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Cell differentiation can turn neurons into everything from clocks that control circadian rhythms to photoreceptors that convert light into electrical-chemical impulses or decision makers that tally votes and decide courses of action. In the retina (often used as a case study because it can be directly and naturally stimulated), there are at least fifty different kinds of neurons specialized to different tasks, such as looking for motion, recognizing colors, detecting objects in low light, and measuring brightness and contrast. In the brain as a whole, there may be as many as 10,000 different kinds of neurons, each contributing to a different aspect of mental life.
"Sometimes guitar riffs get repeated over and over ("vamping," in the lingo of musicians), but generally there is a soloist proving variation that runs above that background, lest the song sound monotonous. Philip Glass's minimalist compositions (such as the soundtrack to 'Koyaanisqatsi') deviate from much of the classical music that preceded them, with much less obvious movement than, say, the Romantic-era compositions that his work seems to rebel against, yet his works, too, consist not only of extensive repetition but also of constant (though subtle) variation. Virtually every song you've ever heard consists of exactly that: themes that recur over and over, overlaid with variations."
To the extent that genomes can be thought of as compressed encodings of biological structures, they are spectacularly efficient. All the trillions of cells in the human body-not just the tens of billions in the brain-are guided in one way or another by the information contained in 30,000 or so genes. The best high-quality set of pictures of the body- the National Institutes of Health Visible Human Project, a series of high-resolution digital photos of slices taken from volunteer Joseph Paul Jernigan (deceased)-takes up about 60 gigabytes, enough (if left uncompressed) to fill about 100 CD-ROMs-and still not enough detail to capture individual cells. The genome, in contrast, contains only about 3 billion nucleotides, the equivalent (at two bits per nucleotide) of less than two-thirds of a gigabyte, or a single CD-ROM.
There are, of course, many reasons to think that brains operate mostly in parallel. Individual neurons are too slow to allow brains to operate in strict serial von Neumann fashion, and ample data suggest that in any given laboratory task (and by extension, any real-world situation) many different parts of the brain are engaged simultaneously.
To mention a colorful example, the nineteenth-century German scientist Karl Vogt once wrote that “thoughts stand in the same relation to the brain as gall does to the liver or urine to the kidneys.” When he expressed this idea in public, a philosopher interjected that the longer one listens to Professor Vogt, the more one tends to believe him. Clearly, more sophisticated ideas and models are in demand.
"Repetition sometimes works in poetry, but rarely in prose. The musical provocateur John Cage once wrote a lecture in which a single page was repeated fourteen times, with the refrain "If anybody is sleep let him go to sleep" (Cage, 1961). Midway through, the artist Jean Reynal stood up and screamed, "John, I dearly love you, but I can't bear another minute.
The CD-ROM's worth of information in the genome really wouldn't be enough to paint a bitmapped picture of an embryo, but it is enough to describe a process for building one. An artist who only wants to paint a picture that looks like a kind of tree has much less to remember than an artist who wants to paint a particular Ponderosa Pine from memory; in a similar way, if some alien's genome had to encode every cell in a body, it would need much more information (many more nucleotides) than our genomes do, because ours specify a general way to build a creature rather than an exact picture of every detail of the finished product. Our genomes are lossy because they specify methods rather than pictures, but it is precisely that lossiness that allows them to so efficiently supervise the construction of complex biological structure.
"To take one example, even a brief exposure to light in a newborn kitten, rat, or monkey can launch a complex cascade of gene expression. The light activates photoreceptors-which send signals-which trigger a pathway-which leads to the expression of neural growth factors and a set of genes known as "immediate early genes" or "early response genes"-each of which, in turn, triggers the expression of many more genes. One study of cichlid fish suggests that a change in social status (from submissive to dominant) is tied to changes in the expression levels of at least fifty-nine different genes-a phenomenon not entirely unrelated to the testosterone rush that Joe-six-pack gets when the home team wins."
"In a 1957 experiment that helped launch the modern study of language acquisition, the late Roger Brown showed that children know that if you say, "Can you see a sib?" you probably have in mind an action or a process. No other mammal seems to be equipped to use such clues for word learning.
Even more dramatically, no other species seems to be able to make much of word order. The difference between the sentence "Dog bites man" and the sentence "Man bites dog" is largely lost on our nonhuman cousins. There is a bit of evidence that Kanzi can pay attention to word order to some tiny extent, but certainly not in anything like as rich a fashion as a three-year-old human child."