The thalamus is crucially embedded in language processing

Introduction: To understand a sentence, it is necessary to analyze its syntactic structure. In scalp EEG, correlates of such analysis can be depicted with event related potentials (ERP): the so called early left anterior negativity (ELAN), presumed to automatically detect phrase violations, and a later centro-parietal positivity (P600), putatively mirroring phrase structure re-analysis or repair. At difference to cortical processing, the specific involvement of subcortical language comprehension is controversial. Therefore, we aimed at recording syntactic error potentials directly from the thalamus in patients with deep brain stimulation (DBS). Methods: In 8 patients with disabling tremor diseases, EEG was continuously recorded from 20 thalamic DBS electrodes, bilaterally implanted into the ventral intermediate nucleus (VIM) and postoperatively connected to externalized leads for a couple of days. In all patients, a simultaneous scalp EEG was derived from 20 scalp sites, widely distributed due to the 10–20 system (sampling rate 2 kHz, bandpass from 0.05–500Hz). Per patient, 148 correct and erroneous sentences were acoustically presented in randomized order (75% correct, 25% syntactically incorrect). 3s after the sentence the in-/correctness had to be indicated by left/right button press. Results Thalamic and scalp ERP were consistently delineated. At scalp level, the frontal ELAN and the parietal P600 typically peaked at 169±34 ms and 803±92 ms, respectively. In the simultaneous thalamic recordings, two negative potentials were identified upon syntactic errors, peaking at 195±45 ms and 591±58 ms. The peak latencies of thalamic ERP differed significantly from those of scalp ERP. Conclusion: At scalp level, the classical ‘syntactic’ EEG pattern was obtained. In subcortical recordings, two additional language-related processes were identified. According to the latencies obtained at either level, it is conceivable that the early thalamic negativity arises along a fronto-thalamic route along which early information of automatic error detection is propagated. The late thalamic negativity is compatible with a thalamo-cortical route, initiating cortical language re-analysis. We conclude that the thalamus is embedded in a cortico-basal network of primary language comprehension.