martes, 18 de diciembre de 2007

Gene Implicated In Human Language Affects Song Learning In Songbirds

Zebra finch. (Credit: iStockphoto/David Gluzman)

ScienceDaily (Dec. 5, 2007) — Do special "human" genes provide the biological substrate for uniquely human traits, like language?

Genetic aberrations of the human FoxP2 gene impair speech production and comprehension, yet the relative contributions of FoxP2 to brain development and function are unknown.

Songbirds are a useful model to address this because, like human youngsters, they learn to vocalize by imitating the sounds of their elders.

Previously, Dr. Constance Sharff and colleagues found that, when young zebra finches learn to sing or when adult canaries change their song seasonally, FoxP2 is up-regulated in Area X, a brain region important for song learning.

Dr. Sebastian Haesler, Dr. Scharff, and colleagues experimentally reduce FoxP2 levels in Area X before zebra finches started to learn their song. They used a virus-mediated RNA interference for the first time in songbird brains.

The birds, with lowered levels of FoxP2, imitated their tutor's song imprecisely and sang more variably than controls.

FoxP2 thus appears to be critical for proper song development.

These results suggest that humans and birds may employ similar molecular substrates for vocal learning, which can now be further analyzed in an experimental animal system.

Journal citation: Haesler S, Rochefort C, Georgi B, Licznerski P, Osten P, et al. (2007) Incomplete and inaccurate vocal imitation after knockdown of FoxP2 in songbird basal ganglianucleus Area X. PLoS Biol 5(12): e321. doi:10.1371/journal.pbio.0050321

Adapted from materials provided by Public Library of Science.

Adapting To Pregnancy Played Key Role In Human Evolution, Study Shows

ScienceDaily (Dec. 13, 2007) — When a pregnant woman leans back, and shifts her weight to stand more comfortably, she is performing a motion that for millions of years has helped to compensate for the strain and weight of childbearing on the body. According to a new study from researchers at Harvard University and the University of Texas at Austin, women's lower spines evolved to be more flexible and supportive than men's to increase comfort and mobility during pregnancy, and to accommodate the special biology of carrying a baby for nine months while standing on two feet.

The study, published in the Dec. 13 Nature, was led by Katherine Whitcome, a postdoctoral researcher in the Department of Anthropology in Harvard's Faculty of Arts and Sciences, with Daniel Lieberman, professor of anthropology at Harvard, and Liza Shapiro, associate professor of anthropology of the University of Texas at Austin.

"Pregnancy presents an enormous challenge for the female body," says Whitcome. "The body must change in dramatic ways to accommodate the baby, and these changes affect a woman's stability and posture. It turns out that enhanced curvature and reinforcement of the lower spine are key to maintaining normal activities during pregnancy."

It has long been appreciated that giving birth to large-brained infants has influenced human pelvic shape, but there has been little attention paid to the major challenge that pregnant bipedal mothers endure when holding up an enormous fetus and placenta well in front of the hip joints. The study is the first of its kind to examine the evolutionary mechanisms that allow women to carry a baby to term, and the way that women's bodies compensate for increased weight in the abdomen during pregnancy.

Walking on two feet, which happened early in human evolution, presents a unique challenge during pregnancy because the center of gravity shifts far in front of the hips, destabilizing the upper body and impairing locomotion. This is not the case for animals that walk predominantly on four legs such as chimpanzees, or even other bipeds.

To accommodate this shifted center of gravity, women's spines have evolved to help offset the additional weight in the abdomen during pregnancy, so that the back muscles are not taxed in counter-balancing the destabilizing effects of the baby's weight.

In both women and men the curvature of the spine in the lower back, called the lordosis, stabilizes the upper body above the lower body. The researchers studied 19 pregnant women between the ages of 20 and 40, and found that when naturally standing, the women lean back, increasing their lordosis by as much as 60 percent by the end of their term. In doing so, pregnant women maintain a stable center of gravity above the hips.

The research also demonstrates, for the first time, that human lumbar vertebrae differ between males and females in ways that decrease the shearing forces that the lumbar extension of pregnancy places on the lower back in pregnant mothers.

"In females, the lordosis is subtly different than that of males, because the curvature extends across three vertebrae, while the male lordosis curves across only two vertebrae," says Whitcome. "Loading across three vertebrae allows an expectant mother to increase her lordosis, realigning her center of gravity above her hips and offsetting the destabilizing weight of the baby."

In addition to the difference in the number of vertebrae across which the lordosis spans, the female joints are relatively larger and flare out further down the spine than those of males improving the spine's strength. All of this contributes to an increased ability to extend the spine, so that the woman can lean back, realign the body's center of gravity, and safely maintain a more stable position. These differences in the lower back may even reinforce her capability to support and carry her baby in her arms after the baby has been born.

When human ancestors first became bipedal, they set the human lineage off on a different evolutionary path from other apes, but in so doing created special challenges for pregnant mothers. One exciting discovery is that the ability of human females to better carry a baby to term while standing on two feet appears to have evolved at least two million years ago. The researchers studied two hominin fossils that were approximately two million years old, one of which - presumably a female - displayed three lordosis vertebrae and one of which - presumably a male - displayed fewer.

