loop, comprises two sub-components: i) the main storage system, the Phonological Short-Term Store (PhSTS), to which auditory verbal stimuli have direct access and where phonologically coded information is retained for a few seconds; ii) a Rehearsal Process (REH), which actively maintains the trace held in the PhSTS, preventing its decay and conveys visual verbal material to the PhSTS, after the process of Phonological Recoding (PhREC, or Grapheme-to-Phoneme Conversion) has taken place. PhREC converts visuo-verbal graphemic representations into phonological ones. The neural correlates of PhSTM include two discrete regions in the left hemisphere: the temporo-parietal junction (PhSTS) and the inferior frontal gyrus in the premotor cortex (REH). The neural basis of PhREC has been much less investigated. A few single case studies of patients made anarthric by focal or degenerative cortical damage, who show a pattern of impairment indicative of a deficit of PhREC, sparing the REH process, suggest that the primary motor cortex (M1) might be involved. To test this hypothesis in healthy participants with a neurophysiological approach, we measured the corticospinal excitability of M1, by means of Transcranial Magnetic Stimulation (TMS)-induced Motor Evoked Potentials (MEPs), during the execution of phonological judgements on auditorily vs. visually presented words (Experiment #1). Crucially, these phonological tasks involve REH, while PhREC is required only with visual presentation. Results show MEPs with larger amplitude when stimuli are presented visually. Task difficulty does not account for this difference and the result is specific for linguistic stimuli, indeed visual and auditory stimuli that cannot be verbalized lead to different behavioral and neurophysiological patterns (Experiment #2). The increase of corticospinal excitability when words are presented visually can be then interpreted as an indication of the involvement of M1 in PhREC. The present findings elucidate the neural correlates of PhREC, suggesting an involvement of the peripheral motor system in its activity.

Positive self-perception and corticospinal excitability: Recalling positive behavior expands peripersonal space boundaries

Converging evidence suggests that peripersonal space has dynamic properties, that can be influenced by motor and cognitive factors. Here, we investigated whether changes in self-perception may impact upon peripersonal representation. Specifically, employing non-invasive brain stimulation, we tested whether corticospinal excitability elicited by objects placed in the vertical peripersonal vs extrapersonal space can be influenced by changes in self-perception after recalling a personal experience inducing the feeling of high power (vs. positivity vs. low power). In a preliminary study (Study 1, N = 39) participants were presented with an object, whose position was manipulated in the horizontal vs vertical space. We assessed corticospinal excitability by measuring Motor Evoked Potentials (MEPs) using Transcranial Magnetic Stimulation with Electromyography co-registration (TMS-EMG). In the horizontal condition, we replicated the well-known motor facilitation induced by objects falling in the peri vs extrapersonal space, while in the vertical dimension MEPs were higher in the extrapersonal space. In the main experiment (Study 2), participants (N = 55) were randomly assigned to feel high power, low power, or a general positive emotion and were asked to observe the same object positioned either in the peripersonal or in the extrapersonal vertical space. Results showed that in the low power condition MEPs were higher in the extrapersonal vs peripersonal, as in Study 1, while in high power and positive conditions MEPs were not influenced by distance. Taken together, our findings suggest a dissociable pattern of motor facilitation underlying vertical vs horizontal space perception and, crucially, that changes in self-perception can influence such a representation.

Uncertainty in contextual and kinematic cues jointly modulates motor resonance in primary motor cortex

Contextual information accompanying others' actions modulates "motor resonance", i.e., neural activity within motor areas that is elicited by movement observation. One possibility is that we weigh and combine such information in a Bayesian manner according to their relative uncertainty. Therefore, contextual information becomes particularly useful when others' actions are difficult to discriminate. It is unclear, however, whether this uncertainty modulates the neural activity in primary motor cortex (M1) during movement observation. Here, we applied single-pulse transcranial magnetic stimulation (TMS) while subjects watched different grasping actions. We operationalized motor resonance as grip-specific modulation of corticomotor excitability measured in the index (FDI) versus the little finger abductor (ADM). We experimentally modulated either the availability of kinematic information ( experiment 1) or the reliability of contextual cues ( experiment 2). Our results indicate that even in the absence of movement kinematics, reliable contextual information is enough to trigger significant muscle-specific corticomotor excitability changes in M1, which are strongest when both kinematics and contextual information are available. These findings suggest that bottom-up mechanisms that activate motor representations as a function of the observed kinematics and top-down mechanisms that activate motor representations associated with arbitrary cues converge in M1. NEW & NOTEWORTHY Our study reveals new neurophysiological insights in support of the Bayesian account of action observation by showing that "motor resonance", i.e., neural activity evoked by observing others' actions, incorporates the uncertainty related to both contextual (prior beliefs) and kinematic (sensory evidence) cues. Notably, we show that muscle-specific modulation of M1 is strongest when context and movement kinematics are available, and it can be elicited even in the absence of movement kinematics.

