How Deep Brain Stimulation Affects Decision-Making in Parkinson’s disease

What's the science?

How much time should we give ourselves to make a decision? For example, when faced with a difficult decision, we might give ourselves more time to garner more evidence before we reach the ‘decision threshold’ and decide. One brain region involved in adjusting our decision threshold (meaning we take more or less time before the decision) is the subthalamic nucleus (STN). Deep brain stimulation (DBS) of the STN is often performed to reduce motor symptoms in Parkinson’s disease, however, a negative side effect can be impairment in adjusting the decision threshold, leading to impulsive responses. This week in Current Biology, Herz and colleagues conducted a study in patients with DBS electrodes for Parkinson’s placed in the STN, in order to assess how stimulation of this site affects the decision threshold.

How did they do it?

Ten patients with Parkinson’s participated in the study after undergoing surgery to implant electrodes for DBS in the STN. Each patient performed a decision-making task: 1) when DBS was off 2) with DBS on continuously and 3) with ‘adaptive DBS’ where DBS only turns on when necessary. The decision-making task involved looking at dots moving on a screen, and deciding whether the majority of dots were moving to the left or to the right. There were two task conditions and two forms of instruction: In the easy condition, 50% of the dots moved in the same direction, while in the difficult condition only 8% of dots moved in the same direction. Participants were also instructed to focus on either speed or accuracy of their decision.

What did they find?

When DBS was off, participants responded more slowly during the difficult task and when instructed to focus on accuracy (versus speed). However, when DBS was on, slowing during a difficult task was diminished, but slowing due to focus on accuracy remained the same. During adaptive DBS, stimulation came on at different times across trials (when beta activity happened to be high). When the DBS stimulation came on during a 400-500 ms time window after the moving dots appeared on the screen, the time required to make a decision (usually increased during the difficult task) was most diminished, suggesting that the effect of stimulation is confined to a short time window. Using ‘drift diffusion modelling’, they found that stimulation affected the decision threshold time specifically, as opposed to, for example, the motor response time. While DBS was off, beta activity increased after presentation of the dots during the difficult condition, and was related to the decision threshold, but these effects were abolished during stimulation. This indicates that DBS may be lowering the decision threshold by changing the relationship between STN activity and threshold adjustments.

What's the impact?

These results are the first to show that the STN may be directly involved in decision thresholds (how much evidence we need before we reach a decision). During a narrow time window, the STN adjusts decision thresholds based on the anticipated difficulty of the decision. This may be a mechanism by which decision-making is impaired in people with Parkinson’s who have DBS.

P. Brown et al., Mechanisms Underlying Decision-Making as Revealed by Deep-Brain Stimulation in Patients with Parkinson’s Disease. Current Biology (2018). Access the original scientific publication here.

Your Brain Reacting to Social Injustice

What's the science?

How do we perceive injustice? Many neuroimaging studies have looked at how we perceive the violation of social norms by analyzing brain activity while participants play a computer game. For example, participants might have the option to punish one player who is acting unfairly (e.g. stealing) towards another. Further, different hormones, like oxytocin, influence our social behaviour, suggesting they can play a role in our perception of injustice. This week in The Journal of Neuroscience, Stallen and colleagues performed a new set of experiments using brain imaging to analyze the perception of injustice.

How did they do it?

First, oxytocin was administered to half of the participants. Next, all participants underwent an fMRI brain scan, while playing three computer games: 1) Participants played against an opponent, called a ‘taker’. The taker had the opportunity to steal up to 100 chips away from the participant, and the participant could then punish the taker by giving up up to 100 of their own chips. For each chip they gave up 3 would be taken from the taker, (injustice happening to them) 2) Participants received 200 chips and observed a taker stealing chips from another player, and could then punish the taker (using up to 100 chips, 3 taken from the taker for each chip given up), (observing social injustice, punishing as a third party) and 3) Participants observed a taker stealing chips from another player, and could compensate the disadvantaged player using up to 100 chips (the disadvantaged player was given 3 chips per chip given up) (observing social injustice, compensating as a third party). Participants knew they would receive real monetary compensation after the games according to their performance, and all games were anonymous.

What did they find?

Participants who received oxytocin were more likely to dole out small punishments, frequently, to a taker who took chips from the participant or another player, versus participants who did not receive oxytocin. When the authors compared trials in which a participant doled out punishment to the taker versus compensating the disadvantaged player, there was greater activity in the ventral striatum -- a brain region involved in processing rewards. The decision to administer punishment was associated with activity in the anterior insula -- a brain region involved in “gut feelings” and decision making involving risk. Activity in the amygdala, a brain region associated with affective arousal, was correlated with the severity of punishment administered but only in experiment #2, when participants observed a taker behaving unfairly towards someone else.

What's the impact?

This is the first study to assess the perception of social justice in situations where an individual is experiencing injustice firsthand compared to observing injustice as a third party. This study suggests two distinct brain mechanisms might be at play during these unjust situations.

A word of caution: Different brain regions are activated in many different situations. Just because a brain region is known to be activated during reward, for example, does not necessarily mean that brain region will always be active during reward processing.

M. Stallen et al., Neurobiological Mechanisms of Responding to Injustice. Journal of Neuroscience. (2018). Access the original scientific publication here.