Towards formal models of inhibitory mechanisms involved in motor imagery: a commentary on Bach et al. (2022).

A vast body of research suggests that the primary motor cortex is involved in motor imagery. This raises the issue of inhibition: how is it possible for motor imagery not to lead to motor execution? Bach et al. (Psychol Res Psychol Forschung. 10.1007/s00426-022-01773-w, 2022, this issue) suggest that the motor execution threshold may be "upregulated" during motor imagery to prevent execution. Alternatively, it has been proposed that, in parallel to excitatory mechanisms, inhibitory mechanisms may be actively suppressing motor output during motor imagery. These theories are verbal in nature, with well-known limitations. Here, we describe a toy-model of the inhibitory mechanisms thought to be at play during motor imagery to start disentangling predictions from competing hypotheses.

Temporally resolved neural dynamics underlying handwriting.

How do the temporal dynamics of neural activity encode highly coordinated visual-motor behaviour? To capture the millisecond-resolved neural activations associated with fine visual-motor skills, we devised a co-registration system to simultaneously record electroencephalogram and handwriting kinematics while participants were performing four handwriting tasks (writing in Chinese/English scripts with their dominant/non-dominant hand). The neural activation associated with each stroke was clearly identified with a well-structured and reliable pattern. The functional significance of this pattern was validated by its significant associations with language, hand and the cognitive stages and kinematics of handwriting. Furthermore, the handwriting rhythmicity was found to be synchronised to the brain's ongoing theta oscillation, and the synchronisation was associated with the factor of language and hand. These major findings imply an implication between motor skill formation and the interplay between the rhythms in the brain and the peripheral systems.

The handwriting brain in middle childhood.

While the brain network supporting handwriting has previously been defined in adults, its organization in children has never been investigated. We compared the handwriting network of 23 adults and 42 children (8- to 11-year-old). Participants were instructed to write the alphabet, the days of the week, and to draw loops while being scanned. The handwriting network previously described in adults (five key regions: left dorsal premotor cortex, superior parietal lobule (SPL), fusiform and inferior frontal gyri, and right cerebellum) was also strongly activated in children. The right precentral gyrus and the right anterior cerebellum were more strongly activated in adults than in children, while the left fusiform gyrus (FuG) was more strongly activated in children than in adults. Finally, we found that, contrary to adults, children recruited prefrontal regions to complete the writing task. This constitutes the first comparative investigation of the neural correlates of writing in children and adults. Our results suggest that the network supporting handwriting is already established in middle childhood. They also highlight the major role of prefrontal regions in learning this complex skill and the importance of right precentral regions and cerebellum in the performance of automated handwriting.

The Scope of Planning Serial Actions during Typing.

Human activities consisting of multiple component actions require the generation of ordered sequences. This study investigated the scope of response planning in highly serial task, typing, by means of ERPs indexing motor response preparation. Specifically, we compared motor-related ERPs yielded by words typed using a single hand against words that had all keystrokes typed with a single hand, except for a deviant one, typed with the opposite hand. The deviant keystroke occurred either early in the typed sequence, corresponding to the second or third letters, or late, corresponding to the penultimate or last letter. Motor-related ERPs detected before response onset were affected only by deviant keystrokes located at the beginning of the sequence, whereas deviant keystrokes located at the end yielded ERPs that were undistinguishable from unimanual responses. These results impose some constraints on the notion of parallel processing of component actions.

Motor control of handwriting in the developing brain: A review.

This review focuses on the acquisition of writing motor aspects in adults, and in 5-to 12-year-old children without learning disabilities. We first describe the behavioural aspects of adult writing and dominant models based on the notion of motor programs. We show that handwriting acquisition is characterized by the transition from reactive movements programmed stroke-by-stroke in younger children, to an automatic control of the whole trajectory when the motor programs are memorized at about 10 years old. Then, we describe the neural correlates of adult writing, and the changes that could occur with learning during childhood. The acquisition of a new skill is characterized by the involvement of a network more restricted in space and where neural specificity is increased in key regions. The cerebellum and the left dorsal premotor cortex are of fundamental importance in motor learning, and could be at the core of the acquisition of handwriting.