Overt orienting of spatial attention and corticospinal excitability during action observation are unrelated

Observing moving body parts can automatically activate topographically corresponding motor representations in the primary motor cortex (M1), the so-called direct matching. Novel neurophysiological findings from social contexts are nonetheless proving that this process is not automatic as previously thought. The motor system can flexibly shift from imitative to incongruent motor preparation, when requested by a social gesture. In the present study we aim to bring an increase in the literature by assessing whether and how diverting overt spatial attention might affect motor preparation in contexts requiring interactive responses from the onlooker. Experiment 1 shows that overt attention—although anchored to an observed biological movement—can be captured by a target object as soon as a social request for it becomes evident. Experiment 2 reveals that the appearance of a short-lasting red dot in the contralateral space can divert attention from the target, but not from the biological movement. Nevertheless, transcranial magnetic stimulation (TMS) over M1 combined with electromyography (EMG) recordings (Experiment 3) indicates that attentional interference reduces corticospinal excitability related to the observed movement, but not motor preparation for a complementary action on the target. This work provides evidence that social motor preparation is impermeable to attentional interference and that a double dissociation is present between overt orienting of spatial attention and neurophysiological markers of action observation.

Assessment of Neuromuscular Function Using Percutaneous Electrical Nerve Stimulation

Percutaneous electrical nerve stimulation is a non-invasive method commonly used to evaluate neuromuscular function from brain to muscle (supra-spinal, spinal and peripheral levels). The present protocol describes how this method can be used to stimulate the posterior tibial nerve that activates plantar flexor muscles. Percutaneous electrical nerve stimulation consists of inducing an electrical stimulus to a motor nerve to evoke a muscular response. Direct (M-wave) and/or indirect (H-reflex) electrophysiological responses can be recorded at rest using surface electromyography. Mechanical (twitch torque) responses can be quantified with a force/torque ergometer. M-wave and twitch torque reflect neuromuscular transmission and excitation-contraction coupling, whereas H-reflex provides an index of spinal excitability. EMG activity and mechanical (superimposed twitch) responses can also be recorded during maximal voluntary contractions to evaluate voluntary activation level. Percutaneous nerve stimulation provides an assessment of neuromuscular function in humans, and is highly beneficial especially for studies evaluating neuromuscular plasticity following acute (fatigue) or chronic (training/detraining) exercise.

Motor interference in interactive contexts

Action observation and execution share overlapping neural substrates, so that simultaneous activation by observation and execution modulates motor performance. Previous literature on simple prehension tasks has revealed that motor influence can be two-sided: facilitation for observed and performed congruent actions and interference for incongruent actions. But little is known of the specific modulations of motor performance in complex forms of interaction. Is it possible that the very same observed movement can lead either to interference or facilitation effects on a temporally overlapping congruent executed action, depending on the context? To answer this question participants were asked to perform a reach-to-grasp movement adopting a precision grip (PG) while: (i) observing a fixation cross, (ii) observing an actor performing a PG with interactive purposes, (iii) observing an actor performing a PG without interactive purposes. In particular, in the interactive condition the actor was shown trying to pour some sugar on a large cup located out of her reach but close to the participant watching the video, thus eliciting in reaction a complementary whole-hand grasp. Notably, fine-grained kinematic analysis for this condition revealed a specific delay in the grasping and reaching components and an increased trajectory deviation despite the observed and executed movement’s congruency. Moreover, early peaks of trajectory deviation seem to indicate that socially relevant stimuli are acknowledged by the motor system very early. These data suggest that interactive contexts can determine a prompt modulation of stimulus–response compatibility effects.

Complementary actions

Complementary colors are color pairs which, when combined in the right proportions, produce white or black. Complementary actions refer here to forms of social interaction wherein individuals adapt their joint actions according to a common aim. Notably, complementary actions are incongruent actions. But being incongruent is not sufficient to be complementary (i.e., to complete the action of another person). Successful complementary interactions are founded on the abilities: (i) to simulate another person's movements, (ii) to predict another person's future action/s, (iii) to produce an appropriate incongruent response which differ, while interacting, with observed ones, and (iv) to complete the social interaction by integrating the predicted effects of one's own action with those of another person. This definition clearly alludes to the functional importance of complementary actions in the perception-action cycle and prompts us to scrutinize what is taking place behind the scenes. Preliminary data on this topic have been provided by recent cutting-edge studies utilizing different research methods. This mini-review aims to provide an up-to-date overview of the processes and the specific activations underlying complementary actions.

The left side of motor resonance

Motor resonance is defined as the internal activation of an observer's motor system, specifically attuned to the perceived movement. In social contexts, however, different patterns of observed and executed muscular activation are frequently required. This is the case, for instance, of seeing a key offered with a precision grip and received by opening the hand. Novel evidence suggests that compatibility effects in motor resonance can be altered by social response preparation. What is not known is how handedness modulates this effect. The present study aimed at determining how a left- and a right-handed actor grasping an object and then asking for a complementary response influences corticospinal activation in left- and right-handers instructed to observe the scene. Transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEPs) were thus recorded from the dominant hands of left- and right-handers. Interestingly, requests posed by the right-handed actor induced a motor activation in the participants' respective dominant hands, suggesting that left-handers tend to mirror right-handers with their most efficient hand. Whereas requests posed by the left-handed actor activated the anatomically corresponding muscles (i.e., left hand) in all the participants, right-handers included. Motor resonance effects classically reported in the literature were confirmed when observing simple grasping actions performed by the right-handed actor. These findings indicate that handedness influences both congruent motor resonance and complementary motor preparation to observed actions.