Human motor cortical organization is influenced by handedness

Sex differences in mild vascular cognitive impairment: A multimodal transcranial magnetic stimulation study

BACKGROUND

Sex differences in vascular cognitive impairment (VCI) at risk for future dementia are still debatable. Transcranial magnetic stimulation (TMS) is used to evaluate cortical excitability and the underlying transmission pathways, although a direct comparison between males and females with mild VCI is lacking.

METHODS

Sixty patients (33 females) underwent clinical, psychopathological, functional, and TMS assessment. Measures of interest consisted of: resting motor threshold, latency of motor evoked potentials (MEPs), contralateral silent period, amplitude ratio, central motor conduction time (CMCT), including the F wave technique (CMCT-F), short-interval intracortical inhibition (SICI), intracortical facilitation, and short-latency afferent inhibition, at different interstimulus intervals (ISIs).

RESULTS

Males and females were comparable for age, education, vascular burden, and neuropsychiatric symptoms. Males scored worse at global cognitive tests, executive functioning, and independence scales. MEP latency was significantly longer in males, from both sides, as well CMCT and CMCT-F from the left hemisphere; a lower SICI at ISI of 3 ms from the right hemisphere was also found. After correction for demographic and anthropometric features, the effect of sex remained statistically significant for MEP latency, bilaterally, and for CMCT-F and SICI. The presence of diabetes, MEP latency bilaterally, and both CMCT and CMCT-F from the right hemisphere inversely correlated with executive functioning, whereas TMS did not correlate with vascular burden.

CONCLUSIONS

We confirm the worse cognitive profile and functional status of males with mild VCI compared to females and first highlight sex-specific changes in intracortical and cortico-spinal excitability to multimodal TMS in this population. This points to some TMS measures as potential markers of cognitive impairment, as well as targets for new drugs and neuromodulation therapies.

Impact of handedness on interlimb transfer depending on the task complexity combined with motor and cognitive skills

PURPOSE

Task complexity could affect acquisition efficiency of motor skills and interlimb transfer; however, how task complexity affects interlimb transfer remains unclear. We hypothesized that left- and right-handed participants may have different interlimb transfer efficiency depending on the task complexity.

METHODS

Left-hand (n = 28) and right-hand (n = 28) dominant participants (age = 24.70 ± 4.02 years, male:female = 28:28) performed a finger sequence test with two levels of complexity (simple: one-digit with four fingers vs. complex: two-digit with five fingers) before and after ten trials of 2-min practice each on the same apparatus. The speed and task errors were measured and analyzed.

RESULTS

Right-handed participants failed to improve performance on their right hand (non-trained hand) after contralateral left-hand practice in the simple finger sequence task. In contrast, the left-handed participants improved performance on non-trained hands both right and left after contralateral practices. In the complex task, however, both the left- and right-handed participants improved performance on non-trained hands by contralateral practices.

CONCLUSION

Our results showed that task complexity of skilled practice gave different effects on interlimb transfer between right- and left-handed subjects. It appears that a certain level of appropriate complexity is necessary to detect inter-limb transfers in motor learning in right-handed subjects.

Neuroenhancement of a dexterous motor task with Anodal tDCS

Motor skill learning can cause structural and functional changes in the primary motor cortex (M1) leading to cortical plasticity that can be associated with the performance change during the motor skill that is practiced. Similarly, anodal transcranial direct current stimulation (a-tDCS) has been shown to facilitate and enhance plasticity in M1, causing even greater motor skill improvement. By using a fine motor task (O'Connor Tweezer Dexterity Task) in combination with a-tDCS we theorized that a-tDCS could increase the speed of skill acquisition. Forty subjects were recruited and randomized into either a-tDCS or SHAM groups. Subjects completed a single session performing the O'Connor Tweezer Dexterity Task with their non-dominant hand while receiving either a-tDCS stimulation or SHAM stimulation of the hand region of M1. The time it took to place 50- pins was assessed before and after 20 minutes of practice with a-tDCS or SHAM. We found that both groups had similar pre-test performance (P=0.94) and they both had a similar amount of practice pins placed (P=0.69). However, the a-tDCS group had a greater improvement than the SHAM group (p=0.028) for overall learning from pretest to posttest. These results suggest that a-tDCS improved the rate of motor learning and fine motor task performance. These results are in line with previous research and demonstrate that a-tDCS applied to M1 can increase manual precision and steadiness needed for delicate tasks and could have implications in the advancement of surgical training as well as in athletic, military, and other occupational settings.

