A Computational View of the Cognitive Semantics of Spatial Prepositions

A COMPUTATIONAL VIEW OF THE COGNITIVE SEMANTICS OF SPATIAL PREPOSITIONS* Patrick Olivier Centre for Intelligent Systems University of Wales Aberystwyth Dyfed, SY23 3DB, UK Internet: plo~aber.ac.uk Abstract This paper outlines the linguistic semantic com- mitments underlying an application which au- tomatically constructs depictions of verbal spa- tial descriptions. Our approach draws on the ideational view of linguistic semantics developed by Ronald Langacker in his theory of Cognitive Grammar, and the conceptual representation of physical objects from the two-level semantics of Bierwisch and Lang. In particular the dimensions of the process of conventwnal imagery are used as a metric for the design of our own conceptual representation. INTRODUCTION An increased interest in ttle semantics of spatial language has accompanied the recent rise in popularity of cognitive linguistics (see [Rudzka-Ostyn1988]), yet computational ap- proaches are thin on the ground. This can in part be accounted for by the rather descriptive and unformalized nature of the theories devel- oped, but is more likely due to the adoption of an ideational view of linguistic meaning which, it seems, is an anathema to computational lin- guists. In this paper we take a serious, if infor- mal, look at Ronald Langacker's theory of Cogni- tive Grammar [Langacker1987], [Langacker1988a], [Langacker1988b], more specifically its commit- ment to conceptualization and the use of conven- tional imagery. The first section of this paper introduces the semantics of projective prepositions (eg. "in front of", "behind", "left of", "right of"), illustrating that these seemingly simple predicates are supris- ingly complex and ambiguous. In the light of this discovery the following sections consider Lan- gacker's view of linguistic meaning, and the design of a conceptual representation for spatial preposi- tions motivated by the consideration of the various *Thi~ research wa~ kindly funded by the Mat- sushita Electric Industrial Company Limited. Jun-ichi Tsujii Centre for Computational Linguistics University of ~anchester Institute of Science and Technology , Manchester, M60 1QD, UK Internet: tsujii~ccl.umist.ac.uk dimensions of conventional imagery. The repre- sentation has been implemented for English spa- tial descriptions and after demonstrating its utility for the automatic depiction of verbal descriptions, we finally contrast our approach against previous at tenapts. THE SEMANTICS OF PROJECTIVE PREPOSITIONS In this section we characterize the components of the spatial meaning of projective prepositions that have motivated our interest in cognitive linguis- tic approaches. Throughout, the decoding prob- lem, that is, generating adequate meanings for a locative expression in a particular situation, is our benchmark for representational adequacy. The spatial meaning Of a projective preposi- tional predication (eg. "the chair is in front of the desk") can include: a constraint on the proximity of the located (LO) (eg. "the chair") and refer- ence objects (RO) (eg. "the desk"); a directional constraint on the LO relative to the RO; and a relative orientation between the speaker, LO and RO. Constraints are of an intrinsically fuzzy na- ture such that different relative positions and ori- entations of the speaker, RO and LO satisfy the predication to different degrees, and combinations of constraints on …

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