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Posts Tagged ‘visual cortex

Sleep-deprived Brains Alternate Between Normal Activity And ‘Power Failure’

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Neuroscience researchers at the Duke-NUS Graduate Medical School in Singapore have shown for the first time what happens to the visual perceptions of healthy but sleep-deprived volunteers who fight to stay awake, like people who try to drive through the night.

The scientists found that even after sleep deprivation, people had periods of near-normal brain function in which they could finish tasks quickly. However, this normalcy mixed with periods of slow response and severe drops in visual processing and attention, according to their paper, published in the Journal of Neuroscience on May 21.

“Interestingly, the team found that a sleep-deprived brain can normally process simple visuals, like flashing checkerboards. But the ‘higher visual areas’ — those that are responsible for making sense of what we see — didn’t function well,” said Dr. Michael Chee, lead author and professor at the Neurobehavioral Disorders Program at Duke-NUS. “Herein lies the peril of sleep deprivation.”

The research team, including colleagues at the University of Michigan and University of Pennsylvania, used magnetic resonance imaging to measure blood flow in the brain during speedy normal responses and slow “lapse” responses. The study was funded by grants from the DSO National Laboratories in Singapore, the National Institutes of Health, the National Institute on Drug Abuse, the NASA Commercialization Center, and the Air Force Office of Scientific Research.

Study subjects were asked to identify letters flashing briefly in front of them. They saw either a large H or S, and each was made up of smaller Hs or Ss. Sometimes the large letter matched the smaller letters; sometimes they didn’t. Scientists asked the volunteers to identify either the smaller or the larger letters by pushing one of two buttons.

During slow responses, sleep-deprived volunteers had dramatic decreases in their higher visual cortex activity. At the same time, as expected, their frontal and parietal ‘control regions’ were less able to make their usual corrections.

Scientists also could see brief failures in the control regions during the rare lapses that volunteers had after a normal night’s sleep. However, the failures in visual processing were specific only to lapses that occurred during sleep deprivation.

The scientists theorize that this sputtering along of cognition during sleep deprivation shows the competing effects of trying to stay awake while the brain is shutting things down for sleep. The brain ordinarily becomes less responsive to sensory stimuli during sleep, Chee said.

This study has implications for a whole range of people who have to struggle through night work, from truckers to on-call doctors. “The periods of apparently normal functioning could give a false sense of competency and security when in fact, the brain’s inconsistency could have dire consequences,” Chee said.

“The study task appeared simple, but as we showed in previous work, you can’t effectively memorize or process what you see if your brain isn’t capturing that information,” Chee said. “The next step in our work is to see what we might do to improve things, besides just offering coffee, now that we have a better idea where the weak links in the system are.”

Michael W. L. Chee, Jiat Chow Tan, Hui Zheng, Sarayu Parimal, Daniel H. Weissman, Vitali Zagorodnov, and David F. Dinges
Lapsing during Sleep Deprivation Is Associated with Distributed Changes in Brain Activation
J. Neurosci. 2008 28: 5519-5528; doi:10.1523/JNEUROSCI.0733-08.2008

Lapses of attention manifest as delayed behavioral responses to salient stimuli. Although they can occur even after a normal night’s sleep, they are longer in duration and more frequent after sleep deprivation (SD). To identify changes in task-associated brain activation associated with lapses during SD, we performed functional magnetic resonance imaging during a visual, selective attention task and analyzed the correct responses in a trial-by-trial manner modeling the effects of response time. Separately, we compared the fastest 10% and slowest 10% of correct responses in each state. Both analyses concurred in finding that SD-related lapses differ from lapses of equivalent duration after a normal night’s sleep by (1) reduced ability of frontal and parietal control regions to raise activation in response to lapses, (2) dramatically reduced visual sensory cortex activation, and (3) reduced thalamic activation during lapses that contrasted with elevated thalamic activation during nonlapse periods. Despite these differences, the fastest responses after normal sleep and after SD elicited comparable frontoparietal activation, suggesting that performing a task while sleep deprived involves periods of apparently normal neural activation interleaved with periods of depressed cognitive control, visual perceptual functions, and arousal. These findings reveal for the first time some of the neural consequences of the interaction between efforts to maintain wakefulness and processes that initiate involuntary sleep in sleep-deprived persons.

