[Script Info]
Title: 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:07.18,0:00:09.71,Default,,0000,0000,0000,,You hear the gentle lap of waves,
Dialogue: 0,0:00:09.71,0:00:11.91,Default,,0000,0000,0000,,the distant cawing of a seagull.
Dialogue: 0,0:00:11.91,0:00:15.82,Default,,0000,0000,0000,,But then an annoying whine\Ninterrupts the peace,
Dialogue: 0,0:00:15.82,0:00:19.46,Default,,0000,0000,0000,,getting closer, and closer, and closer.
Dialogue: 0,0:00:19.46,0:00:21.57,Default,,0000,0000,0000,,Until...Whack!
Dialogue: 0,0:00:21.57,0:00:25.100,Default,,0000,0000,0000,,You dispatch the offending mosquito, \Nand calm is restored.
Dialogue: 0,0:00:25.100,0:00:32.00,Default,,0000,0000,0000,,How did you detect that noise from afar\Nand target its maker with such precision?
Dialogue: 0,0:00:32.00,0:00:35.47,Default,,0000,0000,0000,,The ability to recognize sounds \Nand identify their location
Dialogue: 0,0:00:35.47,0:00:38.52,Default,,0000,0000,0000,,is possible thanks to the auditory system.
Dialogue: 0,0:00:38.52,0:00:43.16,Default,,0000,0000,0000,,That’s comprised of two main parts: \Nthe ear, and the brain.
Dialogue: 0,0:00:43.16,0:00:47.37,Default,,0000,0000,0000,,The ear’s task is to convert sound energy\Ninto neural signals;
Dialogue: 0,0:00:47.37,0:00:52.08,Default,,0000,0000,0000,,the brain’s is to receive and process \Nthe information those signals contain.
Dialogue: 0,0:00:52.08,0:00:53.90,Default,,0000,0000,0000,,To understand how that works,
Dialogue: 0,0:00:53.90,0:00:57.55,Default,,0000,0000,0000,,we can follow a sound \Non its journey into the ear.
Dialogue: 0,0:00:57.55,0:00:59.62,Default,,0000,0000,0000,,The source of a sound creates vibrations
Dialogue: 0,0:00:59.62,0:01:03.32,Default,,0000,0000,0000,,that travel as waves of pressure \Nthrough particles in air,
Dialogue: 0,0:01:03.32,0:01:04.22,Default,,0000,0000,0000,,liquids,
Dialogue: 0,0:01:04.22,0:01:05.72,Default,,0000,0000,0000,,or solids.
Dialogue: 0,0:01:05.72,0:01:07.99,Default,,0000,0000,0000,,But, our inner ear, called the cochlea,
Dialogue: 0,0:01:07.99,0:01:11.24,Default,,0000,0000,0000,,is actually filled \Nwith saltwater-like fluids.
Dialogue: 0,0:01:11.24,0:01:15.85,Default,,0000,0000,0000,,So, the first problem to solve \Nis how to convert those sound waves,
Dialogue: 0,0:01:15.85,0:01:17.53,Default,,0000,0000,0000,,wherever they’re coming from,
Dialogue: 0,0:01:17.53,0:01:19.81,Default,,0000,0000,0000,,into waves in the fluid.
Dialogue: 0,0:01:19.81,0:01:23.83,Default,,0000,0000,0000,,The solution is the ear drum, \Nor tympanic membrane,
Dialogue: 0,0:01:23.83,0:01:26.47,Default,,0000,0000,0000,,and the tiny bones of the middle ear.
Dialogue: 0,0:01:26.47,0:01:30.17,Default,,0000,0000,0000,,Those convert the large movements \Nof the ear drum
Dialogue: 0,0:01:30.17,0:01:33.42,Default,,0000,0000,0000,,into pressure waves \Nin the fluid of the cochlea.
Dialogue: 0,0:01:33.42,0:01:35.99,Default,,0000,0000,0000,,When sound enters the ear canal,
Dialogue: 0,0:01:35.99,0:01:40.01,Default,,0000,0000,0000,,it hits the ear drum and makes it vibrate \Nlike the head of a drum.