"Early human women lived very strenuous, active lives, and pregnant females were forced to cope with the discomfort of childbearing while foraging for food and escaping from predators," Lieberman says. "This evolution of the lower back helped early woman to remain more mobile during pregnancy, which would have been essential to survival, and appears to have been favored by natural selection."

The research was funded by the National Science Foundation, the L.S.B. Leakey Foundation, and the American School of Prehistoric Research.

Adapted from materials provided by Harvard University.

Ape To Human: Walking Upright May Have Protected Heavy Human Babies

For safety, all nonhuman primates carry their young clinging to their fur from birth, and species survival depends on it. (Credit: iStockphoto/Graeme Purdy)

ScienceDaily (Dec. 17, 2007) — The transition from apes to humans may have been partially triggered by the need to stand on two legs, in order to safely carry heavier babies. This theory of species evolution presented by Lia Amaral from the University of São Paulo in Brazil has just been published online in Springer’s journal, Naturwissenschaften.

For safety, all nonhuman primates carry their young clinging to their fur from birth, and species survival depends on it. The carrying pattern changes as the infant grows. Newborns are carried clinging to their mother’s stomach, often with additional support. Months later, infants are carried over the adult body usually on the mother’s back, and this carrying pattern lasts for years in apes. However, this necessity to carry infants safely imposes limits on the weight of the infants.

Through a detailed mechanical analysis of how different types of apes - gibbons, orangutans and gorillas - carry their young, looking at the properties of ape hair, infant grip, adult hair density and carrying position, Amaral demonstrates a relationship between infant weight, hair friction and body angle which ensures ape infants are carried safely.

Amaral also shows how the usual pattern of primate carrying of heavy infants is incompatible with bipedalism. African apes have to persist with knuckle-walking on all fours, or ‘quadruped’ position, in order to stop their young from slipping off their backs.

The author goes on to suggest that the fall in body hair in primates could have brought on bipedality as a necessary consequence, through the strong selective pressure of safe infant carrying, as infants were no longer able to cling to their mother’s body hairs. In the author’s opinion, safe carrying of heavy infants justified the emergence of the biped form of movement. Although an adult gorilla is much heavier than an adult human, its offspring is only half the weight of a human baby.

Amaral concludes that this evolution to bipedality has important consequences for the female of the species. Indeed, it frees the arms and hands of males and juveniles, but females have their arms and hands occupied with their young. This restriction of movement placed limits on food gathering for biped females carrying their infants, and may have been at the origin of group cooperation.

Reference: Amaral LQ (2007). Mechanical analysis of infant carrying in hominoids. Naturwissenschaften (DOI 10.1007/s00114-007-0325-0).

Adapted from materials provided by Springer.

Ape Gestures Offer Clues To The Evolution Of Human Communication

Chimp reaching out. (Credit: Frans de Waal / Yerkes National Primate Research Center, Emory University)

ScienceDaily (May 1, 2007) — Researchers at the Yerkes National Primate Research Center, Emory University, have found bonobos and chimpanzees use manual gestures of their hands, feet and limbs more flexibly than they do facial expressions and vocalizations, further supporting the evolution of human language began with gestures as the gestural origin hypothesis of language suggests. This study appears in the current issue of the Proceedings of the National Academy of Sciences.

Working with two groups of bonobos (13 animals) and two groups of chimpanzees (34 animals), Yerkes researchers Amy Pollick, PhD, and Frans de Waal, PhD, distinguished 31 manual gestures and 18 facial/vocal signals. They found both species used facial/vocal signals similarly, but the same did not hold true for the manual gestures. Rather, the researchers found both within and between species the manual gestures were less closely tied to a particular emotion and, thereby, serve a more adaptable function. For example, a single gesture may communicate an entirely different message depending upon the social context in which it is used.

"A chimpanzee may stretch out an open hand to another as a signal for support, whereas the same gesture toward a possessor of food signals a desire to share," said Pollick. "A scream, however, is a typical response for victims of intimidation, threat or attack. This is so for both bonobos and chimpanzees, and suggests the vocalization is relatively invariant," Pollick continued.

By studying similar types of communication in closely related species, researchers are able to determine shared ancestry. We know gestures are evolutionarily younger than facial expressions and vocalizations, as shown by their presence in apes and humans but not in monkeys. "A gesture that occurs in bonobos and chimpanzees as well as humans likely was present in the last common ancestor," said Pollick. "A good example of a shared gesture is the open-hand begging gesture, used by both apes and humans. This gesture can be used for food, if there is food around, but it also can be used to beg for help, for support, for money and so on. It's meaning is context-dependent," added de Waal.

Looking for further distinctions between species, the researchers found bonobos use gestures more flexibly than do chimpanzees. "Different groups of bonobos used gestures in specific contexts less consistently than did different groups of chimpanzees," said Pollick. The researcher's findings also suggest bonobos and chimpanzees engage in multi-modal communication, combining their gestures with facial expressions and vocalizations to communicate a message. "While chimpanzees produce more of these combinations, bonobos respond to them more often. This finding suggests the bonobo is a better model of symbolic communication in our early ancestors," concluded Pollick.

Adapted from materials provided by Emory University.

