Your ears are not simply “eyes for sound.” Sound contains quite different information about the world than does light. Light tends to be ongoing, whereas sound occurs when things change: when they vibrate, collide, move, break, explode! Audition is the sense of events rather than scenes. The auditory system thus processes auditory information quite differently from how the visual system processes visual information: whereas the dominant role of sight is telling where things are, the dominant role of hearing is telling when things happen [[Hack #44]].
Hearing is the first sense we develop in the womb. The regions of the brain that deal with hearing are the first to finish the developmental process called myelination, in which the connecting “wires” of neurons are finished off with fatty sheaths that insulate the neurons, speeding up their electrical signals. In contrast, the visual system doesn’t complete this last step of myelination until a few months after birth.
Hearing is the last sense to go as we lose consciousness (when you’re dropping off to sleep, your other senses drop away and sounds seem to swell up) and the first to return when we make it back to consciousness.
We’re visual creatures, but we constantly use sound to keep a 360° check on the world around us. It’s a sense that supplements our visual experience—a movie without a music score is strangely dull, but we hardly notice the sound track normally. We’ll look at how we hear some features of that sound track, stereo sound [[Hack #45]], and pitch [[Hack #46]].
And of course, audition is the sense of language. Hacks in this chapter show how we don’t just hear a physical sound but can hear the meanings they convey [[Hack #49]], even on the threshold of perception [[Hack #48]]. Just as with vision, what we experience isn’t quite what is physically there. Instead, we experience a useful aural construction put together by our brains.
We’ll finish up by investigating three aspects of understanding language: of the hidden sound symbolism in words [[Hack #50]], of how we break sentences into phrases, [[Hack #51]], and of how you know excalty waht tehse wrdos maen [[Hack #52]].
Audition is a specialized sense for gathering information from the fourth dimension.
If vision lets you see where something is, hearing tells you when it is. The time resolution of audition is way above that of vision. A cinema screen of 24 images a second looks like a constant display, rather than 24 brief images. A selection of 24 clicks a second sounds like a bunch of clicks—they don’t blur into a constant tone.
Listen to these three sound files:
24 clicks per second, for 3 seconds ( http://www.mindhacks.com/book/44/24Hz.mp3 ; MP3)
48 clicks per second, for 3 seconds ( http://www.mindhacks.com/book/44/48Hz.mp3 ; MP3)
96 clicks per second, for 3 seconds ( http://www.mindhacks.com//book/44/96Hz.mp3 ; MP3)
At a frequency of 24 frames per second, film blurs into a continuous image. At 24 clicks per second, you perceive the sound as separate clicks. At four times that rate, you still hear the sound as discontinuous. You may not be able to count the clicks, but you know that the sound is made up of lots of little clicks, not one continuous hum. Auditory “flicker” persists up to higher frequencies than visual flicker before it is integrated to a continuous percept.
Specialization for timing is evident in many parts of the auditory system. However, it is the design of the sound receptor device (the ears) that is most crucial. In the eye, light is converted to neural impulses by a slow chemical process in the receptor cells. However, in the ear, sound is converted to neural impulses by a fast mechanical system.
Sound vibrations travel down the ear canal and are transmitted by the tiny ear bones (ossicles) to the snail-shaped cochlea, a piece of precision engineering in the inner ear. The cochlea performs a frequency analysis of incoming sound, not with neural circuitry, but mechanically. It contains a curled wedge, called the basilar membrane, which, due to its tapering thickness, vibrates to different frequencies at different points along its length. It is here, at the basilar membrane, that sound information is converted into neural signals, and even that is done mechanistically rather than chemically. Along the basilar membrane are receptors, called hair cells. These are covered in tiny hairs, which are in turn linked by tiny filaments. When the hairs are pushed by a motion of the basilar membrane, the tiny filaments are stretched, and like ropes pulling open doors, the filaments open many minute channels on the hairs. Charged atoms in the surrounding fluid rush into the hair cells, and thus sound becomes electricity, the native language of the brain. Even movements as small as those on the atomic scale are enough to trigger a response. And for low frequency sounds (up to 1500 cycles per second), each cycle of the sound can trigger a separate group of electrical pulses. For higher frequencies, individual cycles are not coded, just the average intensity of the cycles. The cells that receive auditory timing input in the brain can fire at a faster rate than any other neurons, up to 500 times a second.
This arrangement means that the auditory system is finely attuned to frequency and timing information in sound waves. Sounds as low as 20 Hz (1 Hz is one beat per second), and as high as 20,000 Hz can be represented. The timing sensitivity is exquisite; we can detect periods of silence in sounds of as little as 1 millisecond (thousandths of a second). Compare this with your visual system, which requires exposure to an image for around 30 milliseconds to report an input to consciousness. Furthermore, thanks to the specialized systems in the ear and in the brain, timing between the ears is even more exquisite. If sound arrives at one ear as little as 20 microseconds (millionths of a second) before arriving at the other, this tiny difference can be detected [[Hack #45]]. For perspective, an eye blink is in the order of 100,000 microseconds, 5000 times slower.
Although vision dominates many other senses in situations of conflicting information [[Hack #53]], given the sensitivity of our ears, it is not surprising that audition dominates over vision for determing the timing of events
We use this sensitivity to timing in many ways—notably in enjoying music and using the onset of new sounds to warn us that something has changed somewhere.
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