Impulsive Violence: Can Brain Implants Affect Poor Impulse Control?

Poor decision making and impulsive behaviour are hallmarks of youth, sociopathy, and the violent criminal.

Interestingly, Parkinson's Disease patients can also exhibit impulsive behaviours -- often as a side affect of treatment. Both drug treatment for Parkinson's and deep brain stimulation (DBS) via implants can increase impulsive and dysfunctional behaviours in Parkinson's patients. Researchers wanted to know why DBS was causing this impulsivity, and what they could do about it.

For their first experiment, the researchers designed a computerized decision-making experiment. They asked 65 healthy subjects and 14 subjects with Parkinson's disease to choose between pairs of generic line art images while their mPFC brain activity was recorded. Each image was each associated with a level of reward. Over time the subjects learned which ones carried a greater reward.

Sometimes, however, the subjects would be confronted with images of almost equal reward -- a relatively tough choice. That's when scalp electrodes detected elevated activity in the mPFC in certain low frequency bands. Lead author and postdoctoral scholar James Cavanagh found that when mPFC activity was larger, healthy participants and Parkinson's participants whose stimulators were off would take proportionally longer to decide. But when deep brain stimulators were turned on to alter STN function, the relationship between mPFC activity and decision making was reversed, leading to decision making that was quicker and less accurate.

The Parkinson's patients whose stimulators were on still showed the same elevated level of activity in the mPFC. The cortex wanted to deliberate, Cavanagh said, but the link to the brakes had been cut.

"Parkinson's patients on DBS had the same signals," he said. "It just didn't relate to behavior. We had knocked out the network."

In the second experiment, the researchers presented eight patients with the same decision-making game while they were on the operating table in Arizona receiving their DBS implant. The researchers used the electrode to record activity directly in the STN and found a pattern of brain activity closely associated with the patterns they observed in the mPFC.

"The STN has greater activity with greater [decision] conflict," he said. "It is responsive to the circumstances that the signals on top of the scalp are responsive to, and in highly similar frequency bands and time ranges."

A mathematical model for analyzing the measurements of accuracy and response time confirmed that the elevated neural activity and the extra time people took to decide was indeed evidence of effortful deliberation.

"It was not that they were waiting without doing anything," said graduate student Thomas Wiecki, the paper's second author. "They were slower because they were taking the time to make a more informed decision. They were processing it more thoroughly."

The results have led the researchers to think that perhaps the different brain regions communicate by virtue of these low-frequency signals. Maybe the impulsivity side effect of DBS could be mitigated if those bands could remain unhindered by the stimulator's signal. Alternatively, Wiecki said, a more sophisticated DBS system could sense that decision conflict is underway in the mPFC and either temporarily suspend its operation until the decision is made, or stimulate the STN in a more dynamic way to better mimic intact STN function. _SD

We know that the prefrontal cortices (PFCs) are crucial to good executive function and impulse control. But it appears from the experiments above that the PFCs need help from other brain centres, such as the sub-thalamic nuclei (STN). Understanding the interaction of the various brain nuclei in the control of complex behaviour and decision-making, can help designers of brain implants and brain stimulators to avoid unfortunate side effects of implant therapy, and to increase positive serendipitous effects of such therapies.

Given the importance of impulse control in the prevention of crime and violence, it is likely that brain stimulators and implants will take on a greater role in the penal system. An interesting historical example of the electrical control of violent behaviour, is the curious instance of Dr. Jose Delgado and the charging bull. Law enforcement officers and correctional officers would like to be able to stop a charging maniac in his tracks, like Delgado did with the bull. It is likely that someday they will have that power.

It would be best if society set about training its youth to possess sound executive function from the earliest age, so as to avoid that type of dystopian future.