Young Chimps Top Adult Humans In Numerical Memory

Young chimpanzees could grasp many numerals at a glance, with no change in performance as the hold duration -- the amount of time that the numbers remained on the screen -- was varied, the researchers found. (Credit: iStockphoto/Peter-John Freeman)

ScienceDaily (Dec. 9, 2007) — Young chimpanzees have an "extraordinary" ability to remember numerals that is superior to that of human adults, researchers report.

"There are still many people, including many biologists, who believe that humans are superior to chimpanzees in all cognitive functions," said Tetsuro Matsuzawa of Kyoto University. "No one can imagine that chimpanzees--young chimpanzees at the age of five--have a better performance in a memory task than humans. Here we show for the first time that young chimpanzees have an extraordinary working memory capability for numerical recollection--better than that of human adults tested in the same apparatus, following the same procedure."

Chimpanzee memory has been extensively studied, the researchers said. The general assumption is that, as with many other cognitive functions, it is inferior to that of humans. However, some data have suggested that, in some circumstances, chimpanzee memory may indeed be superior to human memory.

In the current study, the researchers tested three pairs of mother and infant chimpanzees (all of which had already learned the ascending order of Arabic numerals from 1 to 9) against university students in a memory task of numerals. One of the mothers, named Ai, was the first chimpanzee who learned to use Arabic numerals to label sets of real-life objects with the appropriate number.

In the new test, the chimps or humans were briefly presented with various numerals from 1 to 9 on a touch-screen monitor. Those numbers were then replaced with blank squares, and the test subject had to remember which numeral appeared in which location and touch the squares in the appropriate order.

The young chimpanzees could grasp many numerals at a glance, with no change in performance as the hold duration--the amount of time that the numbers remained on the screen--was varied, the researchers found. In general, the performance of the three young chimpanzees was better than that of their mothers. Likewise, adult humans were slower than all of the three young chimpanzees in their response. For human subjects, they showed that the percentage of correct trials also declined as a function of the hold duration--the shorter the duration became, the worse their accuracy was.

Matsuzawa said the chimps' memory ability is reminiscent of "eidetic imagery," a special ability to retain a detailed and accurate image of a complex scene or pattern. Such a "photographic memory" is known to be present in some normal human children, and then the ability declines with the age, he added.

The researchers said they believe that the young chimps' newfound ability to top humans in the numerical memory task is "just a part of the very flexible intelligence of young chimpanzees."

The researchers include Sana Inoue and Tetsuro Matsuzawa, of Kyoto University, Japan. This research was published in the December 4th issue of Current Biology, a publication of Cell Press.

Adapted from materials provided by Cell Press.

Wild Chimpanzees Appear Not To Regularly Experience Menopause

Elderly chimpanzee. Wild chimpanzees appear not to regularly experience menopause. (Credit: iStockphoto/Ruben Vicente)

ScienceDaily (Dec. 13, 2007) — A pioneering study of wild chimpanzees has found that these close human relatives do not routinely experience menopause, rebutting previous studies of captive individuals which had postulated that female chimpanzees reach reproductive senescence at 35 to 40 years of age.

Together with recent data from wild gorillas and orangutans, the finding suggests that human females are rare or even unique among primates in experiencing a lengthy post-reproductive lifespan.

"We find no evidence that menopause is common among wild chimpanzee populations," says lead author Melissa Emery Thompson, a postdoctoral researcher in anthropology at Harvard University. "While some female chimpanzees do technically outlive their fertility, it's not at all uncommon for individuals in their 40s and 50s -- quite elderly for wild chimpanzees -- to remain reproductively active."

While wild chimpanzees and humans both experience fertility declines starting in the fourth decade of life, most other human organ systems can remain healthy and functional for many years longer, far outstripping the longevity of the reproductive system and giving many women several decades of post-reproductive life.

By contrast, in chimpanzees reproductive declines occur in tandem with overall mortality. A chimpanzee's life expectancy at birth is only 15 years, and just 7 percent of individuals live to age 40. But females who do reach such advanced ages tend to remain fertile to the end, Emery Thompson and her colleagues found, with 47 percent giving birth once after age 40, including 12 percent observed to give birth twice after age 40.

"Fertility in chimpanzees declines at a similar pace to the decline in survival probability, whereas human reproduction nearly ceases at a time when mortality is still very low," the researchers write in Current Biology. "This suggests that reproductive senescence in chimpanzees, unlike in humans, is consistent with the somatic aging process."

In other words, human evolution has resulted in an extended life span without complementary extended reproduction.

"Why hasn't reproduction kept pace with the general increase in human longevity? It may be because there hasn't been anything for natural selection to act on, though there is heritable variation in age of menopause," Emery Thompson says. "However, it may be that the advantage older females gain by assisting their grandchildren outstrips any advantage they might get by reproducing themselves."

The oldest known wild chimpanzee, who died earlier this year at approximately age 63, gave birth to her last offspring just eight years ago, at about 55. Female chimpanzees only give birth every 6 to 8 years, on average, and they generally begin reproducing at age 13 to 15. This makes the chimpanzee reproductive profile much longer and flatter than that of humans, whose procreation is concentrated from age 25 to 35.

