Subcortical nuclei and language processing

Authors: Elissa-Marie Cocquyt, speech therapist and postdoctoral researcher at the University of Groningen & Adrià Rofes, assistant professor of Neurolinguistics at the University of Groningen.

The human brain is one of the most wonderful organs. In general, our brain consists of a left and right part, of which the outermost layer is called the ‘cortex’. The functions of the cortex have been extensively investigated and comprise all aspects of human behavior, from walking to your car and driving to the grocery store to remembering the name of your neighbor and the ability to chat with him ¹. An accurate execution of these daily activities relies on key interactions between the cortex and little structures in the depth of the brain: the ‘subcortical nuclei’ ². In this article, we will delve into the intricacies of the subcortical nuclei. In particular, we will emphasize their potential contribution in language processing. We will do so, by targeting a specific patient population, i.e. Parkinson’s disease (PD), and a treatment option, i.e. Deep Brain Stimulation (DBS).

PD is the second most common neurodegenerative disease worldwide, after Alzheimer’s Disease ³. It is typically described as a motor disorder due to the presence of a resting tremor, rigidity, slower/smaller movements, and postural imbalance ⁴. Importantly, PD is not limited to motor symptoms. Sleep and smell disorders, fatigue, anxiety and depression (to name a few) can also be observed, sometimes even before the onset of the motor symptoms ⁵. PD has an important impact on the communicative abilities of patients as well. This is partially due to the speech¹ problems they experience, typically including a reduction in loudness, pitch variations and articulation precision. These symptoms are related to the motor symptoms mentioned before ⁶. In addition, PD can compromise language² production and comprehension.

Specific to language, frequently reported symptoms are (1) difficulties in complex (e.g. embedded³) sentence comprehension (e.g. ^{7 8}), (2) a reduced accuracy (compared to healthy controls) on verbal fluency (i.e. generating as much words from a specific category as possible in a given time) and picture naming tasks, especially when action verbs (e.g. jumping, writing, running etc.) are targeted (e.g. ^{9 10 11}) and (3) less information transfer during spontaneous language production (story retelling or picture description) tasks, meaning that less of what they say is considered informative (e.g. ^{12 13}).

Equally important, the presence of cognitive (e.g. attention, inhibition, flexibility, working memory) disorders can heavily influence the results obtained during language tasks. Indeed, in current research, it remains an open question whether the observed language difficulties can be fully explained by the presence of cognitive disorders ¹⁴.

Looking into the brain of people with PD, one of the key characteristics is a significant loss of specific (‘dopaminergic’) nerve cells in the substantia nigra (a subcortical nucleus), which on his turn influences the interactions with other subcortical structures and the cortex. Disturbances in these interactions give rise to the multitude of symptoms observed in people with PD ¹⁵. Until today, there is no definite cure for PD. Hence, the available treatment options focus on relieving and controlling the (motor) symptoms, by influencing the disturbed brain interactions. As a first treatment, many patients will receive medication (e.g. levodopa), which is supposed to compensate for the loss of dopaminergic nerve cells ¹⁶.

For those individuals in whom medication effects become unpredictable, DBS surgery can be considered. This is a medical procedure during which electrodes are implanted in specific subcortical nuclei. In case of PD, the target region for electrode placement frequently is the subthalamic nucleus (STN) in both the left and right part of the brain (Figure 1). The electrodes are connected by leads with a neurostimulator, which, on his turn, is implanted in the abdominal wall or chest. Depending on the neurostimulator settings, an electrical current can be sent to the STN ^{17 18}.

DBS surgery and STN stimulation result in a long-term improvement of the motor symptoms in people with PD, and allow a reduction of medication treatment ¹⁹. Nonetheless, the effects of STN DBS on language functioning are less clear. In general, most studies comparing language functions before and after DBS surgery focused on verbal fluency tasks, and reported a decline in performance (weeks to months) after surgery (for a review, see ²⁰). Fewer follow-up studies made use of other language tasks, which is an important limitation. In these studies, the effects of STN DBS were variable: both stable performances (e.g. for picture naming ²¹), accuracy improvements (e.g. for spontaneous language production, ²²) and declines (e.g. on decision tasks targeting words with multiple meanings (e.g. ‘table’ ²³) have been observed after (compared to before) DBS surgery.

These results provide evidence that the STN may be part of the brain networks responsible for language functions, but also show that the effect of STN DBS varies according to the type of task ²⁴. Importantly, however, caution is needed when comparing the results of different studies due to the many differences in their approach (method), and differences between patients. For example, the decision when to perform the language evaluation (before and after surgery) could influence the obtained results: Moretti et al. ²⁵ showed that it could take up to twelve months after surgery before a beneficial effect of STN DSB on complex sentence comprehension is observed. Also, the specific characteristics of each individual patient (e.g. disease duration and severity, medication intake, STN stimulation parameters, cognitive abilities) can give rise to unique patterns of language functioning, and unique effects of STN DBS ²⁶.

In summary, the multitude of symptoms in people with PD, and the diverse effects of DBS stimulation, make it clear that subcortical brain structures, although small in size, play a big role in human functioning. Focusing on communication, exciting research lines on language processing (beyond verbal fluency), the link with cognition, as well as the long-term effects of DBS in this population are underway ²⁷. Going in depth (of the brain) can lead to a better understanding of the complex interplay between the cortex and subcortical nuclei, and provide clinicians with better tools to counsel people with PD.

Acknowledgements

Both authors received funding from the Dutch Research Council (NWO, 406.XS.01.050).

Notes

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