Becoming Recursive: Toward a Computational Neuroscience Account of Recursion in Language and Thought

After remaining more or less in the background for several decades, the issue of recursion in human languages has returned to the fore. The current, vigorous debate is about not just recursion, but the very idea of linguistic universals as a coherent scientific notion, and the nature of language itself. The prevailing view is that recursion – the ability to produce an unbounded number of well-formed sentences through the combinatorial operations of syntax – is a fundamental property of the “faculty of language in the narrow sense” (FLN), i.e., that recursion is what differentiates human language from other forms of animal communication (Hauser, Chomsky, and Fitch 2002). As an explicit challenge this view, Everett (2005) has documented the syntax of Pirahã, a language that appears to have no embedded clauses, recursive possessives, or other syntactic structures suggesting this capacity. Everett offers Pirahã as a fatal counterexample to the universality of recursion, and questions the utility of searching for other sorts of linguistic universals as well. Although this debate has involved linguistics, and sometimes politics, we suggest that it might also be fruitfully informed by neuroscience. In particular, we will present a model of event semantics based on neurally plausible role/filler bindings, and describe how recursion, like other syntactic processes, can be viewed as the by-product of “unpacking” and serializing the contents of these representations. A rough outline of our argument: we start by enumerating some basic assumptions or ground rules about what any model of human language and cognition should look like. We then provide a review of simple vector arithmetic for the benefit of readers unfamiliar with this branch of mathematics, and we show how high-dimensional vectors can be used as a plausible model of role/filler semantics and other mental representations without violating any of our stated assumptions. Next we sketch the rudiments of a model of language that treats grammar as a process of transduction between these mental representations and sequences of symbols. We present a brief account of how this language model can be related to traditional syntactic categories like agent and theme, and conclude by setting the model in the context of current research on the sources of linguistic universals.