Effect of training status on beta-range corticomuscular coherence in agonist vs. antagonist muscles during isometric knee contractions.

Antagonist muscle co-activation is thought to be partially regulated by cortical influences, but direct motor cortex involvement is not fully understood. Corticomuscular coherence (CMC) measures direct functional coupling of the motor cortex and muscles. As antagonist co-activation differs according to training status, comparison of CMC in agonist and antagonist muscles and in strength-trained and endurance-trained individuals may provide in-depth knowledge of cortical implication in antagonist muscle co-activation. Electroencephalographic and electromyographic signals were recorded, while 10 strength-trained and 11 endurance-trained participants performed isometric knee contractions in flexion and extension at various torque levels. CMC magnitude in 13-21 and 21-31 Hz frequency bands was quantified by CMC analysis between Cz electroencephalographic electrode activity and all electromyographic signals. CMC was significant in both 13-21 and 21-31 Hz frequency bands in flexor and extensor muscles regardless of participant group, torque level, and direction of contraction. CMC magnitude decreased more in antagonist than in agonist muscles as torque level increased. Finally, CMC magnitude was higher in strength-trained than in endurance-trained participants. These findings provide experimental evidence that the motor cortex directly regulates both agonist and antagonist muscles. Nevertheless, the mechanisms underlying muscle activation may be specific to their function. Between-group modulation of corticomuscular coherence may result from training-induced adaptation, re-emphasizing that corticomuscular coherence analysis may be efficient in characterizing corticospinal adaptations after long-term muscle specialization.

Brain correlates of phonological recoding of visual symbols.

Learning to read involves setting up associations between meaningless visual inputs (V) and their phonological representations (P). Here, we recorded the brain signals (ERPs and fMRI) associated with phonological recoding (i.e., V-P conversion processes) in an artificial learning situation in which participants had to learn the associations between 24 unknown visual symbols (Japanese Katakana characters) and 24 arbitrary monosyllabic names. During the learning phase on Day 1, the strength of V-P associations was manipulated by varying the proportion of correct and erroneous associations displayed during a two-alternative forced choice task. Recording event related potentials (ERPs) during the learning phase allowed us to track changes in the processing of these visual symbols as a function of the strength of V-P associations. We found that, at the end of the learning phase, ERPs were linearly affected by the strength of V-P associations in a time-window starting around 200ms post-stimulus onset on right occipital sites and ending around 345ms on left occipital sites. On Day 2, participants had to perform a matching task during an fMRI session and the strength of these V-P associations was again used as a probe for identifying brain regions related to phonological recoding. Crucially, we found that the left fusiform gyrus was gradually affected by the strength of V-P associations suggesting that this region is involved in the brain network supporting phonological recoding processes.

Functional specificity in the motor system: Evidence from coupled fMRI and kinematic recordings during letter and digit writing.

A few intriguing neuropsychologial studies report dissociations where agraphic patients are severely impaired for writing letters whereas they write digits nearly normally. Here, using functional magnetic resonance imaging (fMRI) together with graphic tablet recordings, we tested the hypothesis that the motor patterns for writing letters are coded in specific regions of the cortex. We found a set of three regions that were more strongly activated when participants wrote letters than when they wrote digits and whose response was not explained by low-level kinematic features of the graphic movements. Two of these regions (left dorsal premotor cortex and supplementary motor complex) are part of a motor control network. The left premotor activation belongs to what is considered in the literature a key area for handwriting. Another significant activation, likely related to phoneme-to-grapheme conversion, was found in the right anterior insula. This constitutes the first neuroimaging evidence of functional specificity derived from experience in the cortical motor system.

Effects of biscriptuality on graphomotor coordination dynamics.

Biscriptuality is the ability to write in two different writing systems. The aim of this study was to examine the effects of biscriptuality on graphomotor coordination dynamics in right-handed adults. Thirty-four French monoscriptuals and 34 French-Arabic biscriptual participants traced series of loops in two writing directions and in two directions of rotation. We found that biscriptuals displayed a general advantage over monoscriptuals in terms of tracing frequency, while both groups displayed a preference for the left-to-right direction. These results provide novel evidence on the effects of writing direction and type of expertise on graphomotor performance by showing that biscriptuality could be an asset. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Distinct neural mechanisms support inner speaking and inner hearing.

