Decline and fall: A biological, developmental, and psycholinguistic account of deliberative language processes and ageing

Abstract

Background: This paper reviews the role of deliberative processes in language: those language processes that require central resources, in contrast to the automatic processes of lexicalisation, word retrieval, and parsing.

Aims: We describe types of deliberative processing, and show how these processes underpin high-level processes that feature strongly in language. We focus on metalinguistic processing, strategic processing, inhibition, and planning. We relate them to frontal-lobe function and the development of the fronto-striate loop. We then focus on the role of deliberative processes in normal and pathological development and ageing, and show how these processes are particularly susceptible to deterioration with age. In particular, many of the commonly observed language impairments encountered in ageing result from a decline in deliberative processing skills rather than in automatic language processes.

Main Contribution: We argue that central processing plays a larger and more important role in language processing and acquisition than is often credited.

Conclusions: Deliberative language processes permeate language use across the lifespan. They are particularly prone to age-related loss. We conclude by discussing implications for therapy.

Keywords:

• Control of language
• Central executive
• Fronto-striate loop
• Language across the lifespan
• Deliberative language
• Metalinguistic processing
• Ageing
• Parkinson's disease
• Alzheimer's disease
• Speech therapy

Acknowledgments

Lesley Jessiman is now at the University of the West of Scotland. We would like to thank our research volunteers for their efforts in providing these data. Some of the material in this paper formed part of the PhD research of the second author who was funded by the Royal Society of Edinburgh/Lloyds Bank Trust. We also gratefully acknowledge the financial assistance of the Parkinson's Disease Society of the United Kingdom and the Carnegie Trust for the Universities of Scotland. We are also most grateful to Merrill Garrett and the NSF who provided funds for a visit to Tucson partly to discuss the ideas in this paper. We would like to thank three anonymous reviewers for their detailed and insightful comments on an earlier version of this manuscript.