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Summary: Deep brain stimulation (DBS) in the subthalamic nucleus, a treatment for Parkinson’s disease, may affect more than just motor control. This treatment, which alleviates Parkinson’s symptoms such as tremors, also appears to affect patients’ ability to shift attention between tasks.
The study experimented with patients with Parkinson’s disease, tracking how their attention shifted when the DBS device was active rather than in standby mode. The results suggest that while DBS improves motor function, it may interfere with the brain’s ability to redirect thoughts and attention, which may explain why some patients experience cognitive and behavioral side effects.
Key facts:
- Deep brain stimulation in the subthalamic nucleus is effective in controlling Parkinson’s symptoms, but may also affect cognitive functions related to attention and impulse control.
- The study used auditory distraction to measure changes in attention in patients with Parkinson’s disease and found that patients with active DBS had difficulty shifting their attention.
- This research suggests that the subthalamic nucleus plays a critical role in both motor and non-motor systems, including the control of thought and attention.
Source: University of Iowa
Researchers at the University of Iowa have linked a brain region to how people redirect thoughts and attention when distracted in a new study. This connection is important because it provides insight into the cognitive and behavioral side effects of the method used to treat patients with Parkinson’s disease.
The subthalamic nucleus is a pea-sized area of the brain involved in the motor control system, that is, our movements. In people with Parkinson’s disease, these movements are impaired: Researchers believe that the subthalamic nucleus, which normally acts as a brake on sudden movements, exerts too much influence. Researchers believe it is this overactive brake that contributes to the tremors and other movement disorders associated with the disease.
In recent years, doctors have treated patients with Parkinson’s disease using deep brain stimulation, an electrode implanted in the subthalamic nucleus that rhythmically generates electrical signals, causing the area of the brain to release inhibition, allowing freedom of movement. A deep brain stimulation system is similar to a heart pacemaker; Once implanted, it operates continuously.
“Frankly, this technique is truly miraculous,” says Jan Wessel, assistant professor of psychology, brain and neurosciences at Iowa.
“People come in with Parkinson’s disease, the surgeons turn on the electrode, and their tremors go away. Suddenly they can keep their hands steady and go play golf. It’s one of those blockbuster treatments where when you see it in action, it really makes you believe in what the neuroscience community is doing.”
However, some patients treated with deep brain stimulation suffer from an inability to focus and impulsive thoughts, sometimes leading to risky behaviors such as gambling and substance use. Researchers began to wonder: Does the role of the subthalamic nucleus in movement mean that this same area of the brain may be involved in thoughts and impulse control?
Wessel decided to find out. His team designed an experiment to assess the attention span of more than a dozen patients with Parkinson’s disease while deep brain stimulation therapy was either activated or inactive.
Participants, equipped with brainwave-tracking skullcaps, were instructed to focus their attention on a computer screen while brainwaves in their visual cortex were monitored.
On about one occasion in five, participants randomly heard a chirping sound designed to divert their visual attention away from the screen to a newly introduced auditory distractor.
In a 2021 study, Wessel’s team found that brain waves in participants’ visual cortex decreased when they heard chirping sounds, meaning their attention was distracted by the sound.
By reversing the times when there was chirping and when there was no sound, the researchers could see when attention was distracted and when visual focus was maintained.
For this study, the team turned its attention to Parkinson’s disease groups. When deep brain stimulation was not performed and the chirping sound was heard, Parkinson’s patients switched their attention from the visual system to the auditory system – just as the control group did in the previous study.
But when participants with Parkinson’s disease were asked to chirp while deep brain stimulation was activated, those participants did not divert their visual attention.
“We found that they can no longer interrupt or suppress their focus of attention in the same way,” says Wessel, co-author of the study.
“An unexpected sound occurs and they are still completely focused on their visual system. They didn’t take their attention away from the visuals.”
This difference confirmed the role of the subthalamic nucleus in how the brain and body communicate not only through movement, as previously known, but also through thoughts and attention.
