It’s no secret that people slow down mentally after making a mistake. Monkeys do, too. Neuroscientists call it post-error slowing or PES.

What’s not clear are the neurological processes that drive PES.

A new study that addresses a long-standing debate on the value of PES could offer insights into conditions that impair judgments, such as Alzheimer’s disease and ADHD, researchers say. Explains study coauthor Braden Purcell, postdoctoral fellow at New York University:

“Our research reveals that a combination of changes in the brain slow us down after mistakes. One gathers more information for the decision to prevent repeating the same mistake again. A second change reduces the quality of evidence we obtain, which decreases the likelihood we will make an accurate choice."

Adds Roozbeh Kiani, an assistant professor in NYU’s Center for Neural Science and the study’s other coauthor:

“In the end, these two processes cancel each other out, meaning that the deliberative approach we take to avoid repeating a mistake neither enhances nor diminishes the likelihood we’ll repeat it.”

Humans vs. Monkeys

The researchers took a closer look at the process through a series of experiments involving monkeys and humans. Both watched a field of noisy moving dots on a computer screen and reported their decision about the net direction of motion with their gaze.

The experimenters controlled the difficulty of each decision with the proportion of dots that moved together in a single direction. For instance, a large proportion of dots moving to the right provided very strong evidence for a rightward choice, but a small proportion provided only weak evidence.

Humans and monkeys showed strikingly similar behavior. After errors, both slowed down the decision-making process, but the pattern of slowing depended on the difficulty of the decision.

Slowing was maximum for more difficult decisions, suggesting longer accumulation of information. However, the overall accuracy of their choices did not change, indicating the quality of accumulated sensory information was lower.

Brain activity observed from the monkeys while they performed the task shed light on what was happening in the brain. Specifically, the researchers analyzed neural responses from a region of parietal cortex involved in accumulating information in their task.

Evidence Accumulation Rates

During decision-making, these neurons represent evidence accumulation by increasing their activity over time at a rate that depends on the quality of evidence. Specifically, stronger motion leads to faster ramping and weaker motion leads to slower ramping.

After mistakes, the exact same motion stimulus produced neural activity that ramped more slowly—consistent with impaired quality of sensory evidence. Critically, however, the neurons showed significant increase in how much evidence was accumulated before a decision, preventing a reduction in the overall accuracy.

“Patients with ADHD or schizophrenia often do not slow down after errors and this has been interpreted as an impaired ability to monitor one’s own behavior,” explains Purcell. “Our results suggest that this absence of slowing may reflect much more fundamental changes in the underlying decision-making brain networks.

By better understanding the neural mechanisms at work after we make a mistake, we can begin to see how these afflictions impair this process.”

Purcell, Braden A. et al. Neural Mechanisms of Post-error Adjustments of Decision Policy in Parietal Cortex Neuron DOI:

Photo: bark/Flickr

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