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Toward a connectionist model of recursion in human linguistic performance

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Nội dung chi tiết: Toward a connectionist model of recursion in human linguistic performance

Toward a connectionist model of recursion in human linguistic performance

Toward a connectionist model of recursion in human linguistic performancereserved.Toward a Connectionist Model of Recursion in Human Linguistic PerformanceMorten II. ChristiansenSouthern Illinois UniversityNick ChaterUniver

sity of WarwickNaturally occurring speech contains only a limited amount of complex recursive structure, and this is reflected in the empirically docu Toward a connectionist model of recursion in human linguistic performance

mented difficulties that people experience when processing such structures. We present a connectionist model of human performance in processing recurs

Toward a connectionist model of recursion in human linguistic performance

ive language structures. The model is trained on simple artificial languages. We find that the qualitative performance profile of the model matches hu

Toward a connectionist model of recursion in human linguistic performances and right-branching recursive constructions. We analyze how these differences in performance are reflected in the internal representations of the mo

del by performing discriminant analyses on these representations both before and after training. Furthermore, we show how a network trained to process Toward a connectionist model of recursion in human linguistic performance

recursive structures can also generate such structures in a probabilistic fashion. This work suggests a novel explanation of people's limited recursi

Toward a connectionist model of recursion in human linguistic performance

ve performance, without assuming the existence of a mentally represented competence grammar allowing unbounded recursion.I. INTRODUCTIONNatural langua

Toward a connectionist model of recursion in human linguistic performanceple are only able to deal easily with relatively simple recursive structures. Thus, for example, a doubly center-embedded sentence like (1) below is e

xtremely difficult to understand.(I)The mouse that the cat that the day chased bit ran away.Direct all correspondence to: Morten II. Christiansen. Dep Toward a connectionist model of recursion in human linguistic performance

artment of Psychology. Southern Illinois University.Carbondale. IL 62901-6502; E-Mail: niorten@siu.edu.1571 58CHRISTIANSEN AND CHATERIn this paper, we

Toward a connectionist model of recursion in human linguistic performance

present a connectionist network which models the limited human abilities to process and generate recursive constructions. The “quasi-recursive” natur

Toward a connectionist model of recursion in human linguistic performanceatural language originates not from the project of trying to understand human linguistic performance which is the focus of this paper, but from the ve

ry different enterprise of specifying a “competence grammar" a set of rules and or principles which specify the legal strings of a language. It is sta Toward a connectionist model of recursion in human linguistic performance

ndardly assumed that, if the competence grammar allows a recursive construction to apply at all, it can apply arbitrarily many times. Thus, if (2) is

Toward a connectionist model of recursion in human linguistic performance

sanctioned by a recursive analysis with one level of recursion, then the grammar must thereby also sanction (1) with two levels of recursion and (3) w

Toward a connectionist model of recursion in human linguistic performanceway.Thus, the very idea that natural language is recursive requires a broadening of the notion of which sentences are in the language, to sentences li

ke (3) which would presumably never be uttered or understood. In order to resolve the difference between language so construed and the language that h Toward a connectionist model of recursion in human linguistic performance

umans are able to produce and comprehend, a distinction is typically made between linguistic competence and human performance. C ompetence in this con

Toward a connectionist model of recursion in human linguistic performance

text refers to a speaker hearer’s knowledge of the language, and is the subject of linguistic inquiry. In contrast, psycholinguists study performance—

Toward a connectionist model of recursion in human linguistic performanceuse of that knowledge. Il is here lhal "performance factors”, such as memory limitations, can be invoked to show that some sentences, while consistent

with linguistic competence, will never actually be said, or understood, rhe competence performance distinction is also embodied in many symbolic mode Toward a connectionist model of recursion in human linguistic performance

ls of language processing, such as CC-READER (.lust & Carpenter, 1992). In this model, grammatical competence consists of a set of recursive productio

