Speech Production From a Developmental Perspective

Abstract

Purpose Current approaches to speech production aim to explain adult behavior and so make assumptions that, when taken to their logical conclusion, fail to adequately account for development. This failure is problematic if adult behavior can be understood to emerge from the developmental process. This problem motivates the proposal of a developmentally sensitive theory of speech production. The working hypothesis, which structures the theory, is that feedforward representations and processes mature earlier than central feedback control processes in speech production. Method Theoretical assumptions that underpin the 2 major approaches to adult speech production are reviewed. Strengths and weaknesses are evaluated with respect to developmental patterns. A developmental approach is then pursued. The strengths of existing theories are borrowed, and the ideas are resynthesized under the working hypothesis. The speech production process is then reimagined in developmental stages, with each stage building on the previous one. Conclusion The resulting theory proposes that speech production relies on conceptually linked representations that are information-reduced holistic perceptual and motoric forms, constituting the phonological aspect of a system that is acquired with the lexicon. These forms are referred to as exemplars and schemas, respectively. When a particular exemplar and schema are activated with the selection of a particular lexical concept, their forms are used to define unique trajectories through an endogenous perceptual-motor space that guides implementation. This space is not linguistic, reflecting its origin in the prespeech period. Central feedback control over production emerges with failures in communication and the development of a self-concept.

Figure 1.

The ecological dynamics and information-processing approaches to speech production both assume three major levels of analysis: a phonological level where abstract form representations are associated with conceptual meaning, a speech plan level where abstract forms are elaborated for implementation, and an implementation level where articulatory action is formulated and adjusted in real time to achieve the plan. The two approaches otherwise adopt very different fundamental assumptions, resulting in different theories of representation, sequencing, and control. In particular, the ecological dynamics approach emphasizes speech as action and assumes gestalt articulatory representations, emergent sequential structure, and self-organized articulation. In contrast, the information-processing approach emphasizes the importance of discrete elements and assumes executive control over sequencing and implementation, thus promoting a strong role for perception in production while assuming that the two processes are distinct. Solid lines with arrows represent feedforward processes; dotted lines with arrows represent feedback processes.

Figure 2.

The onset of speech coincides with attempts to produce specific meanings (i.e., concepts) associated with perceptual word forms learned from the ambient language (left). Specifically, infants engage their perceptual–motor map to derive a best motoric approximation of the exogenous perceptual form or “perceptual exemplar.” The shape of the approximation will depend on how the map has been warped through vocal exploration, which itself is constrained by motor development. The motor routines used to convey specific concepts are abstracted and stored during production (right). These abstractions, or “motor schemas,” are associated with the concept attempted and so serve as one half of the phonological representation of a word. Solid lines with arrows represent feedforward processes; dotted lines with arrows represent feedback processes.

Figure 3.

Following early word production, the next major developmental change is hypothesized to occur when motorically driven homophony begins to threaten the young child's ability to effectively communicate. At this stage, the child begins to focus on how words should sound. As a result, production shifts from an entirely feedforward process to one where feedforward routines are adjusted to match perceptual representations. The adjustment process, carried out through interactions between the endogenous perceptual–motor map and the repository of exogenous word form representations or “perceptual exemplars,” sets the stage for state feedback control, which nonetheless begins with a delayed comparison between the perceptual trace and target—absent adjustment (left). Solid lines with arrows represent feedforward processes; dotted lines with arrows represent feedback processes.

Figure 4.

During the preschool years, children begin to self-monitor based on external perceptual feedback to identify deviations between how they sound and who they want to sound like. The perceived deviations highlight aspects of the stored perceptual representations, driving the perceptual–motor mapping and resulting endogenous motoric representations (i.e., schemas) ever more toward matching exogenous perceptual goals (i.e., exemplars). Solid lines with arrows represent feedforward processes; dotted lines with arrows represent feedback processes.

Figure 5.

The adult model of speech production implied by the developmental model outlined in this review article. Solid lines with arrows represent feedforward processes; dotted lines with arrows represent feedback processes. The linkages between the repository of lexical concepts and motor schemas and between lexical concepts and perceptual exemplars represent the conceptual and phonological aspects of the lexicon.