Mapping of Motor Function with Neuronavigated Transcranial Magnetic Stimulation: A Review on Clinical Application in Brain Tumors and Methods for Ensuring Feasible Accuracy

Navigated transcranial magnetic stimulation (nTMS) has developed into a reliable non-invasive clinical and scientific tool over the past decade. Specifically, it has undergone several validating clinical trials that demonstrated high agreement with intraoperative direct electrical stimulation (DES), which paved the way for increasing application for the purpose of motor mapping in patients harboring motor-eloquent intracranial neoplasms. Based on this clinical use case of the technique, in this article we review the evidence for the feasibility of motor mapping and derived models (risk stratification and prediction, nTMS-based fiber tracking, improvement of clinical outcome, and assessment of functional plasticity), and provide collected sets of evidence for the applicability of quantitative mapping with nTMS. In addition, we provide evidence-based demonstrations on factors that ensure methodological feasibility and accuracy of the motor mapping procedure. We demonstrate that selection of the stimulation intensity (SI) for nTMS and spatial density of stimuli are crucial factors for applying motor mapping accurately, while also demonstrating the effect on the motor maps. We conclude that while the application of nTMS motor mapping has been impressively spread over the past decade, there are still variations in the applied protocols and parameters, which could be optimized for the purpose of reliable quantitative mapping.

Handedness effects on motor imagery during kinesthetic and visual-motor conditions

Recent studies show that during a simple movement imagery task, the power of sensorimotor rhythms differs according to handedness. However, the effects of motor imagery perspectives on these differences have not been investigated yet. Our study aimed to check how handedness impacts the activity of alpha (8-13 Hz) and beta (15-30 Hz) oscillations during creating a kinesthetic (KMI) or visual-motor (VMI) representation of movement. Forty subjects (20 right-handed and 20 left-handed) who participated in the experiment were tasked with imagining sequential finger movement from a visual or kinesthetic perspective. Both the electroencephalographic (EEG) activity and behavioral correctness of the imagery task performance were measured. After the registration, we used independent component analysis (ICA) on EEG data to localize visual- and motor-related EEG sources of activity shared by both motor imagery conditions. Significant differences were obtained in the visual cortex (the occipital ICs cluster) and the right motor-related area (right parietal ICs cluster). In comparison to right-handers who, regardless of the task, demonstrated the same pattern in the visual area, left-handers obtained higher power in the alpha waves in the VMI task and better performance in this condition. On the other hand, only the right-handed showed different patterns in the alpha waves in the right motor cortex during the KMI condition. The results indicate that left-handers imagine movement differently than right-handers, focusing on visual experience. This provides new empirical evidence on the influence of movement preferences on imagery processes and has possible future implications for research in the area of neurorehabilitation and motor imagery-based brain-computer interfaces (MI-BCIs).

Handedness Development: A Model for Investigating the Development of Hemispheric Specialization and Interhemispheric Coordination

The author presents his perspective on the character of science, development, and handedness and relates these to his investigations of the early development of handedness. After presenting some ideas on what hemispheric specialization of function might mean for neural processing and how handedness should be assessed, the neuroscience of control of the arms/hands and interhemispheric communication and coordination are examined for how developmental processes can affect these mechanisms. The author’s work on the development of early handedness is reviewed and placed within a context of cascading events in which different forms of handedness emerge from earlier forms but not in a deterministic manner. This approach supports a continuous rather than categorical distribution of handedness and accounts for the predominance of right-handedness while maintaining a minority of left-handedness. Finally, the relation of the development of handedness to the development of several language and cognitive skills is examined.

