Posts tagged handedness
Articles and news from the latest research reports.
More left-handed men are born during the winter
Men born in November, December or January are more likely of being left-handed than during the rest of the year. While the genetic bases of handedness are still under debate, scientists at the Faculty of Psychology, University of Vienna, obtained indirect evidence of a hormonal mechanism promoting left-handedness among men. Psychologist Ulrich Tran and his colleagues published their findings in the scientific journal “Cortex”.
Various manual tasks in everyday life require the use of the right hand or are optimized for right-handers. Around 90 percent of the general population is right-handed, only about 10 percent is left-handed. The study of Ulrich Tran, Stefan Stieger, and Martin Voracek comprised two large and independent samples of nearly 13000 adults from Austria and Germany. As in modern genetic studies, where a discovery-and-replication-sample design is standard, the use of two samples allowed testing the replicability and robustness of findings within one-and-the-same study. Overall, 7.5 percent of women and 8.8 percent of men were left-handed. “We were surprised to see that this imbalance was caused by more left-handed men being born specifically during November, December, and January. On a monthly average, 8.2 percent of left-handed men were born during the period February to October. During November to January, this number rose to 10.5 percent”, according to Ulrich Tran, lead author of the study.
A hormonal cause during embryonic development
"Presumably, the relative darkness during the period November to January is not directly connected to this birth seasonality of handedness. We assume that the relative brightness during the period May to July, half a year before, is its distal cause", explains Ulrich Tran. A theory, brought forth in the 1980s by US neurologists Norman Geschwind and Albert Galaburda, posits that testosterone delays the maturation of the left brain hemisphere during embryonic development. The left brain hemisphere is dominant among right-handers, the right brain hemisphere is dominant among left-handers. Intrauterine testosterone levels are higher in the male fetus, because of its own testosterone secretion, than in the female fetus. However, the testosterone level of the mother and external factors may also affect intrauterine testosterone levels. Specifically, more daylight may increase testosterone levels, making a seasonality effect plausible.
Previous studies on the subject provided mixed and inconsistent evidence. There was no clear indication which season has an effect, and whether seasonality affects men, women or both sexes equally. According to the current findings, there is a small, but robust and replicable, effect of birth seasonality on handedness, affecting only men. These results are consistent with a hormonal basis of handedness, corroborating thus an old and controversial theory. However, the exact way of causation needs to be investigated in future studies.
(Source: medienportal.univie.ac.at)
Left-handed fetuses could show effects of maternal stress on unborn babies
Fetuses are more likely to show left-handed movements in the womb when their mothers are stressed, according to new research.
Researchers at Durham and Lancaster universities say their findings are an indicator that maternal stress could have a temporary effect on unborn babies, adding that their research highlights the importance of reducing stress during pregnancy.
However, the researchers emphasised that their study was not evidence that maternal stress led to fixed left-handedness in infants after birth. They said that some people might be genetically predisposed to being left-handed and that there are examples where right and left-handedness can switch throughout a person’s life.
Using 4d ultrasound scans, the researchers observed 57 scans of 15 healthy fetuses, recording 342 facial touches.
The fetuses were scanned at four different stages between 24 and 36 weeks of pregnancy. Researchers also asked the mothers of these babies how much stress they had experienced in the four weeks between each of the scans.
The researchers found that the more stress mothers reported, the more frequently fetuses touched their faces with their left hands. They added that a significant number of touches by the fetuses of stressed mothers were done with their left, rather than right hands - therefore fetal touches of their own faces, indicated a left-handed tendency.
As right-handedness is more common in the general population, the researchers had expected to see more of a bias towards right-handed movements in the fetuses as they grew older. The high percentage of left-handed behaviour, observed only when mothers reported being stressed, led them to conclude that maternal stress has an effect on the lateral behaviour of the babies they scanned.
The findings are published in the journal Laterality: Asymmetries of Body, Brain and Cognition.
Lead author Dr Nadja Reissland, in Durham University’s Department of Psychology, said: “Our research suggests that stressed mothers have fetuses who touch their face relatively more with their left hand.
“This suggests maternal stress could be having on effect on the child’s behaviour in the womb and highlights the importance of reducing maternal stress in pregnancy.
“Such measures may include increased emphasis on stopping stressful work early, the inclusion of relaxation classes in pre-natal care and involvement of the whole family in the pre-natal period.
“While we observed a higher degree of left-handed behaviour in the fetuses of stressed mothers than had been expected, we are not saying that maternal stress leads to a child becoming left-handed after birth, as there could be a number of reasons for this.
