One of last year's most popular posts was about the idea of free will. Neuroscience-based models like Two Minds Theory are often interpreted as implying that people don't make decisions about their own behavior. And in fact, when it comes to the Narrative mind I would argue that this is true. I have likened the Narrative system's role in behavior to that of a sports commentator: You can listen to it for the play-by-play and get a good idea of what's going on, but it isn't actually moving the ball around the field. Within the largely unconscious Intuitive mind, though, I think there's still plenty of room for free will. I have argued for returning to a form of virtue ethics based on the idea that our choices can be seen in our habitual patterns of responding even if our actions don't result from conscious choice. This week I present a kind of intellectual cage match between two celebrated figures in the history of neuroscience, who have provided us with dramatically different pictures of the Narrative system's role in decision making. It's Phineas Gage versus Benjamin Libet: Who will win?
Phineas Gage (pictured above) has been described as "neuroscience's most famous patient." He was a railroad worker who had a terrible accident in 1848, in which a railroad spike was driven through his skull with severe damage to the forward part of his brain. It's from this case that scientists developed the original idea of executive functioning as a skill dependent on the brain's prefrontal cortex (PFC). Mr. Gage's PFC was badly damaged, and his behavior showed the effects. He became impulsive, aggressive, and profane. Friends and coworkers said that he seemed like a different person. The central theme of the change was that Mr. Gage no longer seemed to have any self-control: Whatever thought came into his head, he acted it out immediately.
This case is one of the key sources of evidence that led researchers to conclude that the PFC was the area of the brain responsible for initiating and governing behavior -- in other words, an "executive" role. An executive is someone who manages, directs, or oversees the work of others, and the PFC was believed to be the brain's executive manager. Other sources of evidence include studies of cocaine users, sleep-deprived individuals, and adolescents, all of whom show decreased PFC activity as well as difficulties regulating their own behavior. Findings regarding the PFC are strong and consistent, and perfectly in line with the anecdotal observations of Mr. Gage's friends. People with damaged or inactive PFCs are impulsive, socially inappropriate, and prone to taking dangerous risks.
So the PFC is the seat of reason, right? It's Plato's charioteer, the human soul that keeps the horses of passion and reason running in the same direction? Not so fast. In the other corner of this match-up we have Benjamin Libet (pictured above), a modern neuroscience researcher who provided important new data about the PFC. The graph to the right of Dr. Libet's picture illustrates the time between the start of a neural impulse (the "reaction potential" or RP) and an action. The neural signal begins about 500 milliseconds before the actual action begins. In this study, participants were asked to also indicate when they decided to move their hand, in a paradigm where they were asked to do so "whenever they felt like it." To test how early the conscious wish occurred, participants continuously watched a red dot circling on a screen, and noted the position at the time when they felt the urge to move their hand. In that way, no additional action was required; when the participant later reported the position of the dot, the researcher could work backwards to determine at what precise time the dot had been in that location. The shocking finding of Libet's research was that people's conscious awareness of an intention to move occurred only 200 milliseconds before the action began, more than 300 milliseconds after electrical sensors showed their motor neurons already starting to fire. Libet's findings are both revolutionary and controversial. But overall, many people have used these results to argue that free will is an illusion.
Two Minds Theory suggests that the PFC is heavily involved in the Narrative System, which involves slow, logical, and methodical thought -- what Daniel Kahneman called "system 2," humans' higher rational mind. On first glance, that might seem to line us up on the side of Phineas Gage in an argument about free will. And without a functioning PFC, people clearly do act irrationally. But Libet himself argued that the PFC still has a veto role over behavioral impulses that originate elsewhere in the brain. For instance, perhaps the impulse to move at -500 ms is actually the start of the behavior, but the conscious awareness at -200 ms gives the person just enough time to inhibit the movement if they so choose. Two Minds Theory suggests a temporal delay between our two mental systems, in which the Intuitive mind triggers an immediate response to a situation, and the Narrative mind only later has a chance to weigh in. Libet's data fit well with at least the timing of the Narrative response.
My own argument is that Libet's study does rule out Narrative-system control over behavior, or conscious free will. Our actions are not based on what we think in words, and therefore the Narrative mind can't be considered an "executive" in any sense. But Libet's study doesn't invalidate the idea of non-conscious free will, and it does still suggest a role for the PFC. One clear strength of the Narrative system is social imagination, the ability to envision possible futures and the way that people might interact with us based on a proposed course of action. What might happen is that the Intuitive mind initiates a behavior, but before it is actually produced the Intuitive mind also makes a quick search of our established Narratives to make sure it isn't going to do anything with the potential for disastrous consequences.
Essentially, the Narrative mind can serve as a censor jumping in at the last minute to prevent us from saying or doing something disastrous. If we aren't able to properly develop Narratives because of Gage-type damage to the PFC, or if our Intuitive minds forget to make that last check before behavior, we will tend to be more impulsive and inappropriate as a result. Each mental system does have an important role in behavior. But we also need to be aware that "executive functioning" isn't quite what it sounds like, because our conscious Narrative minds are not directing the action.
Comments
Post a Comment