Toward a connectionist model of recursion in human linguistic performance

n rules w hich are applied to produce slate changes in a separate working memory. By imposing constraints on the capacity of the working memory system

Toward a connectionist model of recursion in human linguistic performancen alternative account of people’s limited ability to do recursion, without assuming an internally represented grammar which allows unbounded recursion

i.c., without invoking the competencc/performance distinction.In light of this discussion, it is clear that, from the point of view of modeling psych Toward a connectionist model of recursion in human linguistic performance

ological processes, wre need not take the purported unbounded recursive structure of natural language as axiomatic. Nor need we take for granted the s

Toward a connectionist model of recursion in human linguistic performance

uggestion that a speaker hearer's knowledge of language captures such infinite recursive structure. Rather, the view’ that “unspeakable” sentences whi

Toward a connectionist model of recursion in human linguistic performancew' dominant in linguistics and many areas of the psychology of language. The challenge for a computational model such as the coimectionist model we pr

opose isA CONNECTIONIST MODEL OF RECURSION1 59to account for those aspects of human comprehension production performance which are suggestive of the s Toward a connectionist model of recursion in human linguistic performance

tandard recursive picture. If this can be done without making the assumption that the language processor really implements recursion, or that arbitrar

Toward a connectionist model of recursion in human linguistic performance

ily complex recursive structures arc really sentences of the language, then it presents an alternative to adopting this assumption. Therefore, in asse

Toward a connectionist model of recursion in human linguistic performancedle recursive structures: we need not require that conncclionisl systems be able to handle recursion in full generality.In this paper, we shall consid

er the phenomenon of natural language recursion in a ‘pure* and highly simplified form. Specifically, we train connectionist networks on small artific Toward a connectionist model of recursion in human linguistic performance

ial languages, which exhibit the different types of recursive structure found in natural language. We do tills in order to address directly the classi

Toward a connectionist model of recursion in human linguistic performance

c arguments by Chomsky (1957) that recursion in natural language in principle rules out associative and finite state models of language processing. In

Toward a connectionist model of recursion in human linguistic performancermits us to address the in principle viability of comiectionist networks in handling recursion, in much the same way as simple artificial languages ha

ve been used, for example, to assess rhe feasibility of symbolic parameter-setting approaches to the learning of linguistic structure (Gibson & Wexler Toward a connectionist model of recursion in human linguistic performance

. 1994; Niyogi & Berwick. 199Ố).The structure of this paper is as follows. We begin by distinguishing varieties of recursion in natural language, cons

Toward a connectionist model of recursion in human linguistic performance

idering the three kinds of recursion discussed in Chomsky (1957). We then summarize past connectionist research dealing with natural language recursio

Toward a connectionist model of recursion in human linguistic performanceations using conncclionisl networks trained on these languages, rhe results suggest that the networks are able to handle recursion to a degree compara

ble with humans. We close by drawing conclusions for the prospects of conncelionist models of language processing.II. VARIETIES OF RECURSIONChomsky (1 Toward a connectionist model of recursion in human linguistic performance

957) proposed that a recursive generative grammar consists of a set of phrase structure rules, complemented by a set of transformational rules (we sha

Toward a connectionist model of recursion in human linguistic performance

ll not consider transformational rules further below). Phrase structure rules have the form A —> BC, with the interpretation that the symbol A can be

Toward a connectionist model of recursion in human linguistic performancelf (c.g., A —> /?/!). rhe new symbol can then itself be replaced by a further application of the recursive rule, and so on. Recursion can also arise t

hrough the application of a recursive set of rules, none of which need individually be recursive. When such rules are used successively to expand a pa Toward a connectionist model of recursion in human linguistic performance

rticular symbol, the original symbol may eventually be derived. A recursive construction in a natural or artificial language is one that is modeled us

Toward a connectionist model of recursion in human linguistic performance

ing recursive rules: a language has recursive structure if it contains such constructions.160CHRISTIANSEN AND CHATERs --> NP VPNP --> N (comp S)VP -4