Emery Thompson and her colleagues gathered data from six wild chimpanzee populations in Tanzania, Uganda, Guinea, and Gambia. They compared these chimpanzees' fertility patterns to those seen among two well-studied human foraging populations, in Botswana and Paraguay.

This research was described recently in the journal Current Biology.

Emery Thompson's co-authors are Richard W. Wrangham of Harvard; James H. Jones of Stanford University; Anne E. Pusey and Jane Goodall of the Jane Goodall Institute; Stella Brewer-Marsden and David Marsden of the Chimpanzee Rehabilitation Trust; Tetsuro Matsuzawa, Toshisada Nishida, and Yukimaru Sugiyama of Kyoto University; and Vernon Reynolds of Oxford University. Their work was supported by numerous organizations, with primary funding coming from the National Science Foundation, MEXT Japan, the Jane Goodall Institute, and the Louis Leakey Foundation.

Adapted from materials provided by Harvard University.

Losses Of Long-established Genes Contribute To Human Evolution

Human chromosomes. (Credit: Jane Ades, NHGRI)

ScienceDaily (Dec. 15, 2007) — While it is well understood that the evolution of new genes leads to adaptations that help species survive, gene loss may also afford a selective advantage. A group of scientists at the University of California, Santa Cruz led by biomolecular engineering professor David Haussler has investigated this less-studied idea, carrying out the first systematic computational analysis to identify long-established genes that have been lost across millions of years of evolution leading to the human species.

Haussler and five others in his group--postdoc Jingchun Zhu, graduate students Zack Sanborn and Craig Lowe, technical projects manager Mark Diekhans, and evolutionary biologist Tom Pringle--are co-authors on the paper*.

"The idea that gene losses might contribute to adaptation has been kicked around, but not well studied," said Zhu, who is first author of the paper. "We found three examples in the literature, and all of them could have medical implications."

To find gene losses, Zhu employed a software program called TransMap that Diekhans had developed. The program compared the mouse and human genomes, searching for genes having changes significant enough to render them nonfunctional somewhere during the 75 million years since the divergence of the mouse and the human.

"This is the first study designed to search the entire genome for recent loss of genes that do not have any near-duplicate copies elsewhere in the genome," said Haussler. "These are likely to be the more important gene losses."

Genes can be lost in many ways. This study focused on losses caused by mutations that disrupt the open reading frame (ORF-disrupting mutations). These are either point mutations, where events such as the insertion or substitution of a DNA base alter the instructions delivered by the DNA, or changes that occur when a large portion of a gene is deleted altogether or moves to a new place on the genome.

"We used the dog genome as an out-group to filter out false positives," Sanborn explained, because the dog diverged from our ancient common ancestor earlier than the mouse. "If a gene is still living in both dog and mouse but not in human, it was probably living in the common ancestor and then lost in the human lineage."

Using this process, they identified 26 losses of long-established genes, including 16 that were not previously known.

The gene loss candidates found in this study do not represent a complete list of gene losses of long-established genes in the human lineage, because the analysis was designed to produce more false negatives than false positives.

Next they compared the identified genes in the complete genomes of the human, chimpanzee, rhesus monkey, mouse, rat, dog, and opossum to estimate the amount of time the gene was functional before it was lost. This refined the timing of the gene loss and also served as a benchmark for whether the gene in question was long-established, and therefore probably functional, or merely a loss of a redundant gene copy. Through this process, they found 6 genes that were lost only in the human.

One previously unknown loss, the gene for acyltransferase-3 (ACYL3), particularly caught their attention. "This is an ancient protein that exists throughout the whole tree of life," said Zhu. Multiple copies of the ACYL3 gene are encoded in the fly and worm genomes. "In the mammalian clade there is only one copy left, and somewhere along primate evolution, that copy was lost."

"In our analysis, we found that this gene contains a nonsense mutation in human and chimp, and it appears to still look functional in rhesus," said Sanborn. Further, they found that the mutation is not present in the orangutan, so the gene is probably still functional in that species.

"On the evolutionary tree leading to human, on the branch between chimp and orangutan sits gorilla," explained Sanborn. Knowing if the gene was still active in gorilla would narrow down the timing of the loss.

Sanborn took to the wet laboratory to sequence the corresponding region in a DNA sample from a gorilla. The gorilla DNA sequence showed the gene intact, without the mutation, so the loss likely occurred between the speciation of gorilla and chimpanzee.

"Acyltransferase-3 was not the only lost gene that doesn't have any close functional homologues in the human genome. A highlight of our research was that we were able to find a list of these 'orphan losses,'" said Zhu. "Some of them have been functional for more than 300 million years, and they were the last copies left in the human genome." While the copies of these genes remaining in the human genome appear to be nonfunctional, functional copies of all of them exist in the mouse genome.

"These orphan genes may be interesting candidates for experimental biologists to explore," said Zhu. "It would be interesting to find out what was the biological effect of these losses. Once their function is well characterized in species that still have active copies, we could maybe speculate about their effects on human evolution."

*Their findings appear in the December 14 issue of PLoS Computational Biology.

This research was funded by the National Human Genome Research Institute, the National Institutes of Health, the National Cancer Institute, and the Howard Hughes Medical Institute.

