Dissociation between Simple and Complex Speech Motor Tasks within the Bilateral Motor Thalamus

Although clinical and experimental evidence highlight the role of the thalamus in voluntary movement production, the involvement of the thalamus in complex motor tasks such as speech production remains to be elucidated. The present study examined neural activity within the bilateral thalamus in 13 participants (seven females) with essential tremor undergoing awake deep brain stimulation implantation surgery, using three speech tasks of varied complexity [vowel vocalization, a diadochokinetic task (DDK), and sentence repetition]. Low-frequency neural activity (delta/theta band) activity was significantly increased during sentence and DDK compared with vowel vocalization in the bilateral motor thalamus and, to a lesser extent, increased for sentence repetition compared with DDK. Moreover, there was prominent prespeech beta band activity, with a greater decrease in the power of beta activity for sentence compared with DDK and vowel vocalization. The greater low-frequency activity in more complex speech tasks may reflect the allocation of additional cognitive resources to monitor the execution of speech motor plans through cortico-thalamo-cortical pathways in a temporally precise manner. The greater decrease in the power of beta activity prior to the onset of sentence repetition may imply greater involvement of the bilateral thalamus in the planning of complex speech tasks. These findings provide new insights into the role of the bilateral thalamus in speech production and may have clinical implications for neurological disorders that affect speech production.