Speech Planning in Neuroimaging

Neuroimaging data contribute in a substantial way to a better understanding of the cerebral mechanisms underlying speech motor control, which can be subdivided into at least a phonetic planning and a motor execution stage. A series of functional imaging studies delineated a widely accepted minimal network of speech motor control encompassing various brain structures. However, there is still an ongoing discussion about the possible neuroanatomical correlates subserving speech motor programming. Functional imaging investigations and clinical studies analysing lesion locations in patients with and without a speech programming deficit (apraxia of speech) point to a left lateralised representation of phonetic encoding processes. Within this hemisphere the perisylvian cortex, i.e. the ventral part of premotor cortex and Broca's area seem to be of special relevance for speech motor programming. Results of imaging studies investigating the hypothesised role of the anterior insula in this context are still conflicting and inconsistent. Whereas the insula is considered in some studies as a crucial area for speech motor programming, i.e. for establishing new motor plans, others suggest a contribution of the insula to articulatory processes. Another controversial issue addressed recently by various imaging studies is related to the influence of sublexical frequencies on phonetic encoding mechanisms or associated hemodynamic activation patterns, respectively. So far, data showed a rather weak support for a possible impact of syllable frequencies but nonetheless it can not be excluded that (other) sublexical frequency variables may affect the process of speech motor programming.