Summary: Deep brain stimulation (DBS) in the subthalamic nucleus, a treatment for Parkinson’s disease, may influence more than just motor control. This treatment, which mitigates Parkinson’s symptoms such as tremors, also appears to affect patients’ ability to shift their attention between tasks.
The study conducted experiments with Parkinson’s patients, tracking how their attention shifted when the DBS device was active versus idle. Findings indicate that while DBS aids motor function, it may impede the brain’s ability to redirect thoughts and attention, possibly explaining why some patients experience cognitive and behavioral side effects.
Key Facts:
- Deep brain stimulation in the subthalamic nucleus is effective for controlling Parkinson’s symptoms but may also impact cognitive functions related to attention and impulse control.
- The study used auditory distractions to measure shifts in attention among Parkinson’s patients, finding that those with active DBS had difficulty switching their focus.
- This research suggests the subthalamic nucleus plays a critical role in both motor and non-motor systems, including thought and attention management.
Source: University of Iowa
Researchers at the University of Iowa in a new study have linked a region in the brain to how humans redirect thoughts and attention when distracted. The connection is important because it offers insights into cognitive and behavioral side effects to a technique being used to treat patients with Parkinson’s disease.
The subthalamic nucleus is a pea-sized brain region involved in the motor-control system, meaning our movements. In people with Parkinson’s disease, those movements have been compromised: Researchers believe the subthalamic nucleus, which normally acts as a brake on sudden movement, is exerting too much influence. That overactive brake, researchers think, is what contributes to the tremors and other motor deficiencies associated with the disease.
In recent years, clinicians have treated Parkinson’s patients with deep-brain stimulation, an electrode implanted in the subthalamic nucleus that rhythmically generates electrical signals, causing the brain region to loosen its braking, freeing up movement. The deep brain stimulation system is like a pacemaker for the heart; once implanted, it runs continuously.
“The technique is truly miraculous, frankly,” says Jan Wessel, associate professor in the departments of Psychological and Brain Sciences and Neurology at Iowa.
“People come in with Parkinson’s, surgeons turn the electrode on, and their tremor goes away. Suddenly they can hold 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.”
Yet some patients treated with deep brain stimulation have been beset by an inability to focus attention and impulsive thoughts, sometimes leading to risky behaviors such as gambling and substance use. Researchers began to wonder: Did the subthalamic nucleus’ role in movement also mean this same brain region may deal with thoughts and impulse control?
Wessel decided to find out. His team designed an experiment gauging the focus of attention of more than a dozen Parkinson’s patients when the deep brain stimulation treatment was either activated or idle.
The participants, outfitted with a skull cap to track their brain waves, were instructed to fix their attention on a computer screen while the brain waves in their visual cortex were being monitored.
About one in five times, in a random order, the participants heard a chirping sound, meant to divert their visual attention from the screen to the newly introduced audial distraction.
In a 2021 study, Wessel’s group established that brain waves in participants’ visual cortex dropped when they heard a chirp, meaning their attention had been diverted by the sound.
By interchanging instances when there was a chirp or no sound, the researchers could see when attention had been diverted, and when the focus of visual attention had been maintained.
The team turned their attention to the Parkinson’s groups for this study. When the deep brain stimulation was idle and the chirp was sounded, the Parkinson’s patients diverted their attention from the visual to the auditory systems—just as the control group had done in the previous study.
But when the chirp was introduced to the Parkinson’s participants with deep brain stimulation activated, those participants did not divert their visual attention.
“We found they no longer can break or suppress their focus of attention in the same way,” says Wessel, the study’s corresponding author.
“The unexpected sound happens and they’re still full-on attending to their visual system. They haven’t diverted their attention from the visual.”
The distinction confirmed the subthalamic nucleus’ role in how the brain and body communicate not only with movement—as previously known—but also with thoughts and attention.
“Until now, it was very unclear why those with Parkinson’s disease had issues with thoughts, such as why they performed worse on attention tests,” Wessel says.