Adapted from materials provided by University of California - Santa Cruz.

Neuronas espejo: ¿la clave del autismo?

Un mal funcionamiento del sistema de las neuronas espejo podría estar implicado en algunas enfermedades mentales como el autismo

Las neuronas espejo son uno de los más sorprendentes descubrimientos de la neurociencia en la última década. Todavía profundamente inmersos en su estudio, los neurocientíficos están encontrando indicios de que están relacionadas con la capacidad humana de la empatía, con el aprendizaje, con la socialización en general e incluso con el desarrollo del lenguaje.

* Fecha de publicación: 13 de diciembre de 2007

En 1995, en el laboratorio del neurofisiólogo Giacomo Rizzolatti, en la Universidad de Parma (Italia), se produjo uno de esos acontecimientos casuales que derivan en grandes descubrimientos. Los investigadores estudiaban la acción de neuronas motoras en macacos, y para ello habían implantado en el cerebro de los animales finísimos electrodos capaces de registrar actividad de una única neurona. Cuando el animal cogía un cacahuete situado a su alcance, una de sus neuronas motoras emitía un impulso eléctrico que los investigadores detectaban.

Un día entró en el laboratorio otro investigador y cogió él mismo un cacahuete; la sorpresa fue mayúscula: la neurona motora del macaco había emitido su impulso exactamente como lo hacía cuando el propio animal realizaba la acción... ¡pero el macaco no se había movido! Esa observación casual ha llevado al descubrimiento del hoy llamado sistema de neuronas espejo, integrado por células nerviosas que 'disparan' -emiten un impulso eléctrico- ya sea cuando el sujeto realiza una acción o cuando observa a otros hacerla.
Predecir intencionalidad

Numerosos grupos estudian ahora las neuronas espejo. Se han hallado también en el hombre, en casi las mismas áreas cerebrales que en los macacos, aunque no con registros de actividad de una sola neurona -algo no factible en humanos- sino con técnicas no invasivas de registro de actividad cerebral (mediante electroencefalograma o resonancia magnética). Se sabe ya que estas neuronas se activan no sólo a través de estímulos visuales, sino también auditivos. Por ejemplo, una neurona de mono que dispara cuando él mismo rasga un papel, disparará cuando vea a una persona u otro mono rasgar un papel, pero también cuando sólo escuche el sonido.

Por eso se ha postulado que una de las funciones de este sistema podría ser ayudar a interpretar las acciones de los demás. También se ha visto que, dentro del sistema de neuronas espejo, algunas de las células podrían estar especializadas en detectar intencionalidad, para predecir las acciones siguientes del sujeto observado. Y se ha analizado mucho la relación del sistema con las emociones, estudiando la actividad cerebral de sujetos mientras observaban o imitaban expresiones faciales y corporales asociadas a distintas emociones.

Una de las funciones del sistema de neuronas espejo podría ser la de ayudar a interpretar las acciones de los demás

Los resultados han mostrado la existencia de interconexiones entre el sistema de neuronas espejo y varias estructuras en el cerebro profundo implicadas en las emociones, el llamado sistema límbico. Además, como señalan Marco Iacoboni y Mirella Dapretto, investigadores de la Universidad de California (Los Ángeles, EE.UU.), en un reciente artículo en 'Nature Reviews Neuroscience', «la actividad en toda la red de interconexiones entre neuronas espejo y estructuras límbicas aumentó durante la imitación de las conductas, como se ha demostrado habitualmente en las áreas donde están las neuronas espejo», al realizar otras acciones no relacionadas con la emoción.

Tanto la capacidad de predecir intencionalidad como la de reconocer e interiorizar el estado emocional de los demás son habilidades consideradas necesarias para la socialización. De ahí la idea de un posible vínculo entre un sistema de neuronas espejo dañado y el autismo, enfermedad caracterizada por el aislamiento social de los enfermos.
Entre neuronas espejo y autismo

«La hipótesis [de la relación entre neuronas espejo y autismo] ha sido desarrollada en profundidad sugiriendo que el sistema de neuronas espejo permite crearse un modelo del comportamiento de otras personas, a través de un mecanismo de representación interna de estados corporales asociados a acciones y emociones», escriben Iacoboni y Dapretto. Esta representación interna proporcionaría una forma directa de experimentar lo que sienten los demás.

Esta presunción se está poniendo a prueba mediante distintos abordajes. Por ejemplo, un reciente estudio con resonancia magnética de Mirella Dapretto midió directamente la actividad del sistema de neuronas espejo en niños mientras observaban e imitaban expresiones faciales que reflejaban distintas emociones. «Los niños con un espectro de autismo demostraban menos actividad en el sistema de neuronas espejo en comparación con los niños de desarrollo normal», explica Dapretto. «Es más, la actividad medida durante las tareas en neuronas espejo de niños con autismo se correlacionaba con el grado de gravedad del trastorno, medido con las escalas usadas habitualmente en la clínica».

Estos datos apoyan la idea de que un mal funcionamiento en el sistema de neuronas espejo es un problema importante en el autismo, afirma Dapretto. Y sugiere, además, que el registro de actividad de las neuronas espejo en tareas de imitación o sociales podría usarse como biomarcador del grado de profundidad del trastorno.

