Dissociation Between Simple and Complex Speech Motor Tasks Within Bilateral Motor Thalamus

Although clinical and experimental evidence highlight the role of thalamus in voluntary movement production, the involvement of thalamus in complex motor tasks such as speech production remains to be elucidated. The present study examined neural activity within bilateral thalamus in 13 participants (7 female) 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 to vowel vocalization in the bilateral motor thalamus, and to a lesser extent increased for sentence repetition compared to DDK. Moreover, there was prominent pre-speech beta band activity, with a greater decrease in the power of beta activity for sentence compared to 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 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. Significance statement The cortical organization of speech production has been extensively investigated, however the role of subcortical structures in speech remains unclear. The present study investigated the modulation of neural activity in bilateral motor thalamus during three speech production tasks with varying complexity. Findings revealed differential modulation of theta and beta band activity in bilateral motor thalamus for simple compared to complex speech tasks. The present study demonstrated that involvement of the bilateral thalamus depends on the complexity of the speech task and has translational implications.