“Our study explains why: While removing the inhibitory influence of the subthalamic nucleus on the motor system is helpful in treating Parkinson’s, removing its inhibitory influence from nonmotor systems (such as thoughts or attention) can have adverse effects.”
Wessel firmly believes deep brain stimulation should continue to be used for Parkinson’s patients, citing its clear benefits to aiding motor-control functions.
“There may be different areas of the subthalamic nucleus that stop the motor system and that stops the attentional system,” he says.
“That’s why we’re doing the basic research, to find out how do we fine-tune it to get the full benefit to the motor system without accruing any potential side effects.”
The study, “The human subthalamic nucleus transiently inhibits active attentional processes,” was published online March 4 in the journal Brain.
The first author is Cheol Soh, from the Department of Psychological and Brain Sciences at Iowa. Contributing authors, all from Iowa, include Mario Hervault, Nathan H. Chalkley, and Cathleen M. Moore, from the Department of Psychological and Brain Sciences; Jeremy Greenlee and Andrea Rohl, from the Department of Neurosurgery; and Qiang Zhang and Ergun Uc, from the Department of Neurology.
Funding: The National Institutes of Health, and the National Science Foundation, through a CAREER award to Wessel, funded the research.
About this neuroscience research news
Author: Richard Lewis
Source: University of Iowa
Contact: Richard Lewis – University of Iowa
Image: The image is credited to Neuroscience News
Original Research: Closed access.
“The human subthalamic nucleus transiently inhibits active attentional processes” by Jan Wessel et al. Brain
Abstract
The human subthalamic nucleus transiently inhibits active attentional processes
The subthalamic nucleus (STN) of the basal ganglia is key to the inhibitory control of movement. Consequently, it is a primary target for the neurosurgical treatment of movement disorders like Parkinson’s Disease, where modulating the STN via deep-brain stimulation (DBS) can release excess inhibition of thalamo-cortical motor circuits.
However, the STN is also anatomically connected to other thalamo-cortical circuits, including those underlying cognitive processes like attention. Notably, STN-DBS can also affect these processes.
This suggests that the STN may also contribute to the inhibition of non-motor activity, and that STN-DBS may cause changes to this inhibition. We here tested this hypothesis in humans.
We used a novel, wireless outpatient method to record intracranial local field potentials (LFP) from STN DBS implants during a visual attention task (Experiment 1, N=12). These outpatient measurements allowed the simultaneous recording of high-density EEG, which we used to derive the steady-state visual evoked potential (SSVEP), a well-established neural index of visual attentional engagement.
By relating STN activity to this neural marker of attention (instead of overt behavior), we avoided possible confounds resulting from STN’s motor role. We aimed to test whether the STN contributes to the momentary inhibition of the SSVEP caused by unexpected, distracting sounds.
Furthermore, we causally tested this association in a second experiment, where we modulated STN via DBS across two sessions of the task, spaced at least one week apart (N=21, no sample overlap with Experiment 1).
The LFP recordings in Experiment 1 showed that reductions of the SSVEP after distracting sounds were preceded by sound-related γ-frequency (>60Hz) activity in the STN. Trial-to-trial modeling further showed that this STN activity statistically mediated the sounds’ suppressive effect on the SSVEP.
In Experiment 2, modulating STN activity via DBS significantly reduced these sound-related SSVEP reductions. This provides causal evidence for the role of the STN in the surprise-related inhibition of attention.
These findings suggest that the human STN contributes to the inhibition of attention, a non-motor process. This supports a domain-general view of the inhibitory role of the STN.
Furthermore, these findings also suggest a potential mechanism underlying some of the known cognitive side-effects of STN-DBS treatment, especially on attentional processes.
Finally, our newly-established outpatient LFP recording technique facilitates the testing of the role of subcortical nuclei in complex cognitive tasks, alongside recordings from the rest of the brain, and in much shorter time than perisurgical recordings.
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