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cognition lecture 4

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PSYC 213
Daniel J Levitin

January 14 , 2010 Lecture 04 Review Bandura postulated the social learning theory, where people could learn not by receiving rewards and punishments themselves, but by watching other people receive rewards and punishments. This is one of the theories that are frequently flashed when people talk about kids learning violence from TVs. Continuation from last class Today we are going to be talking about functional neuroimaging techniques in cognitive neuroscience. CAT scan is basically X-ray, and just gives you tissue information. It is typically not used in inferring function at all. PET (positron emission tomography) was the first type of scanning that could actually pick up brain metabolism. The idea is that the more part of the brain is engaged in a specific task, the more it will consume oxygen or glucose. PET scans are typically based on glucose by injecting patients with a radioactive tracer that is short-lived. That tracer binds with glucose, and then you scan the person and see where glucose was more absorbed in the brain. That is how you get a picture of the metabolic brain processing. (The professors shows an example of a PET scan) This is an example of a PET scan in a study that looked at different word patterns. What you notice is that there is not so much anatomy. There are only squiggly lines representing the brain. That is one of the major problems with PET; it doesnt actually give us a nice anatomical picture of the brain. What they typically do is combine it with CT, which can give you 3D images of the brain and tissue. Then the changes are overlaid on top. Another major disadvantage of PET is that it uses a radioactive tracer. People actually have to be injected with something so that you can image brain metabolism. fMRI is the newest and has already replaced PET. It is something, when it came up, that was a very exciting research tool. Everyone started using it and experimenting with it; sooner or later the media picked up on it. It is very commonly used today and you will hear stories (ex. this is your brain when youre jealous, when youre in love). It is very common today to use fMRI to start linking low-level functions and higher level functions as well. fMRI works differently than PET. It is magnetic based and gives exceptional resolution of tissue. Obviously, we can use MRI to scan other parts of the body, but for the purpose of this class we will be talking about the brain. (Professor shows a picture of an fMRI machine). That is the machine, you go into the tube and they scan you. It has less than 1 millimeter of spatial resolution. You get images that look like this (Professor shows an fMRI scan). The metabolic activity that it picks up is different than PET. fMRI picks up the differences in oxygenated versus deoxygenated blood. The idea is that the more a brain area is active, the more oxygen that will be available to that area. fMRI measures BOLD signal (Blood Oxygenation Level Dependent): This BOLD signal is extracted post-op. When you go in for scanning, all they can see is anatomy. Functional data is analyzed afterwards. Its not like PET where its immediately obvious; these data are analyzed after the scans have been collected. There are different protocols for running fMRI compared to regular MRI. These are the types of pictures that you would see in the media. What do you think these brain images are that we are seeing here? Are these peoples brains that have been scanned? What are we seeing here? What happens is that, in a study you usually scan 10-15 people. All of their brains get normalized to a template. There are two templates that exist that have these normalized coordinates of brain structures in humans. Every subjects brain is squished to this template. This is what you see here: the template, not an individual brain. What are these blue and red blobs that you see? What is plotted? This is very important! The media and everyone seem to think that this just lights up, this is what the brain area does. These are statistical maps. You see that t-value on the scale? Those of you who have taken statistics know what a t-test is. Basically, this is showing the value of a statistic, and statistically how different a particular area responded relative to baseline. This has nothing to do with oxygenated blood or blood volume. These are statistical maps or statistical tests. The important part here is the baseline. In MRI, the idea is that you have to have a control condition that is in every way identical to your test condition except this function that you are testing. When you analyze the data, you subtract the two (by superimposing). All the noise will get subtracted out, and the only difference that you will see is the function that you are trying to image. We cut the brain up in boxels (3D pixels); there are small cubes and the BOLD activity is measured in every cube to see if it passes a certain threshold and if it does, then we plot it. It is all relative to baseline. Choosing your baseline in these experiments is very important. This person has plotted certain things in blue and certain things in red. Depending on the study that you read, these colors could mean all kinds of different things. Here, blue is brain regions more active to location view rather than colour cues. They have chosen to represent in blue here increase in particular conditions. Red is an increase in other conditions. Red doesnt mean increase and blue decrease all the time. This is very important that you know what you are looking at when you are bombarded by fMRI studies. fMRI has phenomenal spatial resolution (it can tell us within 1 millimeter the structure of tissues, or any organ tissue), however it has really bad temporal resolution. This means resolution in time. Whatever its measuring, so we think it is measured oxygenated
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