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DNA Blueprints Guide The Construction Of Specific Human Structures
Chad Mirkin discusses using DNA to build a three-dimensional structure out of gold, likening the process to building a house. Starting with basic materials such as bricks, wood, siding, stone and shingles, a construction team can build many different types of houses out of the same building blocks.
The article includes an audio recording of the full interview. Photo courtesy of the UCSD School of Medicine.
| The Brain's Hierarchy Organizes Artifacts, Like A Dictionary |
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| Medicine - Soc & Psych | |||
| TS-Si News Service | |||
| Wednesday, 30 April 2008 17:00 | |||
 Oxford English Dictionary (OED). The Oxford University Press (OUP) publishes the OED, a comprehensive dictionary of the English language. The OED, as of 30 November 2005, contained about 301,100 main entries. The dictionary's latest, complete print edition (Second Edition, 1989) was printed in 20 volumes, comprising 291,500 entries in 21,730 pages.
WordNet. A combination of dictionary and thesaurus that supports automatic text analysis and artificial intelligence applications. It is a semantic lexicon that groups English words into sets of synonyms called synsets, provides short, general definitions, and records the various semantic relations between these synonym sets.
WordNet was created and is being maintained at the Cognitive Science Laboratory of Princeton University. The database and software tools have been released under a BSD style license and can be downloaded and freely used. But how can dictionaries be useful when everyone knows they can turn into massively circular goose chases? The definition of A refers to B, but the definition of B refers to C, and so on. This goes on until the definition of n closes the loop by referring to A. Careful reading can give a near approximation of what you desire, but humans generally seek certainty in their lives.
Economically organized hierarchies in WordNet and the Oxford English Dictionary. Mark A. Changizi. Journal of Cognitive Systems Research 9(3): 214-228. (June 2008). [ Download PDF ] Perhaps the general acceptance of a dictionary is a clue to how the human brain reconciles these issues. Mark Changizi, assistant professor of cognitive science at Rensselaer Polytechnic Institute (RPI), is well aware of the seemingly circular nature of dictionaries.
 The latest edition of the Oxford English Dictionary (OED) boasts 22,000 pages of definitions. While that seems far from succinct, new research suggests the reference manual is meticulously organized to be as concise as possible. It is a format that mirrors the way our brains make sense of and
categorize the countless words in our vast vocabulary.
But, he says, new research “… suggests that all words are grounded in a small set of atomic words — and it’s likely that the dictionary’s large-scale organization has been driven over time by the way humans mentally systematize words and their meanings.” Changizi’s research appears in the Journal of Cognitive Systems Research.
Changizi finds dictionaries do not possess many circularities. Instead, dictionaries are highly hierarchical, with words being defined via words that are simpler, and lower in the hierarchy. At the bottom are a small number of "most fundamental words" no longer having definitions using simpler terms — that's where most of the circularities lie.
Changizi shows how dictionaries are built like an inverted pyramid. The most complex words (e.g., “albacore” and “antelope”) sit at the top and are defined by words that are more basic, and thus lower on the pyramid. Eventually all words are linked to a small number of words — called “atomic words,” (such as “act” and “group”) — that are so fundamental they cannot be defined by simpler terms. The number of levels of definition it takes to get from a word to an atomic word is called the “hierarchical level” of the word.
The research findings indicate that the dictionaries we use every day utilize approximately the optimal number of hierarchical levels. They provide a visual roadmap of how the lexicon itself has culturally evolved over tens of thousands of years to help lower the overall “brain space” required to encode it. Changizi says many other human inventions — such as writing and other human visual signs — have been designed either explicitly or via cultural selection over time so as to minimize their demands on the brain.
By conducting a series of calculations based on the estimation that the most complex words in the dictionary total around 100,000 different terms, and that the number of atomic words range from 10 to 60, Changizi was able to devise three signature features present in the most efficient dictionaries — as well as in their human counterpart, the brain.
Furthermore, there's more than one way to build a hierarchical dictionary. One could use the most fundamental words to define all the other words in the dictionary, so that there would be just two hierarchical levels: the small set of fundamental (or atomic) words, and the set of everything else.
Alternatively, dictionaries could use the most fundamental words to define an intermediate level of words, and in turn use these words to define the rest. That would make three levels, and, clearly, greater numbers of levels are possible.
Mark Changizi's main discovery is to show that having just the right number of hierarchical levels can greatly reduce the overall size of the dictionary. A dictionary with just two hierarchical levels, for example, would have to be more than three times larger than an optimal one that uses around seven levels. And, most importantly, he provides evidence that actual dictionaries have approximately the optimal number of hierarchical levels. He discovered that the total number of words across all the definitions in the dictionary (and thus the size of the dictionary) changes in relation to the total number of hierarchical levels present. Optimal dictionaries should have approximately seven hierarchical levels, according to Changizi. “The presence of around seven levels of definition will reduce the overall size of the dictionary, so that it is about 30 percent of the size it would be if there were only two hierarchical levels,” Changizi said.
Additionally, users will find that there are progressively more words at each successive hierarchical level, and that each hierarchical level contributes mostly to the definitions of the words just one level above their own, according to Changizi, who put his three predictions to the test by studying actual dictionaries.
The Oxford English Dictionary (OED) and WordNet® — a large, online lexical database of English, developed at Princeton University — were found to possess all three signatures of an economically organized dictionary, and thus were organized in such a way as to economize the amount of dictionary space required to define the lexicon, according to Changizi.
What it means is that language has culturally evolved over the centuries and millenia not only to have the words we need, but also to have an overall organization. “Somehow, over centuries, these revered reference books have achieved near-optimal organization,” Changizi said. Optimality in this case is determined in terms of how words get their meanings from other words —that helps minimize the overall size of the dictionary. It simultaneously helps us efficiently encode the lexicon in our heads.
“That optimality can likely be attributed to the fact that cultural selection pressures over time have shaped the organization of our lexicon so as to require as little mental space and energy as possible.”
Changizi believes his research has potential applications in the study of childhood learning, where scientists could analyze how students learn vocabulary words and possibly develop ways to optimize that learning process.
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