Immediate-head parsing for language models

We present two language models based upon an "immediate-head" parser --- our name for a parser that conditions all events below a constituent c upon the head of c. While all of the most accurate statistical parsers are of the immediate-head variety, no previous grammatical language model uses this technology. The perplexity for both of these models significantly improve upon the trigram model base-line as well as the best previous grammar-based language model. For the better of our two models these improvements are 24% and 14% respectively. We also suggest that improvement of the underlying parser should significantly improve the model's perplexity and that even in the near term there is a lot of potential for improvement in immediate-head language models.