Dialogue: 0,0:01:40.01,0:01:43.94,Default,,0000,0000,0000,,The vibrating eardrum jerks a bone \Ncalled the hammer,
Dialogue: 0,0:01:43.94,0:01:48.68,Default,,0000,0000,0000,,which hits the anvil and\Nmoves the third bone called the stapes.
Dialogue: 0,0:01:48.68,0:01:53.04,Default,,0000,0000,0000,,Its motion pushes the fluid \Nwithin the long chambers of the cochlea.
Dialogue: 0,0:01:53.04,0:01:54.39,Default,,0000,0000,0000,,Once there,
Dialogue: 0,0:01:54.39,0:01:59.18,Default,,0000,0000,0000,,the sound vibrations have finallly\Nbeen converted into vibrations of a fluid,
Dialogue: 0,0:01:59.18,0:02:03.20,Default,,0000,0000,0000,,and they travel like a wave \Nfrom one end of the cochlea to the other.
Dialogue: 0,0:02:03.20,0:02:07.79,Default,,0000,0000,0000,,A surface called the basilar membrane \Nruns the length of the cochlea.
Dialogue: 0,0:02:07.79,0:02:11.80,Default,,0000,0000,0000,,It’s lined with hair cells that have \Nspecialized components
Dialogue: 0,0:02:11.80,0:02:13.54,Default,,0000,0000,0000,,called stereocillia,
Dialogue: 0,0:02:13.54,0:02:17.94,Default,,0000,0000,0000,,which move with the vibrations of the\Ncochlear fluid and the basilar membrane.
Dialogue: 0,0:02:17.94,0:02:22.26,Default,,0000,0000,0000,,This movement triggers a signal \Nthat travels through the hair cell,
Dialogue: 0,0:02:22.26,0:02:24.15,Default,,0000,0000,0000,,into the auditory nerve
Dialogue: 0,0:02:24.15,0:02:28.30,Default,,0000,0000,0000,,then onward to the brain, \Nwhich interprets it as a specific sound.
Dialogue: 0,0:02:28.30,0:02:31.72,Default,,0000,0000,0000,,When a sound makes \Nthe basilar membrane vibrate,
Dialogue: 0,0:02:31.72,0:02:34.37,Default,,0000,0000,0000,,not every hair cell moves -
Dialogue: 0,0:02:34.37,0:02:39.24,Default,,0000,0000,0000,,only selected ones, \Ndepending on the frequency of the sound.
Dialogue: 0,0:02:39.24,0:02:41.72,Default,,0000,0000,0000,,This comes down to some fine engineering.
Dialogue: 0,0:02:41.72,0:02:45.44,Default,,0000,0000,0000,,At one end, \Nthe basilar membrane is stiff,
Dialogue: 0,0:02:45.44,0:02:50.29,Default,,0000,0000,0000,,vibrating only in response to short\Nwavelength, high-frequency sounds.
Dialogue: 0,0:02:50.29,0:02:52.74,Default,,0000,0000,0000,,The other is more flexible,
Dialogue: 0,0:02:52.74,0:02:57.51,Default,,0000,0000,0000,,vibrating only in the presence of longer\Nwavelength, low-frequency sounds.
Dialogue: 0,0:02:57.51,0:03:00.46,Default,,0000,0000,0000,,So, the noises made by the seagull \Nand mosquito
Dialogue: 0,0:03:00.46,0:03:03.54,Default,,0000,0000,0000,,vibrate different locations \Non the basilar membrane,
Dialogue: 0,0:03:03.54,0:03:06.75,Default,,0000,0000,0000,,like playing different keys on a piano.
Dialogue: 0,0:03:06.75,0:03:08.66,Default,,0000,0000,0000,,But that’s not all that’s going on.
Dialogue: 0,0:03:08.66,0:03:12.64,Default,,0000,0000,0000,,The brain still has another \Nimportant task to fulfill:
Dialogue: 0,0:03:12.64,0:03:15.58,Default,,0000,0000,0000,,identifying where a sound is coming from.
Dialogue: 0,0:03:15.58,0:03:19.61,Default,,0000,0000,0000,,For that, it compares the sounds \Ncoming into the two ears
Dialogue: 0,0:03:19.61,0:03:22.13,Default,,0000,0000,0000,,to locate the source in space.
