Findings Friday: The aging brain is a distracted brain

As the brain ages, it becomes more difficult for it to shut out irrelevant stimuli—that is, it becomes more easily distracted. Sitting in a restaurant, having a conversation with your table partner right across the table from you, presents as a new challenge when the restaurant is buzzing with activity.

However, the aging brain does not have to be the distracted brain. Training the mind to shut out irrelevant stimuli is possible, even for the older brain.

Brown University scientists conducted a study involving seniors and college-age students. The experiment was a visual one.

Participants were presented with a letter and number sequence, and asked to report only the numbers, while simultaneously disregarding a series of dots. The dots sometimes moved randomly, and at other times, moved in a clear path. The latter scenario makes the dots more difficult to ignore, as the eye tends to want to watch the dots move.

The senior participants tended to unintentionally learn the dot motion patterns, which was determined when they were asked to describe which way the dots were moving. The college age participants were better able to ignore the dots, and focus on the task at hand (the numbers).

Another study also examined aging and distractibility, or an inability to maintain proper focus on a goal due to attention to irrelevant stimuli. Here, aging brains were trained to be more focused. The researchers used older rats, as well as older humans. Three different sound were played during the experiment, with a target tone presented. Awards were given when the target tone was identified and the other tones ignored. As subjects improved, the tasks became challenging, with the target tone becoming less distinguishable to from the other tones.

However, after training, both the rats and the humans made fewer errors. In fact, electrophysiological brain recordings indicated that neural responses to the non-target, or distracting, tones were decreased.

Interestingly, the researchers indicated that ignoring a task is not the flip side of focusing on a task. Older brains can be just as efficient at focusing as younger brains. The issue in aging brains, however, lies in being able to filter out distractions. This is where training comes in: strengthening the brain’s ability to ignore distractors; not necessarily enhancing the brain’s ability to focus.

The major highlights of the study include training older humans with respect to enhanced aspects of cognitive control, and the adaptive distractor training that sought to selectively suppress distractor responses.

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Brain Imaging Alphabetical Soup: Making Sense of CAT, PET, MRI, fMRI, SPECT: fMRI

This article will focus on fMRI imaging.

fMRI, or functional magnetic resonance imaging, uses MRI equipment. fMRI is sensitive to the amount of deoxyhemoglobin in the blood. We know that when neurons consume oxygen, the neurons convert oxyhemoglobin to deoxyhemoglobin. Now, deoxyhemoglobin has magnetic properties, and these magnetic properties therefore introduce distortions in the local magnetic field. This distortion can be measured, and the measurement therefore gives an indication of how much deoxyhemoglobin in is in the blood. This technique is termed BOLD, for blood oxygen-level-dependent contrast.

The BOLD signal evolves over time in response to an increase in neural activity. This is called the hemodynamic response time, or HRF. HRF has three phases:

 

  1. The initial dip: as neurons consume oxygen, there is a small rise in deoxyhemoglobin, which results in a reduction of the BOLD signal.
  2. Overcompensation: in response to the increased consumption of oxygen, blood to a brain region increases. The increase in blood flow is greater than the increase in blood consumption, and therefore, the BOLD signal increases. This is the component that the fMRI normally measures.
  3. Undershoot: Blood flow and oxygen consumption dip before returning back to original levels.

 

The temporal resolution of fMRI is several seconds. This is better than PET scans, but it is still slow compared to the speeds at which cognitive processes take place.

Brain Imaging Alphabetical Soup: Making Sense of CAT, PET, MRI, fMRI, SPECT: PET scans

Structural imaging measures permanency. In other words, it measures the permanent, structural characteristics of the brain. Functional imaging measures more of the variability, the moment-to-moment changes of the brain that are associated with various cognitive processes, such as attention.

We know the brain consumes about 20% of the body’s total energy. A lot of this energy consumption is due to the maintenance of the membrane potential, of ion channels/gates. Now, most of the energy the brain receives is due to the vast vasculature of the brain. So, when the brain’s metabolic needs increases, you can see the increase of the blood supply to that specific region of the brain that is being most used in a given task.

PET scans are used in these situations, since these sorts of scans measure blood flow to a region directly. Other techniques use oxygenation to a brain region via fMRI.

This article will focus on PET scans.

PET, or positron emission tomography, uses radioactive tracers that have been injected into a subject’s bloodstream. The greater the blood flow to a brain region, the greater the signal emitted by the tracer in that specific region. Most commonly used tracers are oxygen-15 and fluorine-18. Oxygen-15 is usually given in the form of water, while fluorine-18 is usually given in the form of glucose.

Interestingly, though, other tracers can be used, such as radioactive neurotransmitters, which are used more commonly to study neural pathways and to study drug effects on the brain (which makes sense, considering drugs alter neurochemistry, which is based on neurotransmitters release and reuptake).

Now, with PET scans, it takes 30 seconds for the tracer to enter the brain, and then another 30 seconds for the radioactivity to reach its peak. See a problem with this?

These two 30-second windows are crucial, because in these windows, there are changes in blood flow related to cognitive tasks. Therefore, the temporal resolution (which is the accuracy that you can measure WHEN a cognitive activity is taking place) of a PET scan is about 30 seconds, which makes it unviable to use in the timing of things.

Regardless, this is how a PET scan works:

The tracer is injected into the bloodstream and it converted back from the unstable radioactive form to the stable form. As it does this, it emits a positron that ends up colliding with an electron. This releases two photons that can then be detected by machinery. A spatial image of the brain is thus constructed.