The effect of handedness on mental rotation of hands: a systematic review and meta-analysis

Body-specific mental rotation is thought to rely upon internal representations of motor actions. Handedness is a source of distinctly different motor experience that shapes the development of such internal representations. Yet, the influence of handedness upon hand mental rotation has never been systematically evaluated. Five databases were searched for studies evaluating hand left/right judgement tasks in adults. Two independent reviewers performed screening, data extraction, and critical appraisal. Eighty-seven datasets were included, with 72 datasets pooled; all had unclear/high risk of bias. Meta-analyses showed that right-handers were faster, but not more accurate, than left-handers at hand mental rotation. A unique effect of handedness was found on performance facilitation for images corresponding to the dominant hand. Meta-analyses showed that right-handers were quicker at identifying images of right hands than left hands-a dominance advantage not evident in left-handers. Differing hand representations (more lateralised hand dominance in right-handers) likely underpin these findings. Given potential differences between hand preference and motor performance, future research exploring their distinct contributions to mental rotation is warranted.

Primary hand motor representation areas in healthy children, preadolescents, adolescents, and adults

The development of the organization of the motor representation areas in children and adolescents is not well-known. This cross-sectional study aimed to provide an understanding for the development of the functional motor areas of the upper extremity muscles by studying healthy right-handed children (6-9 years, n = 10), preadolescents (10-12 years, n = 13), adolescents (15-17 years, n = 12), and adults (22-34 years, n = 12). The optimal representation site and resting motor threshold (rMT) for the abductor pollicis brevis (APB) were assessed in both hemispheres using navigated transcranial magnetic stimulation (nTMS). Motor mapping was performed at 110% of the rMT while recording the EMG of six upper limb muscles in the hand and forearm. The association between the motor map and manual dexterity (box and block test, BBT) was examined. The mapping was well-tolerated and feasible in all but the youngest participant whose rMT exceeded the maximum stimulator output. The centers-of-gravity (CoG) for individual muscles were scattered to the greatest extent in the group of preadolescents and centered and became more focused with age. In preadolescents, the CoGs in the left hemisphere were located more laterally, and they shifted medially with age. The proportion of hand compared to arm representation increased with age (p = 0.001); in the right hemisphere, this was associated with greater fine motor ability. Similarly, there was less overlap between hand and forearm muscles representations in children compared to adults (p<0.001). There was a posterior-anterior shift in the APB hotspot coordinate with age, and the APB coordinate in the left hemisphere exhibited a lateral to medial shift with age from adolescence to adulthood (p = 0.006). Our results contribute to the elucidation of the developmental course in the organization of the motor cortex and its associations with fine motor skills. It was shown that nTMS motor mapping in relaxed muscles is feasible in developmental studies in children older than seven years of age.

Altered motor system function in post-concussion syndrome as assessed via transcranial magnetic stimulation

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Study examining corticospinal and cortical activity in post-concussion.

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Reduction in GABAB-mediated inhibition observed.

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These changes were associated with depression-related symptoms.

Objective

It is unclear why specific individuals incur chronic symptoms following a concussion. This exploratory research aims to identify and characterize any neurophysiological differences that may exist in motor cortex function in post-concussion syndrome (PCS).

Methods

Fifteen adults with PCS and 13 healthy, non-injured adults were tested. All participants completed symptom questionnaires, and transcranial magnetic stimulation (TMS) was used to measure intracortical and transcallosal excitability and inhibition in the dominant motor cortex.

Results

Cortical silent period (p = 0.02, g = 0.96) and ipsilateral silent period (p = 0.04, g = 0.78) were shorter in the PCS group compared to the control group which may reflect reduced GABA-mediated inhibition in PCS. Furthermore, increased corticomotor excitability was observed in the left hemisphere but not the right hemisphere.

Conclusions

These data suggest that persistent neurophysiological differences are present in those with PCS. The exact contributing factors to such changes remain to be investigated by future studies.