“The research does suggest, however, that a fetus can detect when a mother is stressed and that it responds to this stress.”
Professor Brian Francis, of Lancaster University, emphasised that the study also showed that overall preference for left or right hand varied considerably from scan to scan within each fetus, though fetuses showed more left-hand movements when mothers reported that they had experienced stress. He said: “Overall, there was no consistent handedness preference being shown by the fetuses, with most fetuses switching in preference at least once over the four scans.”
The researchers added that while mothers were asked to report their stress levels in the four weeks between scans, in practice some might have reported the stress they were experiencing at the time of being surveyed.
Previous research has shown that maternal stress in pregnancy leads to increased levels of cortisol – a hormone produced in response to stress - in mothers that could lead to an altered preference for left-sided or right-sided behaviour in fetuses.
The current study did not assess the stress levels of fetuses and Dr Reissland said that future research could examine cortisol levels in fetuses to further determine the effect of stress on lateral behaviour.
Dr Reissland added that further research was also needed to look at whether or not maternal prenatal stress had longer-term effects on the development of infants and children after birth.
(Source: dur.ac.uk)
Babies Don’t Develop Handedness All At Once
Reaching for Froot Loops and grabbing Lego pieces to build a tower are different challenges for toddlers. Depending on what they’re trying to do, tots tend to develop handedness for different tasks at different ages, according to new research.
Most people are right-handed. Babies start using their right hand to reach for cereal nuggets by age 1. However, children take until age 4 to show such a preference when building Lego models. The findings, published in this month’s issue of Developmental Psychobiology, imply tendencies to use one hand more than the other emerge depending on the tasks kids confront, rather than their age.
Preference for the right or left hand is, in part, genetic. Prior studies have shown that some of these one-sided tendencies emerge early. Fetuses suck their right thumb more often than their left; newborns on their back turn to the right more frequently. Most children grow up to be right-handed—in part because of these innate, early leanings, scientists believe.
But the timing of when one hand emerges as the dominant one for most tasks remained unclear.
"As a parent and a scientist, I was surprised to find researchers thought 3-year-olds don’t display a hand preference," said neurobiologist Claudia Gonzalez of the University of Lethbridge in Alberta, Canada.
To study how handedness emerged between ages 1 to 5, Gonzalez and her colleagues assigned about 50 tiny participants to a familiar task: grabbing a colorful object or a tasty tidbit. Children ages 1 to 2 picked up Froot Loops or Cheerios to munch at snack time. Four- and 5-year-olds grasped Lego blocks to build a small model. Three-year-old subjects tackled both tasks.
Even the youngest children had strong right-handed leanings when reaching for food, the team found. Three-year-olds were right-handed eaters, but they were just as likely to use their left hand when playing with blocks. The 4- and 5-year-olds used their left hand to hold the base of their model steady, but they manipulated blocks into the correct positions with their other hand—a clear preference for right-handedness.
"There is a developmental milestone between the ages of 3 and 4 when something clicks," Gonzalez said. "Maybe they become more skilled, or they understand the task better."
Until that developmental “click,” this study shows hand preference isn’t constant across tasks – regardless of a child’s age.
The study “uses a very clever design to get at the question of how handedness varies across tasks,” said Klaus Libertus, an infant development researcher at the University of Pittsburgh. “We did not know handedness is connected to tasks in this way. I would have expected the 3-year-olds to show the same pattern on both tasks, especially since the demands were so similar.”
Developing a hand preference might also correlate with other functions that rely strongly on just one side of the brain, such as language and certain decision-making skills, Gonzalez noted. Preliminary data from children in her lab suggests that when handedness is evident earlier, these other functions also mature more quickly.
Finding the right task to study handedness at different ages will give researchers a firmer grasp on how young brains develop right - or left -handed tendencies, she said.
"You could say hand preference develops before 1, or you could say it doesn’t emerge until age 4—just depending on what task you are looking at," said Gonzalez.
(Source: livescience.com)
Genes for body symmetry may also control handedness
Lefties and righties can thank same DNA that puts hearts on left side for hand dominance
Left- or right-handedness may be determined by the genes that position people’s internal organs.
About 10 percent of people prefer using their left hand. That ratio is found in every population in the world and scientists have long suspected that genetics controls hand preference. But finding the genes has been no simple task, says Chris McManus, a neuropsychologist at University College London who studies handedness but was not involved in the new research.
“There’s no single gene for the direction of handedness. That’s clear,” McManus says. Dozens of genes are probably involved, he says, which means that one person’s left-handedness might be caused by a variant in one gene, while another lefty might carry variants in an entirely different gene.