Humans have the ability to mentally examine speech. This covert form of speech production is often accompanied by sensory (e.g., auditory) percepts. However, the cognitive and neural mechanisms that generate these percepts are still debated. According to a prominent proposal, inner speech has at least two distinct phenomenological components: inner speaking and inner hearing. We used transcranial magnetic stimulation to test whether these two phenomenologically distinct processes are supported by distinct neural mechanisms. We hypothesised that inner speaking relies more strongly on an online motor-to-sensory simulation that constructs a multisensory experience, whereas inner hearing relies more strongly on a memory-retrieval process, where the multisensory experience is reconstructed from stored motor-to-sensory associations. Accordingly, we predicted that the speech motor system will be involved more strongly during inner speaking than inner hearing. This would be revealed by modulations of TMS evoked responses at muscle level following stimulation of the lip primary motor cortex. Overall, data collected from 31 participants corroborated this prediction, showing that inner speaking increases the excitability of the primary motor cortex more than inner hearing. Moreover, this effect was more pronounced during the inner production of a syllable that strongly recruits the lips (vs. a syllable that recruits the lips to a lesser extent). These results are compatible with models assuming that the primary motor cortex is involved during inner speech and contribute to clarify the neural implementation of the fundamental ability of silently speaking in one's mind.

Writing letters in two graphic systems: Behavioral and neural correlates in Latin-Arabic biscripters.

Biscriptuality is the ability to read and write using two scripts. Despite the increasing number of biscripters, this phenomenon remains poorly understood. Here, we focused on investigating graphomotor processing in French-Arabic biscripters. We chose the French and Arabic alphabets because they have comparable visuospatial complexity and linguistic features, but differ dramatically in their graphomotor characteristics. In a first experiment we describe the graphomotor features of the two alphabets and showed that while Arabic and Latin letters are produced with the same velocity and fluency, Arabic letters require more pen lifts, contain more right-to-left strokes and clockwise curves, and take longer to write than Latin letters. These results suggest that Arabic and Latin letters are produced via different motor patterns. In a second experiment we used functional magnetic resonance imaging to ask whether writing the two scripts relies upon partially distinct or fully overlapping neural networks, and whether the elements of the previously described handwriting network are recruited to the same extent by the two scripts. We found that both scripts engaged the so-called "writing network", but that within the network, Arabic letters recruited the left superior parietal lobule (SPL) and the left primary motor cortex (M1) more strongly than Latin letters. Both regions have previously been identified as holding scale-invariant representations of letter trajectories. Arabic and Latin letters also activated distinct regions that do not belong to the writing network. Complementary analyses indicate that the differences observed between scripts at the neural level could be driven by the specific graphomotor features of each script. Overall, our results indicate that particular features of the practiced scripts can lead to different motor organization at both the behavioral and brain levels in biscripters.

Brain responses to handwritten and printed letters differentially depend on the activation state of the primary motor cortex.

Previous studies demonstrated that visual perception of handwritten letters activates the left primary motor cortex more strongly than printed letters. Here, we used EEG to record cortical responses evoked by single letters to directly test if their visual processing is actually influenced by their motor content. We manipulated the "motor familiarity" of letters that we considered high for letters written by the observers themselves, medium for letters written by other individuals, and low for printed, machine designed letters. In order to relate the effects of motor familiarity to the activation of the primary motor cortex, we also directly manipulated its availability during the visual task: we computed Event-Related Potentials (ERPs) over the posterior cortex during a dual task where participants had to observe the letters while performing unrelated self-paced brief movements of the right hand approximately every 5s (allowing the primary motor cortex to successively activate and "idle"). At 300-350 ms and 500-600 ms after stimulus onset, the amplitude of the ERP components markedly reflected the level of motor familiarity of the observed letter. Nonetheless, this was true only when the primary motor cortex was in an "idling" state: when the motor cortex was in an activation state, this motor familiarity effect was dropped. This clearly indicates that, at these latencies, the motor information embedded in letters is processed in the brain and that this processing depends on the activation state of the left primary motor cortex.

