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INTRODUCTION

From the beginning of the computer era, futurists have dreamed of the conversational computer—a machine that we could engage in spoken natural language conversation. For instance, Turing's famous "test" of computational intelligence imagined a computer that could conduct such a fluent English conversation that people could not distinguish it from a human. However, despite prolonged research and many notable scientific and technological achievements, until recently there have been few human-computer dialogues, none spoken. This situation has begun to change, as steady progress in speech recognition and natural language processing technologies, supported by dramatic advances in computer hardware, has made possible laboratory prototype systems with which one can engage in simple question-answer dialogues. Although far from human-level conversation, this initial capability is generating considerable interest and optimism for the future of human-computer interaction using voice.

This paper aims to identify applications for which spoken interaction may be advantageous, to situate voice with respect to alternative and complementary modalities of human-computer interaction, and to discuss obstacles that exist to the successful deployment of spoken language systems because of the nature of spoken language interaction.

Two general sorts of speech input technology are considered. First, we survey a number of existing applications of speech recognition technologies, for which the system identifies the words spoken, but need not understand the meaning of what is being said. Second, we concentrate on applications that will require a more complete understanding of the speaker's intended meaning, examining future spoken dialogue systems. Finally, we discuss how such speech understanding will play a role in future human-computer interactions, particularly those involving the coordinated use of multiple communication modalities, such as graphics, handwriting, and gesturing. It is argued that progress has been impeded by the lack of adequate scientific knowledge about human spoken interactions, especially with computers. Such a knowledge base is essential to the development of well-founded human-interface guidelines that can assist system designers in producing successful applications incorporating spoken interaction. Given recent technological developments, the field is now in a position to systematically expand that knowledge base.