1 00:00:07,867 --> 00:00:11,598 Imagine, for a second, a duck teaching a French class, 2 00:00:11,598 --> 00:00:15,167 a ping-pong match in orbit around a black hole, 3 00:00:15,167 --> 00:00:17,788 a dolphin balancing a pineapple. 4 00:00:17,788 --> 00:00:21,277 You probably haven't actually seen any of these things, 5 00:00:21,277 --> 00:00:23,937 but you could imagine them instantly. 6 00:00:23,937 --> 00:00:27,618 How does your brain produce an image of something you've never seen? 7 00:00:27,618 --> 00:00:28,978 That may not seem hard, 8 00:00:28,978 --> 00:00:31,948 but that's only because we're so used to doing it. 9 00:00:31,948 --> 00:00:34,629 It turns out that this is actually a complex problem 10 00:00:34,629 --> 00:00:38,818 that requires sophisticated coordination inside your brain. 11 00:00:38,818 --> 00:00:41,758 That's because to create these new, weird images, 12 00:00:41,758 --> 00:00:46,667 your brain takes familiar pieces and assembles them in new ways, 13 00:00:46,667 --> 00:00:49,789 like a collage made from fragments of photos. 14 00:00:49,789 --> 00:00:53,329 The brain has to juggle a sea of thousands of electrical signals 15 00:00:53,329 --> 00:00:58,059 getting them all to their destination at precisely the right time. 16 00:00:58,059 --> 00:00:59,779 When you look at an object, 17 00:00:59,779 --> 00:01:03,658 thousands of neurons in your posterior cortex fire. 18 00:01:03,658 --> 00:01:07,018 These neurons encode various characteristics of the object: 19 00:01:07,018 --> 00:01:11,159 spiky, fruit, brown, green, and yellow. 20 00:01:11,159 --> 00:01:15,540 This synchronous firing strengthens the connections between that set of neurons, 21 00:01:15,540 --> 00:01:20,095 linking them together into what's known as a neuronal ensemble, 22 00:01:20,095 --> 00:01:22,300 in this case the one for pineapple. 23 00:01:22,300 --> 00:01:25,329 In neuroscience, this is called the Hebbian principle, 24 00:01:25,329 --> 00:01:28,839 neurons that fire together wire together. 25 00:01:28,839 --> 00:01:30,949 If you try to imagine a pineapple later, 26 00:01:30,949 --> 00:01:35,850 the whole ensemble will light up, assembling a complete mental image. 27 00:01:35,850 --> 00:01:39,029 Dolphins are encoded by a different neuronal ensemble. 28 00:01:39,029 --> 00:01:41,050 In fact, every object that you've seen 29 00:01:41,050 --> 00:01:45,290 is encoded by a neuronal ensemble associated with it, 30 00:01:45,290 --> 00:01:49,240 the neurons wired together by that synchronized firing. 31 00:01:49,240 --> 00:01:52,510 But this principle doesn't explain the infinite number of objects 32 00:01:52,510 --> 00:01:57,240 that we can conjure up in our imaginations without ever seeing them. 33 00:01:57,240 --> 00:02:02,480 The neuronal ensemble for a dolphin balancing a pineapple doesn't exist. 34 00:02:02,480 --> 00:02:04,922 So how come you can imagine it anyway? 35 00:02:04,922 --> 00:02:07,760 One hypothesis, called the Mental Synthesis Theory, 36 00:02:07,760 --> 00:02:11,130 says that, again, timing is key. 37 00:02:11,130 --> 00:02:13,941 If the neuronal ensembles for the dolphin and pineapple 38 00:02:13,941 --> 00:02:16,172 are activated at the same time, 39 00:02:16,172 --> 00:02:20,761 we can perceive the two separate objects as a single image. 40 00:02:20,761 --> 00:02:24,041 But something in your brain has to coordinate that firing. 41 00:02:24,041 --> 00:02:27,521 One plausible candidate is the prefrontal cortex, 42 00:02:27,521 --> 00:02:31,301 which is involved in all complex cognitive functions. 43 00:02:31,301 --> 00:02:35,172 Prefrontal cortex neurons are connected to the posterior cortex 44 00:02:35,172 --> 00:02:40,040 by long, spindly cell extensions called neural fibers. 45 00:02:40,040 --> 00:02:44,339 The mental synthesis theory proposes that like a puppeteer pulling the strings, 46 00:02:44,339 --> 00:02:47,869 the prefrontal cortex neurons send electrical signals 47 00:02:47,869 --> 00:02:49,582 down these neural fibers 48 00:02:49,582 --> 00:02:53,410 to multiple ensembles in the posterior cortex. 49 00:02:53,410 --> 00:02:56,292 This activates them in unison. 50 00:02:56,292 --> 00:02:59,409 If the neuronal ensembles are turned on at the same time, 51 00:02:59,409 --> 00:03:04,342 you experience the composite image just as if you'd actually seen it. 52 00:03:04,342 --> 00:03:06,551 This conscious purposeful synchronization 53 00:03:06,551 --> 00:03:09,852 of different neuronal ensembles by the prefrontal cortex 54 00:03:09,852 --> 00:03:12,052 is called mental synthesis. 55 00:03:12,052 --> 00:03:13,813 In order for mental sythesis to work, 56 00:03:13,813 --> 00:03:19,303 signals would have to arrive at both neuronal ensembles at the same time. 57 00:03:19,303 --> 00:03:21,073 The problem is that some neurons 58 00:03:21,073 --> 00:03:25,083 are much farther away from the prefrontal cortex than others. 59 00:03:25,083 --> 00:03:28,453 If the signals travel down both fibers at the same rate, 60 00:03:28,453 --> 00:03:31,163 they'd arrive out of sync. 61 00:03:31,163 --> 00:03:33,583 You can't change the length of the connections, 62 00:03:33,583 --> 00:03:37,044 but your brain, especially as it develops in childhood, 63 00:03:37,044 --> 00:03:40,884 does have a way to change the conduction velocity. 64 00:03:40,884 --> 00:03:45,534 Neural fibers are wrapped in a fatty substance called myelin. 65 00:03:45,534 --> 00:03:47,343 Myelin is an insulator 66 00:03:47,343 --> 00:03:51,554 and speeds up the electrical signals zipping down the nerve fiber. 67 00:03:51,554 --> 00:03:55,850 Some neural fibers have as many as 100 layers of myelin. 68 00:03:55,850 --> 00:03:57,754 Others only have a few. 69 00:03:57,754 --> 00:04:00,055 And fibers with thicker layers of myelin 70 00:04:00,055 --> 00:04:04,154 can conduct signals 100 times faster or more 71 00:04:04,154 --> 00:04:06,565 than those with thinner ones. 72 00:04:06,565 --> 00:04:09,995 Some scientists now think that this difference in myelination 73 00:04:09,995 --> 00:04:13,835 could be the key to uniform conduction time in the brain, 74 00:04:13,835 --> 00:04:16,925 and consequently, to our mental synthesis ability. 75 00:04:16,925 --> 00:04:20,255 A lot of this myelination happens in childhood, 76 00:04:20,255 --> 00:04:21,814 so from an early age, 77 00:04:21,814 --> 00:04:26,115 our vibrant imaginations may have a lot to do with building up brains 78 00:04:26,115 --> 00:04:28,381 whose carefully myelinated connections 79 00:04:28,381 --> 00:04:31,824 can craft creative symphonies throughout our lives.