Abstract
The subthalamic nucleus (STN) is proposed to modulate response thresholds and speed–accuracy trade-offs. In situations of conflict, the STN is considered to raise response thresholds, allowing time for the accumulation of information to occur before a response is selected. Conversely, speed pressure is thought to reduce the activity of the STN and lower response thresholds, resulting in fast, errorful responses. In Parkinson’s disease (PD), subthalamic nucleus deep brain stimulation (STN-DBS) reduces the activity of the nucleus and improves motor symptoms. We predicted that the combined effects of STN stimulation and speed pressure would lower STN activity and lead to fast, errorful responses, hence resulting in impulsive action. We used the motion discrimination ‘moving-dots’ task to assess speed–accuracy trade-offs, under both speed and accuracy instructions. We assessed 12 patients with PD and bilateral STN-DBS and 12 age-matched healthy controls. Participants completed the task twice, and the patients completed it once with STN-DBS on and once with STN-DBS off, with order counterbalanced. We found that STN stimulation was associated with significantly faster reaction times but more errors under speed instructions. Application of the drift diffusion model showed that stimulation resulted in lower response thresholds when acting under speed pressure. These findings support the involvement of the STN in the modulation of speed–accuracy trade-offs and establish for the first time that speed pressure alone, even in the absence of conflict, can result in STN stimulation inducing impulsive action in PD.
Highlights
The quicker we make a decision, the more likely we are to make errors, whereas the more accurate we try to be, the longer we take
This was reflected in faster Reaction time (RT), increased percentage error (PE) and reduced response thresholds, when cued for speed, relative to accuracy
subthalamic nucleus (STN)-DBS significantly improved the motor symptoms of Parkinson’s disease (PD), but it resulted in the performance of the patients to become differentially faster (ΔRT = 185.32 ms) and more erroneous (ΔPE = 4.11 %) when cued for speed, as opposed to accuracy
Summary
The quicker we make a decision, the more likely we are to make errors, whereas the more accurate we try to be, the longer we take. This speed–accuracy trade-off (SAT) is a property of decision-making that can be controlled at will, depending on what is deemed important—be it responding. The distance between the baseline and threshold (boundary separation) indicates the amount of information that needs to be accumulated before a decision is made. If the distance between the baseline and threshold is large, the threshold is reached more slowly and decisions are made more accurately (Ratcliff and McKoon 2008; Bogacz et al 2009). According to the ‘STN Theory’ of SAT, ordinarily, in situations of conflict, the STN receives additional excitatory input from the frontal cortex, which raises the response threshold and sends a global ‘no-go’ signal to the output pathways of the basal ganglia, preventing premature responses and allowing time for more information to accumulate before a decision is made (Frank et al 2007)
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