The development of gesture and speech as an integrated system.

Children, even at the one-word stage of language development, spontaneously produce gestures along with their speech, just as adults do. Although there appears to be a brief period prior to the onset of two-word speech during which gesture and speech do not form a well-integrated system, the ability to coordinate gesture and speech to convey a single message--and to "read" others' gestures with their speech to comprehend a message--develops early and is maintained throughout life. Gesture-speech combinations deliver a coherent message to the listener despite the fact that they consist of two different modalities of expression. According to McNeill (1992; Chapter One), this coherence is possible because gesture and speech share a common cognitive representation; that is, before the communication unfolds, gesture and speech are part of a single idea. As expression proceeds the message is parsed, with most information channeled into speech but some information channeled into gesture. Speech conveys information in a segmented, combinatorial format, whereas gesture conveys information in a global, mimetic format (see Goldin-Meadow, McNeill, and Singleton, 1996). Thus gesture and speech need not, and in fact often do not, convey the same information within a single utterance. Because gesture and speech form a unified system, mismatches between them can be a source of insight into the cognitive state of the speaker. And, indeed, it turns out that in both the young, one-word speaker and the older child (and possibly adults as well; Perry and Elder, 1996), a difference--or mismatch--between the information conveyed in gesture and the information conveyed in speech can signal readiness for cognitive growth. Whether the actual production of gesture-speech mismatches contributes to cognitive growth is an open question. That is, does the act of expressing two different pieces of information across modalities but within a single communicative act improve a child's ability to transpose that knowledge to a new level and thus express those pieces of information within a single modality? More work is needed to investigate whether the act of producing gesture-speech mismatches itself facilitates transition. Even if it turns out that the production of gesture-speech mismatches has little role to play in facilitating cognitive change, mismatch remains a reliable marker of the speaker's potential for cognitive growth. As such, an understanding of the relationship between gesture and speech may prove useful in clinical settings. For example, there is some evidence that children with delayed onset of two-word speech fall naturally into two groups: children who eventually achieve two-word speech, albeit later than the norm (that is, late bloomers), and children who continue to have serious difficulties with spoken language and may never be able to combine words into a single string (Feldman, Holland, Kemp, and Janosky, 1992; Thal, Tobias, and Morrison, 1991). Observation of combinations in which gesture and speech convey different information may prove a useful clinical tool for distinguishing, at a relatively young age, children who will be late bloomers from those who will have great difficulty mastering spoken language without intervention (see Stare, 1996, for preliminary evidence that the relationship between gesture and speech in children with unilateral brain damage correlates with early versus late onset of two-word combinations. In sum, for both speakers and listeners, gesture and speech are two aspects of a single process, with each modality contributing its own unique level of representation. Gesture conveys information in the global, imagistic form for which it is well suited, and speech conveys information in the segmented, combinatorial fashion that characterizes linguistic structures. The total representation of any message is therefore a synthesis of the analog gestural mode and the discrete speech mode. (ABSTRACT TRUNCATED)