- In a subset of neurons in the dorsolateral prefrontal cortex (dlPFC), changes in spiking activity correlated with subjective decisions as the participants performed a binary opinion-based task
- Changes in neuronal activity were most prominent before motor selection and were not observed as participants completed a sensory perceptual task
- Participants who had undergone resection of dlPFC lesions showed diminished gradation between opposing decisions, which was consistently observed across two different opinion-based tasks
- Disruption of the dlPFC had little effect on the results on a sensory perceptual task
Subjective decision-making is vital to human behavior because it allows consideration of options under conditions in which there are no explicit correct choices. There is a growing understanding that the frontal cortex has a role in decision-making, but little is known about the contribution of localized areas, such as the dorsolateral prefrontal cortex (dlPFC) or how humans make decisions that are based on personal beliefs.
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Postdoctoral Fellows Mohsen Jamali, MD, PhD and Ben Grannan, MD, neurosurgeon Ziv Williams, MD, in the Department of Neurosurgery at Massachusetts General Hospital, and colleagues have demonstrated that neuronal activity in the dlPFC correlates with variations in subjective decisions as humans perform opinion-based tasks. In Nature Neuroscience, the researchers propose a role for the dlPFC in subjective decision-making and its relation to opinion formation.
In the first study, 11 patients undergoing intraoperative neurophysiology as part of deep brain stimulator placement were shown pictures of seven real-world scenarios and were asked whether they believed each scene to be safe or unsafe.
Each prototypic scene was manipulated five times to vary the degree to which it represented a safe or unsafe situation (e.g., the distance was varied between a jogger running on a street and a car passing nearby). For each participant, the researchers constructed a voting profile, which reflected the participant's choices of safe/unsafe for each specific scene and manipulation.
In part, this analysis involved aligning trials to equipoise, the point at which the participants changed their choices from safe to unsafe per scene. Choices aligned to the left of equipoise (−1 to −4) reflected situations the participants believed to be progressively safer, and those aligned to the right of equipoise (+1 to +4) reflected situations they believed were progressively more unsafe.
Neuronal Activity During the Opinion-based Task
As the participants completed the situational assessments, the researchers recorded activity in 119 well-isolated units in Brodmann's area 9 of the dlPFC. Results included:
- Thirty-one neurons responded differently in trials in which the participants subsequently reported the scenes to safe compared to unsafe
- For 58% of the 31 neurons, responses were positively correlated with the participants' voting profiles—there was a gradual increase in the spiking rate as the participants transitioned from viewing the scenes from safe to unsafe
- For 42% of the neurons, spiking activity was negatively correlated with the voting profiles
- Neuronal responses were most prominent during scene presentation, affirming previous research suggesting that subjective decisions are formulated early on during opinion formulation, before motor selection
Neuronal Activity During a Sensory Perceptual Task
Only one neuron displayed a difference in firing activity when the participants were asked to make forced decisions during a sensory-perceptual task.
Localized Disruption of the dlPFC Affected Subjective Decision-making
The researchers then administered the situational assessment task to four participants who had undergone resection of lesions in Brodmann's area 9 of the dlPFC. These participants showed less gradation between the opposing decisions compared with the 11 original participants.
Selective Effect of dlPFC Disruption on Decision-making
When given an opinion-based task (i.e., deciding whether a picture of a face was trustworthy or not), five participants with dlPFC lesions showed a consistently diminished gradation in voting profile compared with 14 individuals who did not have such lesions.
In contrast, lesions in the dlPFC did not appear to affect the participants' choices on the sensory-perceptual task or their motor selection. Thus, the differences between the participants with and without lesions on the opinion-based task do not seem to be attributable to a sensory or motor deficit.
The researchers explain that in decision theory, little gradation between opposing choice options is often called false dichotomous or black-and-white thinking. This tendency to consider options as "right and wrong" or "good and evil" is not reflected in a person's individual choices, but it becomes apparent when evaluating their complete voting profile.
The study results suggest that dlPFC plays an important role in the process by which opinions are formed. Specifically, the findings suggest a process that could allow single neurons to encode both a graded evaluation of the available choice options as well as their unique point of transition at equipoise and, therefore, allow them to reflect an individual's complete voting profile.
Disrupting this process seems to lead to loss of gradation between opposing choice options, but it does not appear to impact the primary mechanisms by which sensory information is processed or motor selections are made.
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