The idea of motor resonance was born at the time that it was demonstrated that cortical and spinal pathways of the motor system are specifically activated during both action-observation and execution. What is not known is if the human action observation-execution matching system simulates actions through motor representations specifically attuned to the laterality of the observed effectors (i.e., effector-dependent representations) or through abstract motor representations unconnected to the observed effector (i.e., effector-independent representations). To answer that question we need to know how the information necessary for motor resonance is represented or integrated within the representation of an effector. Transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEPs) were thus recorded from the dominant and non-dominant hands of left- and right-handed participants while they observed a left- or a right-handed model grasping an object. The anatomical correspondence between the effector being observed and the observer’s effector classically reported in the literature was confirmed by the MEP response in the dominant hand of participants observing models with their same hand preference. This effect was found in both left- as well as in right-handers. When a broader spectrum of options, such as actions performed by a model with a different hand preference, was instead considered, that correspondence disappeared. Motor resonance was noted in the observer’s dominant effector regardless of the laterality of the hand being observed. This would indicate that there is a more sophisticated mechanism which works to convert someone else’s pattern of movement into the observer’s optimal motor commands and that effector-independent representations specifically modulate motor resonance.
Physical practice with one hand results in performance gains of the other (un-practiced) hand, yet the role of sensory feedback and underlying neurophysiology is unclear. Healthy subjects learned sequences of finger movements by physical training with their right hand while receiving real-time movement-based visual feedback via 3D virtual reality devices as if their immobile left hand was training. This manipulation resulted in significantly enhanced performance gain with the immobile hand, which was further increased when left-hand fingers were yoked to passively follow right-hand voluntary movements. Neuroimaging data show that, during training with manipulated visual feedback, activity in the left and right superior parietal lobule and their degree of coupling with motor and visual cortex, respectively, correlate with subsequent left-hand performance gain. These results point to a neural network subserving short-term motor skill learning and may have implications for developing new approaches for learning and rehabilitation in patients with unilateral motor deficits.
Auditory cues are frequently used to support movement learning and rehabilitation, but the neural basis of this behavioural effect is not yet clear. We investigated the microstructural neuroplasticity effects of adding musical cues to a motor learning task. We hypothesised that music-cued, left-handed motor training would increase fractional anisotropy (FA) in the contralateral arcuate fasciculus, a fibre tract connecting auditory, pre-motor and motor regions. Thirty right-handed participants were assigned to a motor learning condition either with (Music Group) or without (Control Group) musical cues. Participants completed 20minutes of training three times per week over four weeks. Diffusion tensor MRI and probabilistic neighbourhood tractography identified FA, axial (AD) and radial (RD) diffusivity before and after training. Results revealed that FA increased significantly in the right arcuate fasciculus of the Music group only, as hypothesised, with trends for AD to increase and RD to decrease, a pattern of results consistent with activity-dependent increases in myelination. No significant changes were found in the left ipsilateral arcuate fasciculus of either group. This is the first evidence that adding musical cues to movement learning can induce rapid microstructural change in white matter pathways in adults, with potential implications for therapeutic clinical practice.
The vast majority of humans are right-handed, but how and when this bias emerges during human ontogenesis is still unclear. We propose an approach that explains postnatal handedness starting from 18 gestational weeks using a kinematic analysis of different fetal arm movements recorded during ultrasonography. Based on the hand dominance reported postnatally at age 9, the fetuses were classified as right-handed (86%) or left-handed, in line with population data. We revealed that both right-handed and left-handed fetuses were faster to reach to targets requiring greater precision (i.e., eye and mouth), with their dominant (vs. non-dominant) hand. By using either movement times or deceleration estimates, handedness can be inferred with a classification accuracy ranging from 89 to 100% from gestational week 18. The reliability of this inference hints to the yet unexplored potential of standard ultrasonography to advance our understanding of prenatal life.
Labial striations on the anterior teeth have been documented in numerous European pre-Neandertal and Neandertal fossils and serve as evidence for handedness. OH-65, dated at 1.8 mya, shows a concentration of oblique striations on, especially, the left I(1) and right I(1), I(2) and C(1), which signal that it was right-handed. From these patterns we contend that OH-65 was habitually using the right hand, over the left, in manipulating objects during some kind of oral processing. In living humans right-handedness is generally correlated with brain lateralization, although the strength of the association is questioned by some. We propose that as more specimens are found, right-handedness, as seen in living Homo, will most probably be typical of these early hominins.
We describe and analyze a Neandertal postcranial skeleton and dentition, which together show unambiguous signs of right-handedness. Asymmetries between the left and right upper arm in Regourdou 1 were identified nearly 20 years ago, then confirmed by more detailed analyses of the inner bone structure for the clavicle, humerus, radius and ulna. The total pattern of all bones in the shoulder and arm reveals that Regourdou 1 was a right-hander. Confirmatory evidence comes from the mandibular incisors, which display a distinct pattern of right oblique scratches, typical of right-handed manipulations performed at the front of the mouth. Regourdou’s right handedness is consistent with the strong pattern of manual lateralization in Neandertals and further confirms a modern pattern of left brain dominance, presumably signally linguistic competence. These observations along with cultural, genetic and morphological evidence indicate language competence in Neandertals and their European precursors.