Brain Imaging Alphabetical Soup: Making Sense of CAT, PET, MRI, fMRI, SPECT: MRI

I started this mini-series with the articles on how imaging got started, and on CT scans. Now we’ll move on to MRI scans.

MRI, or magnetic resonance imaging, is used to create images of the body’s tissues, specifically of the soft tissues, like organs. X-rays pass through soft tissues undistorted and relatively easily. Now, most human tissue is water-based (which makes sense considering we’re 70% water).  The amount of water in tissues differ. So different tissues will behave differently. These differences can then be used to construct a 3D image.

MRI scans are constructed by applying a strong magnetic field around the body. In water, you can find single protons (Hydrogen atoms, like H20). In MRI scans, it is the H nuclei that create the signal.

So, initially, the fields are randomly oriented. When a strong external magnetic field is applied, a fraction will organize themselves, and align themselves with the external field. This external field is applied in a constant manner throughout the scanning session.

Now, when the protons are aligned, a brief radio frequency is applied. This radio frequency then knocks the protons out of alignment by 90 degrees to their original orientation. So now the protons are spinning in this new state, and as they do this, they produce a detectable change in the magnetic field. This then is what forms the MRI signal.

Eventually, the protons are pulled back into their original alignment (they “relax”). The MRI scanner then repeats the process by sending the radio frequency to excite the protons in different slices of the brain that is being scanned.

Now there are different types of MRI scans, including the T1 and T2 scans.

T1-weighted images are used for structural imaging of the brain. In a T1-weighted image, gray matter looks gray, and white matter looks white. Pretty simple to distinguish.

When the protons are misaligned at 90 degrees to the magnetic field, the MRI signal decays because of interactions with other nearby molecules. This is the T2 image.

 

 

 

Brain Imaging Alphabetical Soup: Making Sense of CAT, PET, MRI, fMRI, SPECT: CAT Scans

We’ve all heard the abbreviations: CAT, PET, MRI, fMRI, SPECT.

Now what the heck do they all mean?

Brain imaging has an interesting start. But generally, brain imaging techniques fall into two broad categories: Structural imaging, and Functional imaging. Structural imaging does just what its name implies: it provides images of the structure of the brain. There’s no indication of blood flow or brain metabolism here; just bone and tissue. Structural images are based on the idea that different bodily tissues have different densities and different physical properties. Structural images provide static, one-shot images of the brain’s structure. They say nothing of function.

Functional imaging also does what its name implies: it images the brain as it functions. Dyes, radioactive tracers, are all used in these imaging modalities to image the brain’s blood flow and metabolism. These images are dynamic, showing changes in brain function over a period of time.

The next few articles in my blog will feature one of the imaging techniques in an article. We’ll start off with CAT scans.

CAT stands for Computerized Axial Tomography. CAT scans are also abbreviated CT scans, for Computerized Tomography. CT scans are possible because different tissues have different X-ray absorptions. How much a tissue absorbs X-rays is related to that tissue’s density. So, bone absorbs the most X-rays, cerebrospinal fluid (CSF) absorbs the least. That’s why on CT scans, bone appears white and the ventricles, which contain CSF, appear black.

There’s a slight danger with CT scans, because the person being imaged is exposed to a slight amount of radiation.

 

CT scan image can be found here: http://vectorblog.org/wp-content/uploads/2011/06/Head_CT_scan.jpg

Mosso and Bertino: Brain Injury to Imaging Inventions

The earliest ways of peering into the brains of people was invasive, and sometimes, fatal. Consequently, most subjects were those who were mentally disabled, those who had mental illnesses. Therefore, we now know more about dysfunction than we do function. However, this isn’t a bad thing, considering that it’s through dysfunction that we can better understand how a healthy brain works.

 

There was an 18oo’s experimenter, Angelo Mosso. Mosso had the chance of encountering a peasant named Bertino. Now, Bertino suffered a head injury several years prior to meeting Mosso. The injury was severe enough that it destroyed his frontal bone of his skull, which cover the frontal lobe. The frontal lobe is for reasoning, decision-making, planning, all the executive functions, and personality. What was most interesting to Mosso, however, was the fact that because Bertino had this bone injury, his frontal lobes were now covered not by bone, but by fibrous tissue. This tissue acted as a window through which Mosso could see Bertino’s brain pulsating.

I’m sure if you saw a pulsating brain, you’d investigate further. And that’s exactly what Mosso did.

So one day, Mosso noticed that tere were changing in the pulsation magnitude: whenever the church bells rung at noon, the pulsating increased. So what does Mosso do?

He asks Bertino a question: does the ringing of the church bells remind you of your obligation to silently recite the Ave Maria?

To which Bertino responds: Yes.

And as Bertino responded, the pulsations increased once more.

So of course Mosso is ultra-curious now. He asks Bertino some math questions, like multiply this by that. Whenever Mosso asked a question, the pulsations increased. Whenever Bertino answered a question, there was another pulse magnitude increase.

 

If you made these observations, what would you hypothesize was going on?

 

This was Mosso’ hypothesis, which proved to be correct: an increase in blood flow to the brain could provide a measure, albeit indirect, of brain function during a specific activity.

 

Mosso was right and people began developing techniques for measuring blood flow to the brain. This led to our current technologies, such as PET, fMRI, and SPECT.

 Image is from Wikipedia, Angelo Mosso article.