Brain Imaging Karl Friston

brain imaging is the measurement of brain responses responses to changes in stimuli changes in mood if anything you think of that the brain can do then you can challenge the brain experimentally and then measure how the brain responds responds to those challenges the imaging that comes from the fact that the brain has a particular architecture a particular Natale so certain parts of the brain are specialized for performing certain functions so for example there's a particular part of my brain largely about here that's specifically engaged by visual motion so watching moving things and analyzing and processing that information to try and infer or work out the causes of those visual visual sensations to know that you have to be able to measure the whole brain and establish which parts of those brains responds selectively to the particular attribute or the particular function that you're interested in so that requires a measurement of brain responses and that's essentially what we do here the Wellcome Trust Center from your imaging the University College London there are a number of different ways that one can measure brain responses they can roughly fall into two classes the first are measurements that depend upon the brain's energy supply and blood supply so clearly if you're using your brain to process sensory information say visual information that requires energy it requires work and in fact the brain accounts for a considerable proportion of the body's energy budget and that work is localized so there's an increase a blushing if you like of different parts of the brain in response to processing that sort of information for example visual motion that can be measured if you can assess the local blood flow or the consequences of that non-invasively by which I mean measuring from outside the skull and well a lot of the work that we do here the Wellcome Trust Center is to measure the blood flow or the hemodynamic with sponsors in terms of subtle changes in magnetic fields or the signals produced by the differences in the magnetic behavior of blood before and after it has given up its oxygen to the nervous tissue so that they can do the presencing do the neuronal firing so that means if we can scan a brain within say 1 or 2 seconds in one condition and then scan again in a second the next little portion of time we can get a picture of fluctuations in blood flow or in the energy consumption at each point in the brain and then by looking at the patterns of fluctuations in the buffer responses and the patterns of stimulation that we provide to our subjects or possibly patients then we can see whether the which that whether that part of the brain is engaged by the particular stimuli so I could present to you for a several seconds a a picture still picture of saved dots and then I could suddenly make the dots move and I can measure your brain activity in terms of its metabolic activity during the period of static viewing and the equivalent activity during the movie the processing of the moving dots and then by comparing the patterns of activity throughout the brain basically by subtracting the stationary condition from the moving dot condition I can isolate those parts of the brain that were more active during the moving dots paradigm and then I can infer that that part of the brain was specialized for presencing visual motion that notion and that basic paradigm can be generalized to any sensory motor cognitive emotional processing or situation that you can imagine so we can look at the correlates the hemodynamic parlance the blood flow correlates of working memory of processing various emotional stimuli like faith fearful faces we can look at the the correlates of being in different brain States for example being depressed or not being depressed or being elated and not being it being elated anxious and in this way what can build up literally a map and very often people have described much of early brain imaging as a cartography a map building exercise that allows you to assign various functions to functionally specialized parts of the brain thereby creating a literary atlas or a map of which parts of the brain do what and ultimately how different parts of the brain talk to each other so you're building a picture of functional anatomy and anatomy of function of processing information I repeat from right from the sensory processing right through to the cognitive operations such as attention and memory to get a holistic and global picture of how the brain works now I said at the beginning there are these brain imaging technologists come in two flavors I've been talking about techniques that rest upon functional magnetic resonance imaging or positron emission tomography so these are literally devices that provide or offer you an image a snapshot of brain activity at one point in time usually over several seconds so what we're not what we are looking at not the actual very fast fluctuations in electromagnetic activity that nerve cells engage in all the fast synaptic presences that media you know processing but the energy supply that is necessary to sustain that level of information processing or message passing the fluctuations in that energy supply in the order of four to five seconds so we're looking at fluctuations over quite extended periods of time we can do that with our exquisite spatial precision so we can you know the little elements that constitute the entire brain imaging can be as small as a few millimeters so we can measure almost down to the resolution of a few millimeters the specific responses to various experimental paradigms though clearly there's very little temporal acuity there's very little temporal precision in these sorts of measurements because the way that your brain works is on a time scale its measured in milliseconds you notice things you if you think and you move on a time scale where things happen several times a second if not several times you know every few hundred milliseconds so now we turn to the other sort of brain imaging which is the measurement of the actual electrical and magnetic signal generated by the nervous activity the nerve selectivity itself and these fluctuating by very quickly so if I were to present to you a single dot visually and I left it on the screen for say 50 milliseconds that would create a barrage of nervous impulses that would propagate from your eyes through various subcortical structures to the back of the brain and then bounce forward and go everywhere each part of the brain taking from it or some people would say trying to predict the causes of that sensory information that are provided to you and in doing that what you see are fast fluctuations in the electromagnetic field that can be picked up by sensors that are placed either on the scalp or for the magnetic sensors slightly distant from the scalp to get a picture from very different points of view of these fast fluctuations ripples in electromagnetic activity that are different at every point in the brain so this would be electromagnetic brain imaging it would be the sort of imaging that you will associate with EEG or Magneto and catalog rafi or electro and Kevlar CUFI the measures respectively the magnetic and the electrical consequences of this nervous activity this this neuronal firing induced by experimental design that's a form of brain imaging which has exquisite temporal precision but you can see immediately that looking from the outside in and a very complicated spatial arrangement of coupled neuronal responses playing out on a timescale of a few hundred milliseconds is a very difficult picture to interpret unless you can go in and assign your measurements to various parts of the brain that's a very difficult problem that's called the the an inverse problem basically trying to reconstruct the pattern of electromagnetic activity across the brain the best explains your sensory measurements from these sensors placed on the outside of the brain however that can be done with some some assumptions so you can build or reconstruct a picture of distributed in your own activity on almost a millisecond by millisecond timescale that allows you then to interrogate I understand not the spatial deployment of neuronal responses but their temporal Anatomy the succession of responses in terms of which areas pass information to other areas and then other areas pass information back creating succession of little waves and sometimes in continuously processing information these waves constitute oscillations of the whole field of reconstructing creating images of the brain in action in an ongoing way by characterizing the neuronal activity that you've induced by asking subjects a to to move coherently moving visual stimuli or dots in terms of the frequencies with which you're engaging the neural activity and there are all sorts of interesting questions about the physiology the anatomy of that message-passing that can be addressed at this fine time scale so in conclusion brain imaging is in the game of acquiring measurements that inform our understanding of how the brain passes messages of a neuronal sort from one part of the brain to the other in order to make sense of the world we've got two ways of doing that we can either look at the spatial deployment of the energetics that are induced by neuronal processing using technologies like that magnetic resonance imaging or positron emission tomography or we can get into the detailed temporal structure by letting looking at electromagnetic responses using sensors that are external to the brain

Channel: Serious Science

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Keywords: science, lecture, Serious Science, brain, British Scientists, neurology, psychology

Id: rgftzF6MwpI

Length: 12min 51sec (771 seconds)

Published: Mon Sep 11 2017