how the brakes of the brain work…
The ability to stop quickly, is at least as important as to get started or to keep going – in brain activity as in real life. Keep from gunning the gas when a pedestrian steps into your path or to bite your tongue mid-sentence when the subject of gossip suddenly comes into view, may depend on a few “cables” in the brain.
Researchers at the University of California, San Diego, have found white matter tracts — like “cables” — forming direct, high-speed connections, between distant regions of the brain, and that appear to play a significant role in the rapid control of behavior. The white matter of the brain contains nerve fibers. Many of these nerve fibers (axons) are surrounded by substance called myelin. The myelin gives the whitish appearance to the white matter. Myelin acts as an insulator, and it increases the speed of transmission of all nerve signals.
White Matter of the Brain
Published in the April 4 issue of the Journal of Neuroscience, the study is the first to identify these white matter tracts in humans, confirming similar findings in monkeys, and the first to relate them to the brain’s activity while people voluntarily control their movements.
Cognitive neuroscientist Adam Aron, an assistant professor of psychology at the University of California, San Diego, said: “We’ve known for some time about key brain areas involved in controlling behavior and now we’re learning how they’re connected and how it is that the information can get from one place to the other really fast.”
“The findings could be useful not only for understanding movement control,” Aron said, “but also ‘self-control’ and how control functions are affected in a range of neuropsychiatric conditions such as addiction, Tourette’s syndrome, stuttering and Attention Deficit Hyperactivity Disorder.”
To reveal the network, Aron and researchers from UCLA, Oxford University and the University of Arizona used diffusion-weighted MRI, in 10 subjects, to demonstrate the “cables” between distant regions of the brain known to be important for control, and they used functional MRI, in 15 other subjects, to show that these same regions were activated when participants stopped their responses on a simple computerized “go-stop” task.
One of the connected regions was the subthalamic nucleus, within the deep-seated midbrain, which is an interface with the motor system and can be considered a “stop button” or the brake itself. A second region was in the right inferior frontal cortex, a region near the temple, where the control signal to put on the brakes probably comes from.
Major Areas of the Brain
“This begs the profound question,” Aron said, “of where and how the decision to execute control arises.”
While this remains a mystery, Aron noted that an additional, intriguing finding of the study was that the third connected node in the network was the presupplementary motor area, which is at the top of the head, near the front. Prior research has implicated this area in sequencing and imagining movements, as well as monitoring for changes in the environment that might conflict with intended actions.
The braking network for movements may also be important for the control of our thoughts and emotions.
There is some evidence for this, Aron said, in the example of Parkinson’s patients. In the advanced stages of disease, people can be completely frozen in their movements, because, it seems, their subthalamic nucleus, or stop button, is always “on.” While electrode treatment of the area unfreezes the patients’ motor system, it can also have the curious effect of disinhibiting them in other ways. In one case, an upstanding family man became manic and hypersexual, and suddenly began stealing money from his wife to pay for prostitutes.
“The study gives us new targets for studying how the brain relates to behavior, personality and genetics,” Aron said. “Variability in the density and thickness of the ‘cable’ connections is probably influenced by genes, and it would be intriguing if these differences explained people’s differing abilities not only to control the swing of a bat but also to control their temper.”
J Neurosci. 2007 Apr 4;27(14):3743-52.
Triangulating a cognitive control network using diffusion-weighted magnetic resonance imaging (MRI) and functional MRI.Aron AR, Behrens TE, Smith S, Frank MJ, Poldrack RA.
Department of Psychology, University of California San Diego, La Jolla, California 92093, USA. firstname.lastname@example.org
The ability to stop motor responses depends critically on the right inferior frontal cortex (IFC) and also engages a midbrain region consistent with the subthalamic nucleus (STN). Here we used diffusion-weighted imaging (DWI) tractography to show that the IFC and the STN region are connected via a white matter tract, which could underlie a “hyperdirect” pathway for basal ganglia control. Using a novel method of “triangulation” analysis of tractography data, we also found that both the IFC and the STN region are connected with the presupplementary motor area (preSMA). We hypothesized that the preSMA could play a conflict detection/resolution role within a network between the preSMA, the IFC, and the STN region. A second experiment tested this idea with functional magnetic resonance imaging (fMRI) using a conditional stop-signal paradigm, enabling examination of behavioral and neural signatures of conflict-induced slowing. The preSMA, IFC, and STN region were significantly activated the greater the conflict-induced slowing. Activation corresponded strongly with spatial foci predicted by the DWI tract analysis, as well as with foci activated by complete response inhibition. The results illustrate how tractography can reveal connections that are verifiable with fMRI. The results also demonstrate a three-way functional-anatomical network in the right hemisphere that could either brake or completely stop responses.
PMID: 17409238 [PubMed – in process]