Dialogue: 0,0:03:22.13,0:03:27.45,Default,,0000,0000,0000,,A sound from directly in front of you will\Nreach both your ears at the same time.
Dialogue: 0,0:03:27.45,0:03:31.06,Default,,0000,0000,0000,,You’ll also hear it at the same intensity \Nin each ear.
Dialogue: 0,0:03:31.06,0:03:34.30,Default,,0000,0000,0000,,However, a low-frequency sound \Ncoming from one side
Dialogue: 0,0:03:34.30,0:03:38.85,Default,,0000,0000,0000,,will reach the near ear microseconds \Nbefore the far one.
Dialogue: 0,0:03:38.85,0:03:42.78,Default,,0000,0000,0000,,And high-frequency sounds will sound\Nmore intense to the near ear
Dialogue: 0,0:03:42.78,0:03:46.01,Default,,0000,0000,0000,,because they’re blocked \Nfrom the far ear by your head.
Dialogue: 0,0:03:46.01,0:03:49.76,Default,,0000,0000,0000,,These strands of information \Nreach special parts of the brainstem
Dialogue: 0,0:03:49.76,0:03:54.12,Default,,0000,0000,0000,,that analyze time and\Nintensity differences between your ears.
Dialogue: 0,0:03:54.12,0:03:58.24,Default,,0000,0000,0000,,They send the results of their \Nanalysis up to the auditory cortex.
Dialogue: 0,0:03:58.24,0:04:01.73,Default,,0000,0000,0000,,Now the brain has \Nall the information it needs:
Dialogue: 0,0:04:01.73,0:04:04.54,Default,,0000,0000,0000,,the patterns of activity \Nthat tell us what the sound is,
Dialogue: 0,0:04:04.54,0:04:08.06,Default,,0000,0000,0000,,and information about \Nwhere it is in space.
Dialogue: 0,0:04:08.06,0:04:10.60,Default,,0000,0000,0000,,Not everyone has normal hearing.
Dialogue: 0,0:04:10.60,0:04:15.05,Default,,0000,0000,0000,,Hearing loss is the third most common \Nchronic disease in the world.
Dialogue: 0,0:04:15.05,0:04:19.12,Default,,0000,0000,0000,,Exposure to loud noises \Nand some drugs can kill hair cells,
Dialogue: 0,0:04:19.12,0:04:22.61,Default,,0000,0000,0000,,preventing signals from travelling \Nfrom the ear to the brain.
Dialogue: 0,0:04:22.61,0:04:27.67,Default,,0000,0000,0000,,Diseases like osteosclerosis freeze \Nthe tiny bones in the ear
Dialogue: 0,0:04:27.67,0:04:29.42,Default,,0000,0000,0000,,so they no longer vibrate.
Dialogue: 0,0:04:29.42,0:04:31.30,Default,,0000,0000,0000,,And with tinnitus,
Dialogue: 0,0:04:31.30,0:04:32.96,Default,,0000,0000,0000,,the brain does strange things
Dialogue: 0,0:04:32.96,0:04:36.28,Default,,0000,0000,0000,,to make us think there’s a sound \Nwhen there isn’t one.
Dialogue: 0,0:04:36.28,0:04:38.21,Default,,0000,0000,0000,,But when it does work,
Dialogue: 0,0:04:38.21,0:04:40.97,Default,,0000,0000,0000,,our hearing is an incredible, \Nelegant system.
Dialogue: 0,0:04:40.97,0:04:44.72,Default,,0000,0000,0000,,Our ears enclose a fine-tuned piece \Nof biological machinery
Dialogue: 0,0:04:44.72,0:04:48.40,Default,,0000,0000,0000,,that converts the cacophony of vibrations \Nin the air around us
Dialogue: 0,0:04:48.40,0:04:51.54,Default,,0000,0000,0000,,into precisely tuned electrical impulses
Dialogue: 0,0:04:51.54,0:04:56.30,Default,,0000,0000,0000,,that distinguish claps, taps, \Nsighs, and flies.