Monday, May 25, 2015

Thought Vector + Deep Learning = Progress in NLP

Richard Socher, an artificial intelligence scientist at Stanford University, recently developed a program called NaSent that he taught to recognise human sentiment by training it on 12,000 sentences taken from the film review website Rotten Tomatoes.

Part of the initial motivation for developing “thought vectors” was to improve translation software, such as Google Translate, which currently uses dictionaries to translate individual words and searches through previously translated documents to find typical translations for phrases. Although these methods often provide the rough meaning, they are also prone to delivering nonsense and dubious grammar. Thought vectors, Hinton explained, work at a higher level by extracting something closer to actual meaning.
The technique works by ascribing each word a set of numbers (or vector) that define its position in a theoretical “meaning space” or cloud. A sentence can be looked at as a path between these words, which can in turn be distilled down to its own set of numbers, or thought vector.

The “thought” serves as a the bridge between the two languages because it can be transferred into the French version of the meaning space and decoded back into a new path between words.

The key is working out which numbers to assign each word in a language – this is where deep learning comes in. Initially the positions of words within each cloud are ordered at random and the translation algorithm begins training on a dataset of translated sentences.

At first the translations it produces are nonsense, but a feedback loop provides an error signal that allows the position of each word to be refined until eventually the positions of words in the cloud captures the way humans use them – effectively a map of their meanings.

Hinton said that the idea that language can be deconstructed with almost mathematical precision is surprising, but true. “If you take the vector for Paris and subtract the vector for France and add Italy, you get Rome,” he said. “It’s quite remarkable.”


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