Estos estudios no tienen sólo un interés básico. Si hay un defecto funcional en una estructura cerebral se plantea también la posibilidad de actuar sobre él. Una posible vía, sugieren los expertos, es la imitación. «Las evidencias acerca del papel del sistema de neuronas espejo en el autismo, y los vínculos entre este mismo sistema y la imitación, sugiere que ésta podría ser usada como forma de tratamiento efectivo en niños con autismo», escribe Iacoboni.

Este experto recuerda que existen ya estudios de comportamiento que apoyan esta idea. Un ejemplo es un trabajo en el que un adulto imitaba las acciones de un grupo de niños, mientras que con el grupo control se relacionaba pero no hacía tareas de imitación; los niños cuyas acciones eran imitadas mostraban una mayor tendencia a iniciar interacciones sociales en sesiones posteriores, en comparación con el grupo de niños que no habían sido imitados por el adulto.


La investigación del sistema de neuronas espejo no ha hecho más que empezar. El equipo liderado por Caroline Catmur, de la Universidad de Oxford (Reino Unido), ha descubierto recientemente que el funcionamiento del sistema no es innato, y que puede ser alterado con un entrenamiento adecuado. Los autores emplearon una técnica de estimulación transcraneal para analizar la corteza motora de voluntarios mientras miraban el vídeo de una mano. Cuando los voluntarios veían moverse el dedo índice, los investigadores pudieron comprobar que la actividad en el músculo abductor de su propio dedo índice era mayor, y lo mismo ocurría en el dedo meñique cuando este dedo se movía en la pantalla.

Este fenómeno es atribuible a las neuronas espejo, aseguran los autores. Pero entonces, a la mitad de los voluntarios se les pidió que extendieran su dedo meñique cuando la imagen mostraba el dedo índice en movimiento, y viceversa. La otra mitad de los sujetos debían mover el mismo dedo que aparecía en pantalla. Tras los ensayos, los autores observaron un cambio en la respuesta espontánea a las imágenes (sin mover los dedos).

Los sujetos que movieron el índice cuando en pantalla se mostraba el meñique y viceversa mostraban ahora mayor actividad en los músculos del dedo distinto al que veían, lo que indicaría que la respuesta del sistema de neuronas espejo se habría revertido. Los investigadores concluyen con este estudio que las propiedades del sistema no son innatas, sino que «pueden ser entrenadas a través de la experiencia sensorimotora». Este hallazgo implica que una interacción social insuficiente y las consiguientes experiencias sensoriales alteradas podrían influir en el desarrollo del sistema de neuronas motoras, por ejemplo, en niños con autismo, según señalan los investigadores.

Los monos pueden realizar sumas mentales con resultados similares a los de universitarios

1. • El estudio ayuda a comprender los orígenes evolutivos compartidos de humanos y animales


Los monos pueden realizar sumas mentales con resultados similares a los de estudiantes universitarios, según ha revelado un estudio estadounidense divulgado por la revista PloS Biology.

Según los científicos del Centro de Neurociencias Cognitivas de la Universidad de Duke (Carolina del Norte), los resultados del estudio ayudan a comprender los orígenes evolutivos compartidos de seres humanos y animales en lo que se refiere a la capacidad aritmética. Otras investigaciones habían determinado ya que animales y seres humanos tienen la capacidad para representar y comparar números.

Los animales, los niños y los adultos pueden diferenciar entre cuatro objetos y ocho objetos, por ejemplo. Sin embargo, hasta ahora no estaba claro si los animales podían realizar operaciones aritméticas de forma mental.

"Sabíamos que los animales pueden reconocer cantidades, pero no había suficientes pruebas de su capacidad para realizar tareas matemáticas, como la suma. Nuestro estudio ha demostrado que sí pueden hacerlo", ha dicho Jessica Cantion, una estudiante de postgrado que ha participado en la investigación.

Frente a una pantalla

Los científicos realizaron un experimento con macacos instalados frente a una pantalla con un número variable de manchas. Las manchas fueron borradas de la pantalla, que fue reemplazada por otra en la que aparecía un número diferente de esas manchas. Después, se les mostraba una tercera pantalla en la que había una caja con la suma de las primeras dos pantallas, así como otra caja con un número diferente. Los monos eran recompensados cada vez que tocaban la pantalla que contenía la suma correcta.

Un grupo de estudiantes universitarios fue sometido a una prueba similar en la que tenían que elegir la suma correcta sin contar las manchas. El nivel de aciertos entre los estudiantes fue de un 94% y el de los macacos de un 76%. La media de tiempo de respuesta entre ambos grupos fue de un segundo.

Lo más interesante, han dicho los científicos, es que los resultados no fueron tan buenos entre los dos grupos cuando las cajas mostraban números muy cercanos. "Si la suma correcta era 11 y el número incorrecto era de 12 manchas, tanto los monos como los estudiantes tardaban más en responder y tenían más errores", ha dicho Elizabeth Branion, profesora auxiliar del Centro de Neurociencias Cognitivas.