Training-related decrease in antagonist muscles activation is associated with increased motor cortex activation: evidence of central mechanisms for control of antagonist muscles.

During human contraction, net joint torque production involves the contribution of the antagonist muscles. Their activation protects the articulations and facilitates movement accuracy, but despite these fundamental roles, little is known about the brain mechanisms underlying their control. In view of previous studies that showed lesser antagonist muscles activation in participants engaged in regular strength training (ST) than in participants actively engaged in endurance disciplines (ED), we used this between-group comparison to investigate the possible role of motor cortex activity on the control of antagonist muscles. Electroencephalographic (EEG) and electromyographic (EMG) activity as well as the net joint torque were recorded, while ten ST and eleven ED participants performed isometric knee muscles exertions at different force levels. EEG data showed a linear increase in the suppression of cortical oscillations in the 21-31 Hz frequency band with increasing force level in ST but not in ED participants. This effect was associated with lesser EMG activation of the antagonist muscles in ST than in ED participants, the difference between groups also increasing with the force level. Both effects were found specifically during flexion exertions, indicating that ST participants developed sharp central adaptations to control the antagonist muscles involved as prime movers in their usual training task. This result suggests that the cortical adaptations induced by regular strength training could exert a specific encoding of the antagonist muscles, leading to the minimization of their activation and improved energetic efficiency of the muscle contraction.

Typing expertise in a large student population.

Typing has become a pervasive mode of language production worldwide, with keyboards fully integrated in a large part of many daily activities. The bulk of the literature on typing expertise concerns highly trained professional touch-typists, but contemporary typing skills mostly result from unconstrained sustained practice. We measured the typing performance of a large cohort of 1301 university students through an online platform and followed a preregistered plan to analyse performance distributions, practice factors, and cognitive variables. The results suggest that the standard model with a sharp distinction between novice and expert typists may be inaccurate to account for the performance of the current generation of young typists. More generally, this study shows how the mere frequent use of a new tool can lead to the incidental development of high expertise.

The Role of Motor Inhibition During Covert Speech Production.

Covert speech is accompanied by a subjective multisensory experience with auditory and kinaesthetic components. An influential hypothesis states that these sensory percepts result from a simulation of the corresponding motor action that relies on the same internal models recruited for the control of overt speech. This simulationist view raises the question of how it is possible to imagine speech without executing it. In this perspective, we discuss the possible role(s) played by motor inhibition during covert speech production. We suggest that considering covert speech as an inhibited form of overt speech maps naturally to the purported progressive internalization of overt speech during childhood. We further argue that the role of motor inhibition may differ widely across different forms of covert speech (e.g., condensed vs. expanded covert speech) and that considering this variety helps reconciling seemingly contradictory findings from the neuroimaging literature.

A new statistical test based on the wavelet cross-spectrum to detect time-frequency dependence between non-stationary signals: application to the analysis of cortico-muscular interactions.

The study of the correlations that may exist between neurophysiological signals is at the heart of modern techniques for data analysis in neuroscience. Wavelet coherence is a popular method to construct a time-frequency map that can be used to analyze the time-frequency correlations between two time series. Coherence is a normalized measure of dependence, for which it is possible to construct confidence intervals, and that is commonly considered as being more interpretable than the wavelet cross-spectrum (WCS). In this paper, we provide empirical and theoretical arguments to show that a significant level of wavelet coherence does not necessarily correspond to a significant level of dependence between random signals, especially when the number of trials is small. In such cases, we demonstrate that the WCS is a much better measure of statistical dependence, and a new statistical test to detect significant values of the cross-spectrum is proposed. This test clearly outperforms the limitations of coherence analysis while still allowing a consistent estimation of the time-frequency correlations between two non-stationary stochastic processes. Simulated data are used to investigate the advantages of this new approach over coherence analysis. The method is also applied to experimental data sets to analyze the time-frequency correlations that may exist between electroencephalogram (EEG) and surface electromyogram (EMG).

What differs in visual recognition of handwritten vs. printed letters? An fMRI study.

