The Science of Discworld II

addition, there are also long-range connections between hypercolumns. These are excitatory, and their effect is to bias neighbouring hypercolumns to perceive the natural continuation of that edge, even if the signal they receive is too weak or ambiguous for them to come to that conclusion unaided.
    This bias can be overcome by a sufficiently strong indication that there is an edge pointing in a different direction; but if the line gets faint, or part of it is missing, the bias automatically makes the brain respond as if the line was continuous. So the brain doesn’t ‘fill in’ the gaps: it is set up not to notice that there are gaps. That’s just one layer of the visual cortex, and it uses a rather simple trick: extrapolation. Wehave little idea, as yet, of the inspired guesswork that goes on in deeper layers of the brain, but we can be sure that it’s even more clever, because it produces such a vivid sensation of a complete image.
    What about hearing? How does that relate to sound? The standard lie-to-children about vision is that the cornea and lens make a picture on the retina, and that allegedly explains vision. Similarly, the corresponding lie-to-children about hearing centres on a part of the ear called the cochlea, whose structure allegedly explains how you analyse sound into different notes. In cross-section, the cochlea looks like a sliced snail-shell, and according to the lie-to-children, there are hair-cells all the way down the spiral attached to a tuned membrane. So different parts of the cochlea vibrate at different frequencies, and the brain detects which frequency – which musical note – it is receiving, by being told which part of the membrane is vibrating. In support of this explanation, we are told a rather nice story about boiler-makers, whose hearing was often damaged by the noise in the factories where they worked. Supposedly, they could hear all frequencies except ones near the frequency that was most common in making boilers. So just one place on their cochlea was burnt out, and the rest worked OK. This proved, of course, that the ‘place’ theory of hearing was correct.
    Actually, this story tells you only how the ear can discriminate notes, not how you hear the noise. To explain that, it is usual to invoke the auditory nerve, which connects the cochlea to the brain. However, there are as many connections, or more, that go in the other direction, from brain to cochlea. You have to tell your ear what to hear .
    Now that we can actually look at what the cochlea does when it’s hearing, we find not one place vibrating for each frequency, but more like twenty. And these places move as you flex your outer ear. The cochlea is phase-sensitive, it can discriminate the kind of difference that makes an ‘ooh’ sound different from an ‘eeh’ at the same frequency. This is the kind of change to the sound that you make when you change the shape of your mouth as you speak. And surprise, surprise, that’s just the difference that the cochlea – after your outer ear and your own particular auditory canal, and your own particular eardrum and those three little bones – can best discriminate. A recording from someone else’s eardrum, played back up against yours, makeslittle sense. You have learned your own ears. But you have taught them, too.
    There are about seventy basic sounds, called phonemes, that Homo sapiens uses in speech. Up to about six months old, all human babies can discriminate all of these, and an electrode on the auditory nerve gives different patterns of electrical activity for each. At about six to nine months old, we start talking scribble, and it very soon becomes English scribble or Japanese scribble. By a year old the Japanese ear cannot distinguish ‘l’ from ‘r’, because both phonemes send the same message from cochlea to brain. English babies can’t discriminate the different clicks of the !Kung San, nor the differences between the distinct ‘r’s in French. So our sense organs do not show us the real world. They stimulate our brains to produce, to invent if you like, an internal world made of the counters, the Lego™ set, that each of us has built up as we mature.
    Such apparently straightforward abilities as vision and hearing are far more complicated than we usually imagine. Our brains are much more than just passive recipients. An awful lot is going on inside our heads,