La diferencia, el lenguaje

La diferencia entre ambos grupos está en el hecho de que los seres humanos han agregado el lenguaje y la escritura lo que cambia la forma en que representamos los números, según los científicos. Gran parte de la capacidad matemática de los adultos humanos reside en el hecho de que pueden representar conceptos numéricos utilizando para ello el lenguaje simbólico.

Por ejemplo, un mono no tiene la capacidad de determinar la diferencia entre 2000 y 2001 objetos, por ejemplo. "Sin embargo, nuestro trabajo ha demostrado que tanto seres humanos como monos pueden manipular mentalmente representaciones de números para generar sumas aproximadas de objetos individuales", ha dicho Brannon.

Fuente: el Periódico de Catalunya

miércoles, 12 de diciembre de 2007

L'evolució genètica multiplica cada cop més els tipus humans

1. • Diversos experts d'EUA afirmen que les variacions genètiques s'han accelerat
2. • Els continents són més diferents ara per les malalties, la dieta i l'entorn


"L'evolució humana s'ha accelerat i és molt més ràpida del que pensàvem, sobretot els últims 10.000 anys, després de la invenció de l'agricultura". De fet, l'antropòleg John Hawks, de la Universitat de Wisconsin (EUA), ha comprovat que els humans presenten avui més diferències genètiques amb els que vivien fa 5.000 anys que aquests últims amb l'home de Neandertal, que va desaparèixer fa 30.000 anys.
En un estudi firmat juntament amb quatre experts més i publicat en l'edició on line de la revista Proceedings of the National Academy of Sciences, els antropòlegs també mostren les diferències en l'evolució d'a-
siàtics, europeus i africans. I afirmen que molts dels recents canvis genètics reflecteixen diferents dietes, així com la resistència a les epidèmies que van acabar amb moltes vides durant el creixement de la població mundial. Els africans, per exemple, han desenvolupat gens de resistència contra la malària. Els europeus tenen un gen que facilita la digestió de la llet en l'edat adulta. I els asiàtics, un altre que asseca la cera de les orelles.
Els canvis es deuen a l'enorme creixement de la població, des d'uns quants milions fins als 6.500 actuals. "Els homes s'han desplaçat d'un lloc a un altre, i s'han hagut d'adaptar a nous entorns", afegeix Henry Harpending, un altre autor de l'estudi i professor a la Universitat de Utah (EUA).

Els investigadors van buscar les mutacions genètiques durant els últims 80.000 anys, i van analitzar l'ADN de 270 persones de diferents parts del món. Les dades van mostrar un catàleg de diferències i similituds genètiques de l'home actual que els serveix per investigar quan un canvi genètic va aparèixer en la seqüència completa d'ADN i començar a traçar la pauta d'aquesta modificació en el passat.
"Aquesta evolució mostra diferències a l'Àfrica, l'Àsia i Europa, però gairebé tots els canvis són únics a cada zona perquè des que l'home se'n va anar de l'Àfrica a altres llocs, fa 40.000 anys, no hi ha hagut gaire flux de gens entre regions", afirma Harpending. Els canvis genètics han estat 100 vegades superiors en els últims 5.000 anys que en qualsevol altre període, asseguren els antropòlegs, que afegeixen que el 7% dels nostres gens estan sota una ràpida evolució. Tot i així, l'ADN humà és idèntic en el 99%.

lunes, 10 de diciembre de 2007

Like Humans, Monkey See, Monkey Plan, Monkey Do

ScienceDaily (Dec. 10, 2007) —

How many times a day do you grab objects such as a pencil or a cup? We perform these tasks without thinking, however the motor planning necessary to grasp an object is quite complex. The way human adults grasp objects is typically influenced more by their knowledge of what they intend to do with the objects than the objects' immediate appearance. Psychologists call this the “end-state comfort effect,” when we adopt initially unusual, and perhaps uncomfortable, postures to make it easier to actually use an object.

For example, waiters will pick up an inverted glass with their thumb pointing down if they plan to pour water into the glass. While grabbing thumb-down may feel awkward at first, it allows the waiter to be more comfortable when the glass is turned over and water poured inside.

Does this occur because motor planning abilities were crucial in facilitating the evolution of complex tool use in humans? If so, then we might predict that only humans would show this ability. Or perhaps this ability would be evidenced in humans and other tool-using species. The way to test this hypothesis, then, is to test whether this is something that other animals, non-tool users, would do.

Pennsylvania State University psychologists, Dan Weiss, Jason Wark, and David Rosenbaum decided to see if cotton-top tamarins (non-tool users) would show the end-state comfort effect. In the first experiment, Weiss and colleagues presented the monkeys with a small cup containing a marshmallow. The cup was either suspended upright or upside down. Would these monkeys, a non-tool using species, adopt an unusual grasping pattern while removing the cup from the apparatus to retrieve the marshmallow?

The results, which appear in the December issue of Psychological Science, a journal of the Association for Psychological Science, are fascinating. The monkeys grabbed the inverted cup with their thumb pointing down, thereby behaving much like human adults. In the second experiment, the monkeys were confronted with a new handle shape and still displayed grasps that were consistent with end-state comfort.

