Tuesday, December 3, 2013

Distributed Representations

Distributed vector representations are a set of techniques which take a domain (usually, words) and embed it into a linear space (representing each word as a large vector of numbers). Useful tasks can then be represented as manipulations of these embedded representations. The embedding can be created in a variety of ways; often, it is learned by optimizing task performance. SENNA demonstrated that representations learned for one task are often useful for others.

There are so many interesting advances being made in distributed vector representations, it seems that a nice toolset is emerging which will soon be considered a basic part of machine intelligence.

Google's word2vec assigns distributed vector representations to individual words and a few short phrases. These representations have been shown to give intuitively reasonable results on analogy tasks with simple vector math: king - man + woman is approximately equal to the vector for queen, for example. This is despite not being explicitly optimized for that task, again showing that these representations tend to be useful for a wide range of tasks.

Similar approaches have aided machine translation tasks by turning word translation into a linear transform from one vector space to another.

One limitation of this approach is that we cannot do much to represent sentences. Sequences of words can be given somewhat useful representations by adding together the individual word representations, but this approach is limited.

Socher's RNN learns a matrix transform to compose two elements together and give them a score, which is then used for greedy parsing by composing together the highest-scoring items, with great success. This gives us useful vector representations for phrases and sentences.

Another approach which has been suggested is circular convolution. This combines vectors in a way which captures ordering information, unlike addition or multiplication. Impressively, the technique has solved Raven progressive matrix problems:


Then there's a project, COMPOSES, which seeks to create a language representation in which nouns get vector representations and other parts of speech get matrix representations (and possibly tensor representations?).


I haven't looked into the details fully, but conceptually it makes sense: the parts of speech which intuitively represent modifiers are linear functions, while the parts of speech which are intuitively static objects are getting operated on by these functions.

The following paper gives a related approach:


Here, everything is represented as a matrix of the same size. Representing the objects as functions is somewhat limiting, but the uniform representation makes it easy to jump to higher-level functions (modifiers on modifiers) without adding anything. This seems to have the potential to enable a surprisingly wide range of reasoning capabilities, given the narrow representation.

As the authors of that last paper mention, the approach can only support reasoning of a "memorized" sort. There is no mechanism which would allow chained logical inferences to answer questions. This seems like a good characterization of the general limitations of the broader set of techniques. The distributed representation of a word, phrase, image, or other object is a static encoding which represents, in some sense, a classification of the object into a fuzzy categorization system we've learned. How can we push the boundary here, allowing for a more complex reasoning? Can these vector representations be integrated into a more generally capable probabilistic logic system?

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