In models of letter recognition, handwritten letters are considered as a particular font exemplar, not qualitatively different in their processing from printed letters. Yet, some data suggest that recognizing handwritten letters might rely on distinct processes, possibly related to motor knowledge. We applied functional magnetic resonance imaging to compare the neural correlates of perceiving handwritten letters vs. standard printed letters. Statistical analysis circumscribed to frontal brain regions involved in hand-movement triggering and execution showed that processing of handwritten letters is supported by a stronger activation of the left primary motor cortex and the supplementary motor area. At the whole-brain level, additional differences between handwritten and printed letters were observed in the right superior frontal, middle occipital, and parahippocampal gyri, and in the left inferior precentral and the fusiform gyri. The results are suggested to indicate embodiment of the visual perception of handwritten letters.

The Human Basal Ganglia Mediate the Interplay between Reactive and Proactive Control of Response through Both Motor Inhibition and Sensory Modulation.

The basal ganglia (BG) have long been known for contributing to the regulation of motor behaviour by means of a complex interplay between tonic and phasic inhibitory mechanisms. However, after having focused for a long time on phasic reactive mechanisms, it is only recently that psychological research in healthy humans has modelled tonic proactive mechanisms of control. Mutual calibration between anatomo-functional and psychological models is still needed to better understand the unclear role of the BG in the interplay between proactive and reactive mechanisms of control. Here, we implemented an event-related fMRI design allowing proper analysis of both the brain activity preceding the target-stimulus and the brain activity induced by the target-stimulus during a simple go/nogo task, with a particular interest in the ambiguous role of the basal ganglia. Post-stimulus activity was evoked in the left dorsal striatum, the subthalamus nucleus and internal globus pallidus by any stimulus when the situation was unpredictable, pinpointing its involvement in reactive, non-selective inhibitory mechanisms when action restraint is required. Pre-stimulus activity was detected in the ventral, not the dorsal, striatum, when the situation was unpredictable, and was associated with changes in functional connectivity with the early visual, not the motor, cortex. This suggests that the ventral striatum supports modulatory influence over sensory processing during proactive control.

The graphemic/motor frontal area Exner's area revisited.

OBJECTIVE: In 1881, Exner first described a "graphic motor image center" in the middle frontal gyrus. Current psycholinguistic models of handwriting involve the conversion of abstract, orthographic representations into motor representations before a sequence of appropriate hand movements is produced. Direct cortical stimulation and functional magnetic resonance imaging (fMRI) were used to study the human frontal areas involved in writing. METHODS: Cortical electrical stimulation mapping was used intraoperatively in 12 patients during the removal of brain tumors to identify the areas involved in oral language (sentence reading and naming) and writing, and to spare them during surgery. The fMRI activation experiment involved 12 right-handed and 12 left-handed healthy volunteers using word dictation (without visual control) and 2 control tasks. RESULTS: Direct cortical electrical stimulation of restricted areas rostral to the primary motor hand area (Brodmann area [BA] 6) impaired handwriting in 6 patients, without disturbing hand movements or oral language tasks. In 6 other patients, stimulation of lower frontal regions showed deficits combining handwriting with other language tasks. fMRI also revealed selective activation during word handwriting in left versus right BA6 depending on handedness. This area was anatomically matched to those areas that affected handwriting on electrical stimulation. INTERPRETATION: An area in middle frontal gyrus (BA6) that we have termed the graphemic/motor frontal area supports bridging between orthography and motor programs specific to handwriting.

Shared premotor activity in spoken and written communication.

The aim of the present study was to uncover a possible common neural organizing principle in spoken and written communication, through the coupling of perceptual and motor representations. In order to identify possible shared neural substrates for processing the basic units of spoken and written language, a sparse sampling fMRI acquisition protocol was performed on the same subjects in two experimental sessions with similar sets of letters being read and written and of phonemes being heard and orally produced. We found evidence of common premotor regions activated in spoken and written language, both in perception and in production. The location of those brain regions was confined to the left lateral and medial frontal cortices, at locations corresponding to the premotor cortex, inferior frontal cortex and supplementary motor area. Interestingly, the speaking and writing tasks also appeared to be controlled by largely overlapping networks, possibly indicating some domain general cognitive processing. Finally, the spatial distribution of individual activation peaks further showed more dorsal and more left-lateralized premotor activations in written than in spoken language.