I'm studying NLP and wrapping my head around the steps of passing through a Multi-Layer Perceptron. Since vectors are magnitude and direction in a space, I'm curious what the center of a word vector represents. In a very simple vector, my word might be 21, -5. Does 0,0 represent anything? If not, could it represent something after training a model?
If I understand correctly, a word that has never been seen before will be given a numerical identity and a vector of M dimensions. This vector then passes into the first layer, which has as many nodes as there are dimensions, so in this case M nodes. Through backpropagation the weights are changed so that similar words "group" together in vector space. (So that means the word vectors themselves are never modified from their initial random value, right?) Please correct me if I've made wrong assumptions here. I would just appreciate some insight.
You can think of word 'vectors', numerically, as just points. It's not really significant that they all 'start' at the origin ([0.0, 0.0, 0.0, ..., 0.0]).
The 'center' of any such vector is just its midpoint, which is also a vector of the same 'directionality' with half the magnitude. Often but not always, word-vectors are only compared in terms of raw-direction, not magnitude, via 'cosine similarity', which is essentially an angle-of-difference calculation that's oblvious to length/magnitude. (So, cosine_similarity(a, b) will be the same as cosine_similarity(a/2, b) or cosine_similarity(a, b*4), etc.) So this 'center'/half-length instance you've asked about is usually less meaningful, with word-vectors, than in other vector models. And in general, as long as you're using cosine-similarity as your main method of comparing vectors, moving them closer to the origin-point is irrelevant. So, in that framing, the origin point doesn't really have a distinct meaning.
Caveat with regard to magnitudes: the actual raw vectors created by word2vec training do in fact have a variety of magnitudes. Some have observed that these magnitudes sometimes correlate with interesting word differences – for example, highly polysemous words (with many alternate meanings) can often be lower-magnitude than words with one dominant meaning – as the need to "do something useful" in alternate contexts tugs the vector between extremes during training, leaving it more "in the middle". And while word-to-word comparisons usually ignore these magnitudes for the purely angular cosine-similarity, sometimes downstream uses, such as text classification, may do incrementally better keeping the raw magnitudes.
Caveat with regard to the origin point: At least one paper, "All-but-the-Top: Simple and Effective Postprocessing for Word Representations" by Mu, Bhat, & Viswanath, has observed that often the 'average' of all word-vectors isn't the origin-point, but significantly biased in one direction – which (in my stylized understanding) sort-of leaves the whole space imbalanced, in terms of whether it's using 'all angles' to represent contrasts-in-meaning. (Also, in my experiments, the extent of this imbalance seems a function of how many negative examples are used in negative-sampling.) They found that postprocessing the vectors to recenter them improved performance on some tasks, but I've not seen many other projects adopt this as a standard step. (They also suggest some other postprocessing transformations to essentially 'increase contrast in the most valuable dimensions'.)
Regarding your "IIUC", yes, words are given starting vectors - but these are random, and then constantly adjusted via backprop-nudges, repeatedly after trying every training example in turn, to make those 'input word' vectors ever-so-slightly better as inputs to the neural network that's trying to predict nearby 'target/center/output' words. Both the networks 'internal'/'hidden' weights are adjusted, and the input vectors themselves, which are essentially 'projection weights' – from a one-hot representation of a single vocabulary word, 'to' the M different internal hidden-layer nodes. That is, each 'word vector' is essentially a word-specific subset of the neural-networks' internal weights.