To find handedness genes, William Brandler, a geneticist at the University of Oxford, and colleagues conducted a statistical sweep of DNA from 3,394 people. Statistical searches such as this are known as genome-wide association studies; scientists often do such studies to uncover genes that contribute to complex diseases or traits such as diabetes and height. The people in this study had taken tests involving moving pegs on a board. The difference in the amount of time they took with one hand versus the other reflected how strongly left- or right-handed they were.
A variant in a gene called PCSK6 was most tightly linked with strong hand preference, the researchers report in the Sept. 12 PLOS Genetics. The gene has been implicated in handedness before, including in a 2011 study by the same research group. PCSK6 is involved in the asymmetrical positioning of internal organs in organisms from snails to vertebrates.
Brandler, who happens to be a lefty, knew the gene wasn’t the only cause of hand preference, so he and his colleagues looked at other genetic variants that didn’t quite cross the threshold of statistical significance. Many of the genes the team uncovered had previously been shown in studies of mice to be necessary for correctly placing organs such as the heart and liver. Four of the genes when disrupted in mice can cause cilia-related diseases. Cilia are hairlike appendages on cells that act a bit like GPS units and direct many aspects of development of a wide range of species, including humans.
One of the cilia genes, GLI3, also helps build the corpus callosum, a bundle of nerves that connects the two hemispheres of the brain. Some studies have suggested that the structure is bigger in left-handers.
It’s still a mystery how these genes direct handedness, says Larissa Arning, a human geneticist at Ruhr University Bochum in Germany. In addition to genes that direct body plans, she says, the study suggests that many more yet-to-be-discovered genes probably play a role in handedness.
Brandler hopes the study will also help remove some of the stigma of being left-handed. Left-handedness isn’t a character flaw or a sign of being sinister, he says: “It’s an outcome of genetic variation.”
(Source: sciencenews.org)
New model of how brain functions are organized may revolutionize stroke rehab
A new model of brain lateralization for movement could dramatically improve the future of rehabilitation for stroke patients, according to Penn State researcher Robert Sainburg, who proposed and confirmed the model through novel virtual reality and brain lesion experiments.
Since the 1860s, neuroscientists have known that the human brain is organized into two hemispheres, each of which is responsible for different functions. Known as neural lateralization, this functional division has significant implications for the control of movement and is familiar in the phenomenon of handedness.
Understanding the connections between neural lateralization and motor control is crucial to many applications, including the rehabilitation of stroke patients. While most people intuitively understand handedness, the neural foundations underlying motor asymmetry have until recently remained elusive, according to Sainburg, professor of kinesiology and neurology and participant in the neuroscience and physiology graduate programs at the University’s Huck Institutes of the Life Sciences.
Research by Sainburg and his colleagues in the Center for Motor Control and published in the journal Brain has revealed a new model of motor lateralization that accounts for the neural foundations of handedness. The discovery could fundamentally change the way post-stroke rehabilitation is designed.
"Each hemisphere of the brain is specialized for different aspects of motor control, and thus each arm is ‘dominant’ for different features of movement," said Sainburg. "The dominant arm is used for applying specific force sequences — such as when slicing a loaf of bread with a knife — and the other arm is used for impeding forces to maintain stable posture, such as holding the loaf of bread. Together these specialized control mechanisms are seamlessly integrated into every day activities.
"Our research has shown that this integration breaks down in neural disorders such as stroke, which produces different motor deficits depending on whether the right or left hemisphere has been damaged," Sainburg continued. "Traditionally, physical rehabilitation professionals have used the same protocols to practice movements of the paretic arm, regardless of the hemisphere that has been damaged. Our research shows that each arm should be treated for different control deficits, and it also indicates that therapists should directly retrain patients in how to use the two arms together in order to recover function."
In preparing to test their model, Sainburg and his team selected study participants from the New Mexico Veterans Administration Hospital and Penn State Milton S. Hershey Medical Center based on specific criteria in order to accurately distinguish the motor control mechanisms specific to each brain hemisphere. Participants were then asked to perform a series of tasks on a virtual reality interface, programmed and designed by Sainburg, which allowed the researchers to record detailed 3D position and motion data. The data for all the participants’ hand trajectories and final positions were then aggregated to compare the effects of left versus right hemisphere damage on different aspects of control.
"Our results indicated that while both groups of patients showed similar clinical impairment in the contralesional arm, this was produced by different motor control deficits," Sainburg said. "Right hemisphere damaged patients were able to make straight movements that were directed toward the targets, but were unable to stabilize their arms in the targets at the end of motion. In contrast, left hemisphere damaged patients were unable to make straight and efficient movements, but had no difficulty stabilizing their arms at the end of motion. These results confirmed that each hemisphere contributes unique control to its contralesional arm, verifying why our arms seem different when we use them for the same tasks."
