28 septiembre 2007

TMS, estimulación magnética transcraneal

Neuroscientists connect neural activity and blood flow in new brain stimulation technique

UC Berkeley News

– Neuroscientists at the University of California, Berkeley, have for the first time measured the electrical activity of nerve cells and correlated it to changes in blood flow in response to transcranial magnetic stimulation (TMS), a noninvasive method to stimulate neurons in the brain.

Their findings, reported in the Sept. 28 issue of the journal Science, could substantially improve the effectiveness of brain stimulation as a therapeutic and research tool.

visual cortex during transcranial magnetic stimulation
Illustration of the visual cortex during transcranial magnetic stimulation (TMS). In this non-invasive brain stimulation technique, pulses of current (arrows) are passed through a figure-eight shaped coil placed above the scalp. The induced electric field elicits long-lasting alterations in neural activity which can be measured with blood flow-based imaging methods. (Elena Allen/UC Berkeley)
With technological advances over the past decade, TMS has emerged as a promising new tool in neuroscience to treat various clinical disorders, including depression, and to help researchers better understand how the brain functions and is organized.

TMS works by generating magnetic pulses via a wire coil placed on top of the scalp. The pulses pass harmlessly through the skull and induce short, weak electrical currents that alter neural activity. Yet the relative scarcity of data describing the basic effects of TMS, and the uncertainty in how the method achieves its effects, prompted the researchers to conduct their own study.

"There are potentially limitless applications in both the treatment of clinical disorders as well as in fundamental research in neuroscience," said Elena Allen, a graduate student at UC Berkeley's Helen Wills Neuroscience Institute (HWNI) and co-lead author of the study. "For example, TMS could be used to help determine what parts of the brain are used in object recognition or speech comprehension. However, to develop effective applications of TMS, it is first necessary to determine basic information about how the technique works."

Other techniques for studying neural activity in humans, such as functional magnetic resonance imaging (fMRI) or electroencephalogram (EEG), only measure ongoing activity. TMS, on the other hand, offers the opportunity to non-invasively and reversibly manipulate neural activity in a specific brain area.

In a set of experiments, the researchers used TMS to generate weak, electrical currents in the brain with quick 2- to 4-second bursts of magnetic pulses to the visual cortex of cats. Direct measurements of the electrical discharge of nerve cells in the region in response to the pulses revealed that TMS predictably caused an initial flurry of neural activity, significantly increasing cell firing rates. This increased activity lasted 30 to 60 seconds, followed by a relatively lengthy 5 to 10 minutes of decreased activity.

What the researchers were able to determine for the first time was that the neural response to TMS correlated directly to changes in blood flow to the region. Using oxygen sensors and optical imaging, the researchers found that an initial increase in blood flow was followed by a longer period of decreased activity after the magnetic pulses were applied.

"This long-lasting suppression of activity was surprising," said Brian Pasley, a graduate student at HWNI and co-lead author of the study. "We're still trying to understand the physiological mechanisms underlying this effect, but it has implications for how TMS could be used in clinical applications."

The critical confirmation of the connection between blood flow and neural activity means that researchers can use TMS to alter neural activity, and then use fMRI, which tracks blood flow changes, to assess how the nerve cells respond over time.

"One of the most exciting applications of TMS is the ability to non-invasively modify neural activity in specific ways," said Pasley. "The brain is malleable, so brain stimulation may be used to alter and promote specific functions, like learning and memory, or suppress abnormal activity that underlies neurological disorders. If we can figure out the right ways to stimulate the brain, TMS will likely be useful in attempts to improve neural function."

The researchers noted that one of the difficulties in using TMS for specific applications is the fact that its effects vary in different brain regions and individuals.

"Using TMS is inherently challenging because its neural effects can be so variable," said Ralph Freeman, UC Berkeley professor of vision science and optometry and principal investigator of the study. "Fortunately, we can determine empirically what the end result is by making measurements with fMRI. This should be valuable to clinicians who must evaluate the effectiveness of a stimulation treatment. In turn, fMRI may serve as a guide to determine adjustments in treatment parameters."

The study was also co-authored by Thang Duong, a UC Berkeley graduate student in vision science. The National Eye Institute of the National Institutes of Health and the National Science Foundation helped support this research.

26 septiembre 2007

El experimento psicológico más importante nunca hecho

The Most Important Psychology Experiment That's Never Been Done

By Brandon Keim September 25, 2007 | 4:57:29 PM

Tunnel What's the most important psychology experiment that hasn't been done?

The British Psychological Society asked this question of leading psychologists and bloggers, and is collecting the responses on their blog. One of the more interesting, suggested by Susan Blackmore: watching death.

The most important experiment that’s never been done is to take fMRI or PET scans of people as they die; either those who really do go on to die, or those who suffer clinical death but are resuscitated. If this were done we would be able to test theories about how NDEs and mystical experiences are generated in the dying brain, and answer questions about the timing of the experiences. Perhaps even this would not resolve the final question once and for all, but it would certainly bring us a lot closer to knowing what happens when we die.

And why has it not been done? Because when someone is dying it is far more important to try to save their life than to do a scientific experiment. Nevertheless it could be done, and I hope that one day the technology will be so unobtrusive and easy to use that the ethical problem will disappear and we will be able to watch the dying brain as easily as we can now watch the living brain.


23 septiembre 2007

El olor de la androsterona

September 19, 2007
Molecular biology

Whether a man smells sweet or stinky is in the odorant receptor of the beholder

Sophie L. Rovner

Genetic differences determine whether a component of male body odor smells like sweat, vanilla, or nothing at all, according to newly published research. The component is androstenone, a steroid derived from testosterone that is present in sweat.

To some people, androstenone smells pleasant, with a sweet, floral, or vanilla-like scent. Others find the compound's odor offensive and liken it to sweat or urine. A third group can't even smell the compound. The wide variability in people's perception of androstenone is due in large part to slight genetic variations that affect the odorant receptor OR7D4, report Leslie B. Vosshall, an associate professor who heads Rockefeller University's Laboratory of Neurogenetics & Behavior; Hiroaki Matsunami, an assistant professor of molecular genetics and microbiology at Duke University Medical Center; and their colleagues (Nature, DOI: 10.1038/nature06162).

The researchers determined that people who find androstenone unpleasant have two single-nucleotide polymorphisms in the gene for the receptor which is expressed in sensory cells in the nose. In vitro studies showed that these mutations severely impair the receptor's function.

Matsunami says the work represents the first demonstration of a link between the performance of a human odorant receptor and "how that odor is perceived.” He adds that "the sex-steroid odors that we tested in humans act as pheromones in pigs, and there has been debate whether these same chemicals act similarly in humans. There is evidence that smelling these odors can affect the mood and physiological state of both men and women.”