“Until now, it was very unclear why people with Parkinson’s disease have problems with thinking, for example why they perform worse on attention tests,” says Wessel.
“Our study explains why: Although eliminating the inhibitory influence of the subthalamic nucleus on the motor system is beneficial in the treatment of Parkinson’s disease, eliminating its inhibitory influence on non-motor systems (such as thoughts or attention) may have adverse consequences.”
Wessel strongly believes that deep brain stimulation should continue to be used in patients with Parkinson’s disease, citing its obvious benefits in improving motor control functions.
“There may be different areas in the subthalamic nucleus that shut down the motor system and the attention system,” he says.
“That’s why we’re doing basic research to figure out how we can tweak it to get the full benefit to the motor system without any potential side effects.”
The study, “The human subthalamic nucleus transiently suppresses active attentional processes,” was published March 4 in the journal. Brain.
The first author is Chool Seo from the Department of Psychology and Brain Sciences at Iowa State. The authors, all from Iowa, include Mario Hervo, Nathan H. Chalkley and Kathleen M. Moore of the Department of Psychological and Brain Sciences; Jeremy Greenlee and Andrea Rohl from the Department of Neurosurgery; and Qiang Zhang and Ergun Uk from the Department of Neurology.
Financing: The National Institutes of Health and the National Science Foundation, through a Career Award to Wessel, funded the research.
About neurobiological research news
Author: Richard Lewis
Source: University of Iowa
Contact: Richard Lewis – University of Iowa
Image: Image courtesy of Neuroscience News.
Original research: Closed access.
“The human subthalamic nucleus temporarily suppresses active attentional processes”, Jan Wessel et al. Brain
Abstract
The human subthalamic nucleus temporarily inhibits active attention processes.
The subthalamic nucleus (STN) of the basal ganglia plays a key role in inhibitory control of movement. Consequently, it is a major target for neurosurgical treatment of movement disorders such as Parkinson’s disease, in which modulation of the STN through deep brain stimulation (DBS) can relieve excess inhibition of thalamo-cortical motor circuits.
However, the STN is also anatomically connected to other thalamo-cortical circuits, including those that underlie cognitive processes such as attention. Notably, STN-DBS can also influence these processes.
This suggests that STN may also contribute to inhibition of non-motor activity and that STN-DBS may cause changes in this inhibition. We tested this hypothesis in humans.
We used a novel wireless ambulatory method to record intracranial local field potentials (LFPs) from STN DBS implants during a visual attention task (Experiment 1, N = 12). These ambulatory measurements allowed simultaneous high-density EEG recordings, which we used to determine the steady-state visual evoked potential (SSVEP), a well-known neural index of visual attentional engagement.
By attributing STN activity to this neural marker of attention (rather than overt behavior), we avoided possible confounds arising from the motor role of the STN. We aimed to test whether the STN contributes to momentary inhibition of SSVEP evoked by unexpected distracting sounds.
Additionally, we tested this relationship in a second experiment, where we modulated the STN via DBS in two sessions of the task, at least one week apart (N = 21, sample not overlapping with Experiment 1).
LFP recordings in Experiment 1 showed that the decrease in SSVEP following distracting sounds was preceded by γ-frequency (>60 Hz) auditory activity in the STN. Trial-by-trial simulations further showed that this STN activity statistically mediated the inhibitory effect of sounds on SSVEP.
In Experiment 2, modulation of STN activity by DBS significantly reduced the sound-related decrease in SSVEP. This provides causal evidence for the role of the STN in surprise-related attentional inhibition.
These findings suggest that the human STN contributes to attentional inhibition, a nonmotor process. This supports the general view of the inhibitory role of STN.
In addition, these results also suggest a potential mechanism underlying some of the known cognitive side effects of STN-DBS treatment, especially on attentional processes.
Finally, our newly developed LFP recording technique in an outpatient setting facilitates testing of the role of subcortical nuclei in complex cognitive tasks alongside rest-of-brain recordings and in a much shorter time than perisurgical recordings.
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