Results mirror those of Sainburg’s prior studies of motor deficits in unilateral stroke patients, focused on the ipsilesional arm, which formed the basis for his model of lateralization.
"Because both arms in stroke patients show motor deficits that are specific to the hemisphere that was damaged, we have concluded that the left arm is not simply controlled with the right hemisphere and vice versa," Sainburg said. "This is a revolutionarily new perspective on sensorimotor control: each hemisphere contributes different control mechanisms to the coordination of both arms, regardless of which arm is considered dominant."
Sainburg and his colleagues are currently designing follow-up studies that will aid the development of new rehabilitation protocols addressing the specific motor deficits associated with each hemisphere.
Right-Handed Males, Left-Handed Females?
This is true for sugar gliders (Petaurus breviceps) and grey short-tailed opossums (Monodelphis domestica), say biologists from Saint Petersburg State University, Russia.
Their study, published in the open access journal BMC Evolutionary Biology, shows that handedness in marsupials is dependent on gender.
This preference of one hand over another has developed despite the absence of a corpus callosum, the part of the brain, which in placental mammals allows one half of the brain to communicate with the other.
Many animals show a distinct preference for using one hand (paw, hoof) over another. This is often related to posture – an animal is more likely to show manual laterality if it is upright, related to the difficulty of the task, more complex tasks show a handed preference, or even with age. As an example of all three: crawling human babies show less hand preference than toddlers.
Some species also show a distinct sex effect in handedness but among non-marsupial mammals this tendency is for left-handed males and right-handed females.
In contrast, the team from Russia shows that male quadruped marsupials, such as who walk on all fours, tend to be right-handed while the females are left-handed, especially as tasks became more difficult.
“Marsupials do not have a corpus callosum – which connects the two halves of the mammalian brain together. Reversed sex related handedness is an indication of how the marsupial brain has developed different ways of the two halves of the brain communicating in the absence of the corpus callosum,” explains senior author Dr Yegor Malashichev.
New Study Reveals How Humans Became Right-Handed
According to a new study led by Dr Gillian Forrester of the University of Sussex, a predominance to be right-handed is not a uniquely human trait but one shared by great apes.
The study, published in the journal Behavioural Brain Research, analyzed hand actions directed towards either objects or individuals in chimpanzees, gorillas and children, and found that all three species are right-handed for actions to objects, but not for actions directed to individuals.
The results support a theory that human right-handedness is a trait developed through tool use that was inherited from an ancestor common to both humans and great apes. The findings challenge a widely held view that right-handed dominance in humans was a species-unique trait linked to the emergence of language.
“Humans have been tool users for 2.5 million years, while the current view is that language only emerged one hundred thousand years ago,” Dr Forrester said. “Our findings provide the first non-invasive results from naturalistic behavior, suggesting that language emerged as a consequence of left hemisphere brain regions that were already evolved to process regular sequences of actions. The structure found in language may have developed from pre-existing brain processes adapted from experience with tool-use.”
Worth a Thousand Words: Handedness in Fish
If you look closely at the image above, you’ll see that the mouths of these cichlid fish, Perissodus microlepis, curve in opposite directions. Similar to left or right handedness in humans, many animals exhibit handedness in behavior or morphology. Whether handed behavior is expressed early in development and produces mouth asymmetry or the opposite, that mouth asymmetry produces handed behavior, however, is not well known. The authors of the study “Handed Foraging Behavior in Scale-Eating Cichlid Fish: Its Potential Role in Shaping Morphological Asymmetry” set out to investigate this question and more.
Most Neanderthals were right handed, just like modern humans, and this tendency suggests that they may have had the capacity for speech, new research claims.
A new investigation by Professor Frayer and an international team led by Virginie Volpato of the Senckenberg Institute in Frankfurt, Germany, has confirmed Regourdou’s right-handedness by looking more closely at the robustness of the arms and shoulders, and comparing it with scratches on his teeth.
'We’ve been studying scratch marks on Neanderthal teeth, but in all cases they were isolated teeth, or teeth in mandibles not directly associated with skeletal material,' said Professor Frayer.
'This is the first time we can check the pattern that’s seen in the teeth with the pattern that’s seen in the arms. We did more sophisticated analysis of the arms — the collarbone, the humerus, the radius and the ulna — because we have them on both sides. And we looked at cortical thickness and other biomechanical measurements. All of them confirmed that everything was more robust on the right side then the left.'