This research is the first to provide evidence for more sophisticated motor planning than has previously been attributed to a nonhuman species. The authors suggest that formulating relatively long-term motor plans is a necessary but not sufficient condition for tool use. “Our results may be taken to suggest that the reason tamarins don’t use tools in the wild is not that they lack the ability to plan ahead, but rather that the scope of their planning is limited,” say the researchers.

Adapted from materials provided by Association for Psychological Science.

martes, 4 de diciembre de 2007

Los chimpancés superan a los humanos en el recuerdo numérico

Un estudio de la Universidad de Kioto demuestra que los chimpancés jóvenes tienen una habilidad para recordar números superior a la de las personas

EUROPA PRESS - Madrid - 03/12/2007

Los chimpancés jóvenes tienen una habilidad para recordar números superior a la de los humanos adultos, según un estudio de la Universidad de Kioto en Japón que se publica en la revista Current Biolog.

Los investigadores sometieron a pruebas de memoria numérica a tres parejas de chimpancés compuestas por una madre y su hijo joven, a los que se había enseñado la numeración arábica ascendente de 0 a 9, y a estudiantes universitarios. Una de las madres chimpancé, llamada Ai, había sido el primer chimpancé que aprendió la numeración arábica para etiquetar conjuntos de objetos cotidianos reales con el número apropiado.

En las pruebas, a los chimpancés o a los humanos se les mostraba varios números del 1 al 9 sobre un monitor de pantalla táctil. Estos números eran reemplazados por cuadrados en blanco y el sujeto de la prueba tenía que recordar qué número aparecía en cada localización y tocar los cuadrados en el orden apropiado.

Los chimpancés jóvenes podían retener muchos números de un vistazo y no mostraban cambios en la realización de la prueba aunque variara la cantidad de tiempo durante el que se mantenían los números sobre la pantalla. En general, los resultados obtenidos por los chimpancés jóvenes eran mejores que los de sus madres.

Asimismo, los humanos adultos fueron más lentos que los tres monos jóvenes en sus respuestas. En el caso de los humanos, el porcentaje de ensayos correcto disminuía cuando los números se mantenían menos tiempo en pantalla.

Según explica Tetsuro Matsuzawa, coautor del trabajo, "existe aún mucha gente, incluyendo a muchos biólogos, que creen que los humanos son superiores a los chimpancés en todas las funciones cognitivas". El investigador señala que nadie se imagina que los chimpancés a los cinco años son mejores que los humanos en una tarea de memoria. El estudio muestra por primera vez, según Matsuzawa, que los chimpancés jóvenes tienen una extraordinaria capacidad para el recuerdo numérico, mejor que la de los adultos humanos que realizaron la misma prueba.

Para Matsuzawa esta capacidad de los chimpancés evoca la memoria eidética, una habilidad especial para recordar con detalle lo visto (memoria fotográfica) u oído y que se cree está presente en los niños humanos para luego disminuir con la edad.

Fuente: El País, 4-12-2007

Els dinosaures eren més grans i veloços del que s'estimava

1. • Un fòssil demostra que els animals tenien les vèrtebres més separades

Recreació de l'hadrosaure Dakota, el fòssil del qual ha permès avançar en el coneixement dels dinosaures. Foto: AP / NATIONAL GEOGRAPHIC SOCIETY


Els museus d'història natural hauran de començar a fer una mica més d'espai per a les seves rèpliques de dinosaures. Científics associats a National Geographic han analitzat les restes de Dakota, un inusual fòssil d'hadrosaure mort fa entre 65 i 67 milions d'anys i els teixits tous del qual s'havien "momificat", i han descobert que les vèrtebres dels dinosaures no estaven tan unides com es pensava fins ara. Fins a un centímetre de distància separa les unes de les altres, cosa que augmentaria la mida dels animals: per exemple, un dinosaure amb 200 vèrtebres faria dos metres més del que s'estimava fins al moment
Aquesta troballa és només un dels resultats d'una completa investigació que es presenta en un nou llibre i en un programa que National Geographic emet als EUA diumenge. La base ha estat l'estudi de Dakota, un fòssil localitzat el 1999 per un adolescent a la granja del seu oncle a Dakota del sud i que és un dels pocs espècimens trobats amb teixits tous com per exemple pell, tendons i lligaments fossilitzats. L'estrany fenomen es deu a una reacció química, segons Phillip Manning, el paleontòleg britànic que ha dirigit l'estudi del fòssil i que firma el llibre. Els minerals en què va quedar enterrat el cadàver es van formar inusualment ràpid, avançant la fossilització abans que els teixits tous s'haguessin degenerat completament.

Aquests teixits han permès calcular per primera vegada aspectes com la massa muscular. I en el cas de l'hadrosaure s'ha descobert que la seva gropa era fins a un 25% més gran del que s'estimava, cosa que li permetia córrer fins a 45 quilòmetres per hora. Això suposa que l'herbívor era fins a 16 quilòmetres per hora més ràpid que el voraç carnívor Tiranosaure Rex.
La velocitat era una de les diverses eines de protecció que tenia l'hadrosaure davant els depredadors. Una altra és una pell ratllada que li permetia protegir-se amb tècniques de camuflatge.

Fuente: El Periódico de Catalunya, 4-12-2007