Significance

This study provides novel evidence of lasting neurophysiological changes in PCS.

Electroacupuncture-Induced Plasticity between Different Representations in Human Motor Cortex

Somatosensory stimulation can effectively induce plasticity in the motor cortex representation of the stimulated body part. Specific interactions have been reported between different representations within the primary motor cortex. However, studies evaluating somatosensory stimulation-induced plasticity between different representations within the primary motor cortex are sparse. The purpose of this study was to investigate the effect of somatosensory stimulation on the modulation of plasticity between different representations within the primary motor cortex. Twelve healthy volunteers received both electroacupuncture (EA) and sham EA at the TE5 acupoint (located on the forearm). Plasticity changes in different representations, including the map volume, map area, and centre of gravity (COG) were evaluated by transcranial magnetic stimulation (TMS) before and after the intervention. EA significantly increased the map volume of the forearm and hand representations compared to those of sham EA and significantly reduced the map volume of the face representation compared to that before EA. No significant change was found in the map volume of the upper arm and leg representations after EA, and likewise, no significant changes in map area and COG were observed. These results suggest that EA functions as a form of somatosensory stimulation to effectively induce plasticity between different representations within the primary motor cortex, which may be related to the extensive horizontal intrinsic connectivity between different representations. The cortical plasticity induced by somatosensory stimulation might be purposefully used to modulate human cortical function.

Motor competence is not enough: Handedness does not facilitate visual anticipation of same-handed action outcome

Visual understanding of others performing an action depends on both an observer's visual and motor experience with that action. With regard to visual anticipation of lateralized action outcome in one-on-one confrontative situations, however, the particular role of motor experience is poorly understood. Here, we considered handedness to test the laterality-specific contribution of visual and motor experience to action outcome anticipation. In two experiments, 55 left- and 114 right-handed handball players predicted the outcome (Exp. 1: throw direction; Exp. 2: type of throw) of videos showing left- and right-handed penalty-throws viewed from a goalkeeper's perspective. Analyses reveal that left- and right-handed participants performed similarly and had more difficulties anticipating the outcome direction, but not type of throw, of left- compared to right-handed penalties. Thus, albeit left- and right-handers differ in their lateralized motor experience, this does not seem to be sufficient to facilitate visual anticipation of same-handed action outcome. Instead, findings lend further support to the specificity of perceptual learning and visual experience arising from both left- and right-handers' predominant exposure to more common right-handed movements.

Psychometric properties of the Spanish version of the Edinburgh Handedness Inventory in a sample of Chilean undergraduates

An individual's nervous and cognitive systems are lateralized, and handedness represents a behavioral manifestation of such organization. Therefore, accurately and reliably measuring handedness has repercussion on our understanding of both the human brain and cognition. The Edinburgh Handedness Inventory (EHI) is the most frequently used instrument to measure handedness both in clinical practice and research. We assessed the psychometric properties of the Spanish version of the EHI in a sample of 348 Chilean university students by confirmatory factor analysis. Cronbach's alpha and composite reliability were calculated to evaluate the internal consistency and reliability of the EHI, while the average variance extracted was estimated to evaluate its convergent validity. A 10-item unifactorial structure was confirmed, with factor loadings ≥0.50, showing excellent goodness-of-fit indicators, very high internal consistency and adequate composite reliability and convergent validity. Socio-demographic variables (sex, area of residence and belonging to an indigenous people or community) did not significantly modulate the EHI scores. Overall, by using this validated version of the EHI to accurately and reliably measure handedness in the greater Spanish population, researchers will be able to produce robust data to tackle the still open questions of lateralization in human cognitive and neural architecture.