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Written by huehueteotl

May 22, 2008 at 7:48 am

Twin Study Indicates Genetic Basis For Processing Faces, Places

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A new fMRI study of twins indicates that the genetic foundation for the brain’s ability to recognize faces and places is much stronger than for other objects, such as words. The authors claim, somewhat pompously, to have digged into the problem of “Nature versus nurture in ventral visual cortex…”. As can be seen in other places, the problem is none. No human brain activity is going to be either of the two, exclusively. Nonetheless, the results are a hint pointing towards the role of genetics in assigning these functions to specific regions of the brain.http://www.nmr.mgh.harvard.edu/~rhoge/HST583/doc/VisualCortex.jpg
“We are social animals who have specialized circuitry for faces and places,” says Arthur W. Toga, PhD, director of the Laboratory of NeuroImaging at UCLA School of Medicine. “Some people are better at recognizing faces and places, and this study provides evidence that it is partially determined by genetics.”Using a functional magnetic resonance imaging (fMRI) scanner, Thad Polk, PhD, Joonkoo Park, and Mason Smith of the University of Michigan, along with Denise Park, PhD, at the University of Illinois at Urbana-Champaign, measured activity in the visual cortex of 24 sets of fraternal and identical twins. The twins watched several series of images: sets of people’s faces, houses, letters strung together, and chairs, as well as scrambled images that served as a baseline measurement.Previous research had identified distinct regions in the visual cortex where different categories of information are processed, a sort of division of labor in the brain that handles information about people, for example, independently of that related to cars.

Polk’s analysis of brain activity patterns from the twins suggests how the organization of these independent regions is shaped. By showing greater similarity in the brain activity of identical twins than their fraternal counterparts when processing faces and places, the results indicate a genetic basis for these functions. Activity in response to words, Polk suggests, may be shaped to a greater degree by one’s experiences and environment.

“Face and place recognition are older than reading on an evolutionary scale, they are shared with other species, and they provide a clearer adaptive advantage,” says Polk. “It is therefore plausible that genetics would shape the cortical response to faces and places, but not orthographic stimuli.”

J Neurosci. 2007 Dec 19;27(51):13921-5.
Nature versus nurture in ventral visual cortex: a functional magnetic resonance imaging study of twins.

Department of Psychology, University of Michigan, Ann Arbor, Michigan 48109, USA. tpolk@umich.edu

Using functional magnetic resonance imaging, we estimated neural activity in twins to study genetic influences on the cortical response to categories of visual stimuli (faces, places, and pseudowords) that are known to elicit distinct patterns of activity in ventral visual cortex. The neural activity patterns in monozygotic twins were significantly more similar than in dizygotic twins for the face and place stimuli, but there was no effect of zygosity for pseudowords (or chairs, a control category). These results demonstrate that genetics play a significant role in determining the cortical response to faces and places, but play a significantly smaller role (if any) in the response to orthographic stimuli.

Written by huehueteotl

December 23, 2007 at 3:15 pm

Neural Activity Connected To Blood Flow In New Brain Stimulation Technique

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Neuroscientists at the University of California, Berkeley, have 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.


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. (Credit: Elena Allen/UC Berkeley)

Their findings could substantially improve the effectiveness of brain stimulation as a therapeutic and research tool.

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.”

Science. 2007 Sep 28;317(5846):1918-21. <!– var Menu17901333 = [ [“UseLocalConfig”, “jsmenu3Config”, “”, “”], [“LinkOut”, “window.top.location=’http://www.ncbi.nlm.nih.gov/sites/entrez?Cmd=ShowLinkOut&Db=pubmed&TermToSearch=17901333&ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_RVAbstractPlus&#8217; “, “”, “”] ] —

Transcranial magnetic stimulation elicits coupled neural and hemodynamic consequences.

Allen EA, Pasley BN, Duong T, Freeman RD.

Helen Wills Neuroscience Institute, Group in Vision Science, School of Optometry, University of California, Berkeley, CA 94720, USA.

Transcranial magnetic stimulation (TMS) is an increasingly common technique used to selectively modify neural processing. However, application of TMS is limited by uncertainty concerning its physiological effects. We applied TMS to the cat visual cortex and evaluated the neural and hemodynamic consequences. Short TMS pulse trains elicited initial activation (approximately 1 minute) and prolonged suppression (5 to 10 minutes) of neural responses. Furthermore, TMS disrupted the temporal structure of activity by altering phase relationships between neural signals. Despite the complexity of this response, neural changes were faithfully reflected in hemodynamic signals; quantitative coupling was present over a range of stimulation parameters. These results demonstrate long-lasting neural responses to TMS and support the use of hemodynamic-based neuroimaging to effectively monitor these changes over time.

Written by huehueteotl

October 11, 2007 at 10:50 am