How do brain scans work? - John Borghi and Elizabeth Waters

As far as we know, 00:13 there's only one thing in our solar system sophisticated enough to study itself: 00:19 the human brain. 00:21 But this self-investigation is incredibly challenging; 00:24 a living brain is shielded by a thick skull, 00:27 swaddled in layers of protective tissue, 00:30 and made up of billions of tiny, connected cells. 00:33 That's why it's so difficult to isolate, observe, and understand diseases 00:38 like Alzheimer's. 00:39 So how do we study living brains without harming their owners? 00:44 We can use a trio of techniques called EEG, 00:47 fMRI, 00:48 and PET. 00:49 Each measures something different and has its own strengths and weaknesses, 00:53 and we'll look at each in turn. 00:55 First is EEG, or electroencephalography, 00:59 which measures electrical activity in your brain. 01:03 As brain cells communicate, they produce waves of electricity. 01:07 Electrodes placed on the skull pick up these waves, 01:10 and differences in the signals detected between electrodes 01:14 provide information about what's happening. 01:17 This technique was invented almost 100 years ago, 01:20 and it's still used to diagnose conditions like epilepsy and sleep disorders. 01:25 It's also used to investigate what areas of the brain are active 01:28 during learning or paying attention. 01:31 EEG is non-invasive, 01:32 relatively inexpensive, 01:34 and fast: 01:36 it can measure changes that occur in just milliseconds. 01:39 Unfortunately, it's hard to determine 01:41 exactly where any particular pattern originates. 01:45 Electrical signals are generated constantly all over the brain 01:48 and they interact with each other to produce complex patterns. 01:52 Using more electrodes or sophisticated data-processing algorithms can help. 01:57 But in the end, while EEG can tell you precisely when certain activity occurs, 02:03 it can't tell you precisely where. 02:06 To do that, you'd need another technique, 02:08 such as functional magnetic resonance imaging, or fMRI. 02:13 fMRI measures how quickly oxygen is consumed by brain cells. 02:17 Active areas of the brain use oxygen more quickly. 02:21 So watching an fMRI scan while a person completes cognitive or behavioral tasks 02:26 can provide information about which regions of the brain might be involved. 02:30 That allows us to study everything from how we see faces 02:33 to how we understand what we're feeling. 02:36 fMRI can pinpoint differences in brain activity to within a few millimeters, 02:41 but it's thousands of times slower than EEG. 02:44 Using the two techniques together 02:46 can help show when, and where, neural activity is occurring. 02:50 The third, even more precise, technique is called positron emission tomography 02:56 and it measures radioactive elements introduced into the brain. 03:00 That sounds much scarier than it actually is; 03:03 PET scans, like fMRI and EEG, are completely safe. 03:08 During a PET scan, a small amount of radioactive material called a tracer 03:13 is injected into the bloodstream, 03:15 and doctors monitor its circulation through the brain. 03:19 By modifying the tracer to bind to specific molecules, 03:22 researchers can use PET to study the complex chemistry in our brains. 03:27 It's useful for studying how drugs affect the brain 03:29 and detecting diseases like Alzheimer's. 03:32 But this technique has the lowest time resolution of all 03:36 because it takes minutes for the tracer to circulate and changes to show up. 03:40 These techniques collectively help doctors and scientists 03:44 connect what happens in the brain with our behavior. 03:47 But they're also limited by how much we still don't know. 03:51 For example, let's say researchers are interested in studying how memory works. 03:55 After asking 50 participants to memorize a series of images while in MRI scanners, 04:00 the researchers might analyze the results 04:03 and discover a number of active brain regions. 04:05 Making a link between memory and specific parts of the brain 04:09 is an important step forward. 04:11 But future research would be necessary 04:12 to better understand what's happening in each region, 04:15 how they work together, 04:17 and whether the activity is because of their involvement in memory 04:20 or another process occurring simultaneously. 04:23 More advanced imaging and analysis technology 04:26 might one day provide more accurate results 04:29 and even distinguish 04:30 the activity of individual neurons. 04:33 Until then, our brains will keep measuring, analyzing, and innovating 04:37 in pursuit of that quest to understand 04:39 one of the most remarkable things we've ever encountered.