A review and synthesis of the first 20 years of PET and fMRI studies of heard speech, spoken language and reading

Abstract

The anatomy of language has been investigated with PET or fMRI for more than 20 years. Here I attempt to provide an overview of the brain areas associated with heard speech, speech production and reading. The conclusions of many hundreds of studies were considered, grouped according to the type of processing, and reported in the order that they were published. Many findings have been replicated time and time again leading to some consistent and undisputable conclusions. These are summarised in an anatomical model that indicates the location of the language areas and the most consistent functions that have been assigned to them. The implications for cognitive models of language processing are also considered. In particular, a distinction can be made between processes that are localized to specific structures (e.g. sensory and motor processing) and processes where specialisation arises in the distributed pattern of activation over many different areas that each participate in multiple functions. For example, phonological processing of heard speech is supported by the functional integration of auditory processing and articulation; and orthographic processing is supported by the functional integration of visual processing, articulation and semantics. Future studies will undoubtedly be able to improve the spatial precision with which functional regions can be dissociated but the greatest challenge will be to understand how different brain regions interact with one another in their attempts to comprehend and produce language.

Fig. 1

Functional–anatomical model proposed by Petersen et al. (1988, 1989).

Fig. 2

Functional model based on neuroimaging studies of language.

Fig. 3

Anatomical model based on neuroimaging studies of language. See Fig. 4 and Table 3 for key to anatomical abbreviations.

Fig. 4

Illustrative sketch of the location of language related activations, based on Price et al. data. This figure was created by overlaying images of activations from many different studies that I have co-authored. Details of the studies can be provided on request to the author. The colours indicate the type of task or processing that caused the activation. The blue areas are activated by auditory stimuli (auditory 1 followed by auditory 2). The red/brown areas are activated by visual stimuli (visual 1 then visual 2). The orange areas are activated by general action selection (hand or speech). The pink and purple areas are involved in different levels of semantic and syntactic processing depending on the task demands. The dark pink areas are sensitive to the semantic content of the stimuli, the light pink areas are those that more activated for semantic than phonological decisions. The light purple areas are activated by spoken and written sentences with the dark purple areas most activated by meaningful and grammatical sentences. The green areas are involved in generating or rehearsing speech. The light green areas are involved in word retrieval, the dark green areas are involved in silent phonological decisions. The khaki green area and PreC/vPM areas are activated by mouth movements during speech. Finally the white areas, corresponding to Broca's area (pOp) and Wernicke's area (pSTS) are involved in both perception and production tasks with familiar stimuli. They may function as convergence zones that receive and send signals to all the other areas involved in perceiving and producing speech. The connections between areas are not shown because we don't yet know enough about how all the areas connect to one another.