A sinister subject: Quantifying handedness-based recruitment biases in current neuroimaging research

Approximately ten per cent of humans are left-handed or ambidextrous (adextral). It has been suggested that, despite their sizable representation at the whole-population level, this demographic is largely avoided by researchers within the neuroimaging community. To date, however, no formal effort has been made to quantify the extent to which adextrals are excluded from neuroimaging-based research. Here, we aimed to address this question in a review of over 1,000 recent articles published in high-impact, peer-reviewed, neuroimaging-focused journals. Specifically, we sought to ascertain whether, and the extent to which adextrals are underrepresented in neuroimaging study samples, and to delineate potential trends in this bias. Handedness data were available for over 30,000 research subjects; only around 3%-4% of these individuals were adextral-considerably less than the 10% benchmark one would expect if neuroimaging samples were truly representative of the general population. This observation was generally consistent across different areas of research, but was modulated by the demographic characteristics of neuroimaging participants. The epistemological and ethical implications of these findings are discussed.

Parallel modulation of interhemispheric inhibition and the size of a cortical hand muscle representation during active contraction

Interhemispheric inhibition (IHI) between motor cortexes is thought to suppress unwanted mirror movements during voluntary behaviors and can be assessed using paired-pulse transcranial magnetic stimulation (TMS). The magnitude of IHI may be related to the size of the cortical representation for a given muscle as a mechanism for facilitating unimanual control. To date, the relationship between IHI and cortical muscle representations remains unknown. Fifteen healthy, right-handed individuals participated in the present study. IHI was examined in the right first dorsal interosseous (FDI) muscle by delivering conditioning TMS to ipsilateral (right) primary motor cortex (M1) followed by a test TMS pulse to contralateral (left) M1. The size of the FDI representation in M1 was determined by delivering suprathreshold TMS over a 5 × 5-cm grid centered on the FDI motor hotspot of the left M1. Both IHI and cortical territory were obtained during three conditions: rest, contralateral (right) FDI contraction, and ipsilateral (left) FDI contraction. Results indicate a significant association between IHI and the size of the FDI representation only in the context of contraction and not when the FDI muscle was relaxed. Specifically, reduced IHI corresponded to larger cortical FDI representations during both contralateral and ipsilateral contraction. These data demonstrate that, for a muscle of the hand, the magnitude of IHI and the cortical territory are associated within the context of muscle contraction. NEW & NOTEWORTHY This study provides evidence from noninvasive brain stimulation that communication between the motor cortexes of the two hemispheres plays a role in shaping the motor cortical map that outputs to a hand muscle during active contraction of that muscle. This relationship exists only when the hand muscle is contracted. The findings presented further our understanding of motor control during unilateral movement and may inform future research targeting clinical populations that exhibit impaired unilateral control.

Localization of Sensorimotor Cortex Using Navigated Transcranial Magnetic Stimulation and Magnetoencephalography

The mapping of the sensorimotor cortex gives information about the cortical motor and sensory functions. Typical mapping methods are navigated transcranial magnetic stimulation (TMS) and magnetoencephalography (MEG). The differences between these mapping methods are, however, not fully known. TMS center of gravities (CoGs), MEG somatosensory evoked fields (SEFs), corticomuscular coherence (CMC), and corticokinematic coherence (CKC) were mapped in ten healthy adults. TMS mapping was performed for first dorsal interosseous (FDI) and extensor carpi radialis (ECR) muscles. SEFs were induced by tactile stimulation of the index finger. CMC and CKC were determined as the coherence between MEG signals and the electromyography or accelerometer signals, respectively, during voluntary muscle activity. CMC was mapped during the activation of FDI and ECR muscles separately, whereas CKC was measured during the waving of the index finger at a rate of 3–4 Hz. The maximum CMC was found at beta frequency range, whereas maximum CKC was found at the movement frequency. The mean Euclidean distances between different localizations were within 20 mm. The smallest distance was found between TMS FDI and TMS ECR CoGs and longest between CMC FDI and CMC ECR sites. TMS-inferred localizations (CoGs) were less variable across participants than MEG-inferred localizations (CMC, CKC). On average, SEF locations were 8 mm lateral to the TMS CoGs (p < 0.01). No differences between hemispheres were found. Based on the results, TMS appears to be more viable than MEG in locating motor cortical areas.