Radiosensitivity is a biological response to radiation. This response depends on many factors such as radiation factors as well as biological system factors. It is shown that identical doses of radiation for the treatment of Cancer patients produce different biological responses that are assumed to be depend on different specifications of the biological systems. However, by elimination of these factors, people may still show different biological responses such as acute and low responses to radiotherapy in similar doses of radiation. Some reports indicate that breast cancer, immune diseases including autoimmune diseases such as lupus, Myasthenia Gravies and even the rate of allergy are more frequent in left-handed compared to right-handed individuals. The main goal of the present study is determination of radiosensitivity in left-handed compared to right-handed in breast cancer women by cytogenetic assay. Peripheral venous blood samples (10 ml) of 30 breast cancer women (10 left- and 20 right-handed) were divided into two identical parts. One part is exposed to 2 Gy Co-60 gamma rays, and the second part is considered as non-exposed controls. Lymphocytes were cultured in standard media, and cytokinesis blocked to score micronuclei in bi-nucleated cells. The frequency of micronuclei in 1,000 cells in each sample is considered as the rate of radiosensitivity and was compared in left- and right-handed breast cancer women by appropriate statistical analysis. Results showed that radiosensitivity index in left-handers is higher than right-handers also mean frequency of MN in exposed group of left-handers compare to right-handers is elevated. It seems that left-handed breast cancer women are more radiosensitive than right-handed. More investigations on right- and left-handed healthy people are ongoing in our laboratory.
Hemispheric lateralization for language production and its relationships with manual preference and manual preference strength were studied in a sample of 297 subjects, including 153 left-handers (LH). A hemispheric functional lateralization index (HFLI) for language was derived from fMRI acquired during a covert sentence generation task as compared with a covert word list recitation. The multimodal HFLI distribution was optimally modeled using a mixture of 3 and 4 Gaussian functions in right-handers (RH) and LH, respectively. Gaussian function parameters helped to define 3 types of language hemispheric lateralization, namely “Typical” (left hemisphere dominance with clear positive HFLI values, 88% of RH, 78% of LH), “Ambilateral” (no dominant hemisphere with HFLI values close to 0, 12% of RH, 15% of LH) and “Strongly-atypical” (right-hemisphere dominance with clear negative HFLI values, 7% of LH). Concordance between dominant hemispheres for hand and for language did not exceed chance level, and most of the association between handedness and language lateralization was explained by the fact that all Strongly-atypical individuals were left-handed. Similarly, most of the relationship between language lateralization and manual preference strength was explained by the fact that Strongly-atypical individuals exhibited a strong preference for their left hand. These results indicate that concordance of hemispheric dominance for hand and for language occurs barely above the chance level, except in a group of rare individuals (less than 1% in the general population) who exhibit strong right hemisphere dominance for both language and their preferred hand. They call for a revisit of models hypothesizing common determinants for handedness and for language dominance.
Laterality in handgrip strength was assessed by analyzing dynamometric data of the right and left hand in three samples of Lithuanian boys and girls aged 7-20 years. In addition, the influence of general physical training on the laterality of handgrip strength was explored in a sample of conscripts. A negative secular trend in handgrip strength of schoolchildren has been detected since 1965, and with increasing age, right-handedness has become more pronounced. Children that were ambidextrous (by grip strength) showed negative deviations in physical status more often than their right- or left-handed peers. During one year of physical training, the conscripts had a larger increase in grip strength of the left than in the right hand, and a marked shift in handgrip laterality toward left-handed and ambidextrous individuals was observed. The different impact of schooling and physical training on handgrip strength laterality might partly explain variations in the prevalence of handedness in different societies with divergent cultures and lifestyles (e.g., more or less sedentary).
Williams syndrome is a neurodevelopmental genetic disorder caused by a hemizygous deletion on chromosome 7q11.23, resulting in atypical brain structure and function, including abnormal morphology of the corpus callosum. An influence of handedness on the size of the corpus callosum has been observed in studies of typical individuals, but handedness has not been taken into account in studies of callosal morphology in Williams syndrome. We hypothesized that callosal area is smaller and the size of the splenium and isthmus is reduced in individuals with Williams syndrome compared to healthy controls, and examined age, sex, and handedness effects on corpus callosal area. Structural magnetic resonance imaging scans were obtained on 25 individuals with Williams syndrome (18 right-handed, 7 left-handed) and 25 matched controls. We found that callosal thickness was significantly reduced in the splenium of Williams syndrome individuals compared to controls. We also found novel evidence that the callosal area was smaller in left-handed participants with Williams syndrome than their right-handed counterparts, with opposite findings observed in the control group. This novel finding may be associated with LIM-kinase hemizygosity, a characteristic of Williams syndrome. The findings may have significant clinical implications in future explorations of the Williams syndrome cognitive phenotype.