Which processes are crucial for effective sentence production? Behavioural and lesion-mapping evidence from chronic aphasia

In this study, we assessed eight individuals with aphasia on seven measures that have previously been associated with damage to anterior language structures. Voxel-based lesion-symptom mapping was used to explore their neural correlates. We identified two distinct clusters of measures. The first, fluency and grammaticality, included measures of speech rate, articulatory agility and grammatical word content. Poor scores on these measures were associated with damage to the left insula and central operculum. The second cluster, context sensitivity, included a measure of the difference in accuracy on subject and object nouns during sentence production, and also a measure of the semantic interference effect in the cyclic naming task. Poor scores were associated with damage to the left inferior and middle frontal gyri, and the postcentral and precentral gyri. Based on these results, we propose a two-factor account of the speech difficulties observed in nonfluent aphasia. The first factor is a general slowing of speech planning processes, most notably articulatory-motor programming, and is associated with damage to the left insula and central operculum. The second factor is a difficulty with top-down control of language, associated with left inferior and middle frontal gyrus damage. We suggest this latter difficulty is a consequence of problems generating and maintaining a robust message-level representation to drive the language element selection process.