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Concept: Right-handedness


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.

Concepts: Left-handedness, Evoked potential, Transcranial magnetic stimulation, Handedness, Right-handedness, Laterality


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.

Concepts: Pregnancy, Childbirth, Embryo, Fetus, Left-handedness, Gestational age, Handedness, Right-handedness


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.

Concepts: Left-handedness, Handedness, Right-handedness


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.

Concepts: Bone, Humerus, Ulna, Left-handedness, Upper limb anatomy, Upper limb, Handedness, Right-handedness


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.

Concepts: Immune system, Cancer, Ionizing radiation, Breast cancer, Chemotherapy, Left-handedness, Handedness, Right-handedness


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.

Concepts: Left-handedness, Handedness, Ambidexterity, Michael Gazzaniga, Roger Wolcott Sperry, Right-handedness, Lateralization of brain function, Popular psychology


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).

Concepts: Left-wing politics, Left-handedness, Right-wing politics, Political spectrum, French Revolution, Handedness, Grip strength, Right-handedness


The objective of this study was to replicate the association between atypical handedness and psychosis-proneness in a representative sample of adolescents from the general population. It expands previous studies by (1) analyzing a variety of atypical handedness indexes (left, mixed, ambiguous, and inconsistent), (2) measuring comprehensively the multidimensionality of psychosis-proneness, and (3) analyzing the association of different patterns of atypical handedness with nonclinical dimensions of both trait (schizotypy) and sub-clinical symptom (psychotic-like experiences) levels. Seven hundred and twenty-eight adolescents were assessed for handedness by the 12-item self-report Annett Hand Preference Questionnaire and for psychosis-proneness by the O-LIFE and CAPE scales. Writing-hand alone did not detect associations between laterality and psychosis-proneness. Mixed- rather than left-handedness was related to psychosis-proneness, and this was more evident when analyzing subjects with ambiguous handedness exclusively. Analyzing subjects with non-ambiguous handedness exclusively, strong-left handedness was related to psychosis-proneness. The positive dimension showed a stronger association than the negative one with atypical handedness. Results partially support mixed-handedness as a marker of developmental disorders underlying both atypical lateralization and psychosis-proneness. Among various possible mixed-handedness patterns, inconsistent hand use across primary actions, and for the same action across time, seems particularly related to psychosis-proneness and thus requires further exploration.

Concepts: Dimension, Left-handedness, Handedness, Ambidexterity, Right-handedness, Cross-dominance, Laterality


Unlike the phonological loop in spoken language monitoring, sign language users' own production provides mostly proprioceptive feedback and only minimal visual feedback. Here we investigate whether sign production influences sign comprehension by exploiting hand dominance in a picture-sign matching task performed by left-handed signers and right-handed signers. Should all signers perform better to right-handed input, this would suggest that a frequency effect in sign perception drives comprehension. However, if signers perform better to congruent-handed input, this would implicate the production system’s role in comprehension. We found evidence for both hypotheses, with variation dependent on sign type. All signers performed faster to right-handers for phonologically simple, one-handed signs. However, left-handed signers preferred congruent-handed input for phonologically complex, two-handed asymmetrical signs. These results are in line with a weak version of the motor theory of speech perception, where the motor system is only engaged when comprehending complex input.

Concepts: Phonology, Linguistics, Language, Left-handedness, Phonetics, Handedness, Right-handedness, Sign


Assessing manual lateralization in non-human primates could be an optimal way to understand the adaptive value of this asymmetry in humans. Though many studies have investigated hand preferences in Old and New World monkeys and apes, fewer studies have considered manual lateralization in strepsirrhines, especially in experimental tasks. This study investigated hand preferences for a unimanual and a bimanual task of 17 captive ring-tailed lemurs (Lemur catta), housed at Parco Natura Viva (VR), Italy. The effect of age on handedness has been also investigated. The lemurs were tested on a unimanual task, in which subjects were restricted to using one hand to retrieve the food inside an apparatus, and on a bimanual task, in which lemurs had to use one hand to keep the apparatus door open while reaching with the other hand to retrieve the food inside it. At the population-level, our results revealed an asymmetrical hand use distribution, in particular a bias toward a right hand preference for food reaching in both the unimanual and the bimanual tasks. Furthermore, at the individual-level, the bimanual task seems to elicit a greater hand preference than the unimanual task. Results of this study underline the importance of experimental tasks in determining hand preference in strepsirrhines. Furthermore, as bimanual tasks elicited a stronger degree of lateralization, they appear to be more suited to investigate manual laterality. Finally, findings from this study highlight the presence of a right hand preference in ring-tailed lemurs, shedding new light on the evolution of human right handedness. Am. J. Primatol. © 2016 Wiley Periodicals, Inc.

Concepts: Left-handedness, Primate, Lemur, Strepsirrhini, Handedness, Ring-tailed Lemur, Lemuridae, Right-handedness