Inner speech in post-stroke aphasia : a behavioural and imaging study
Patients with aphasia often complain that there is a poor correlation between the words they think (inner speech) and the words they say (overt speech). Previous studies show that there are some cases in which inner speech is preserved while overt speech is impaired, and vice versa. However, these studies have various methodological and theoretical drawbacks. In cognitive models of language processing, inner speech is described as either dependent on both speech production and speech comprehension, or on the speech production system alone. Lastly, imaging studies show that inner speech is correlated with activation in various language areas. However, these studies are sparse and many have methodological caveats. Moreover, studies looking at inner speech in stroke patients are rare. This study examined inner speech in post-stroke aphasia using three different methodological approaches. Using cognitive behavioural methods, inner speech was characterised in healthy participants and stroke patients with aphasia. Using imaging, the brain structures which support inner speech were investigated. Two different methods were employed in this instance: Voxel based Lesion Symptom Mapping (VLSM) and Voxel Based Morphometry (VBM). Lastly, functional magnetic resonance imaging (fMRI) was used to study the dynamics of functional activations supporting inner speech production. The study showed that inner speech can remain intact while there is a marked deficit in overt speech. Structural studies suggested an involvement of the dorsal language route in inner speech processing, together with systems supporting motor feedback and executive functions. Functional imaging showed that inner speech processing in stroke is correlated with compensatory peri-lesional and contra-lesional activations. Activations outside the language network might reflect increase in effort or attention, or the use of feed forward and feedback mechanisms to support inner speech production. These results have implications for diagnosis, prognosis and therapy of certain patients with post-stroke aphasia.
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