이 기사는 신경과학 분야에서 자유 의지가 실제로 존재하는지, 아니면 뇌의 신호가 우리의 행동을 미리 결정하는지에 대한 깊은 탐구를 다룹니다. 채프먼 대학교의 우리 마오즈(Uri Maoz) 교수는 뇌에서 '준비 전위(Readiness potential)' 신호가 의식적인 결정을 내리기 수 초 전에 나타난다는 기존 연구를 재해석하며 자유 의지 논쟁에 새로운 전기를 마련했습니다. 이 연구는 인간의 의사 결정 과정을 이해하는 데 중요한 철학적, 과학적 질문을 던집니다.
번역된 본문
우리 마오즈(Uri Maoz)는 박사 과정을 밟던 시절 인간을 대상으로 한 연구를 무척 즐겼다. 그는 계산 신경과학에서 매우 구체적인 주제를 연구했다. 즉, 뇌가 팔을 움직이라고 어떻게 지시하는지, 그리고 회백질이 그 움직임을 어떻게 다시 인식하는지를 연구했다. 어느 날 그의 교수가 학부생 강의를 맡아달라고 했다. 마오즈는 지도교수가 해야 할 일을 정확히 알려주거나 적어도 파워포인트 슬라이드라도 건네줄 것이라고 생각했다. 하지만 아니었다. 학생들과 관련만 있다면 마오즈는 무엇이든 자유롭게 가르칠 수 있었다. "사이보그나 뭐 그런 것처럼 인간 두뇌 증강에 대해 이야기할 수도 있었죠." 그렇지만 그 재미있고 공상과학(SF)에 가까운 주제가 그의 머릿속에 자연스럽게 떠오른 것은 아니었다. 그가 흥분하며 떠올린 아이디어는 "자유 의지의 문제에 대해 신경과학이 무슨 말을 할 수 있을까!"였다. 인간이 어떻게, 혹은 정말로 결정을 내리는지(예를 들어, 학부 강의에서 무엇을 논의할지에 대한 결정)는 그가 스무 살 초반에 읽었던 한 기사에서 '어쩌면 인간은 결정을 내리지 않을 수도 있다'고 암시한 이래로 줄곧 그의 머릿속에 맴돌던 문제였다. 이 질문은 자연스럽게 다른 의문점들을 낳았다. 애초에 그 기사를 읽을지 말지 선택할 자유의지가 과연 있었던 걸까? 자신의 인생에서 내린 결정에 대해 자신이 책임이 있는지, 아니면 통제하고 있다는 착각만 했던 건지 어떻게 알 수 있을까? 현재 캘리포니아 채프먼 대학교 교수인 마오즈는 "그후로는 돌아갈 수 없었다"고 말했다. 그는 인간 움직임에 대한 박사 과정 연구를 마쳤지만, 이후 신경 연결망을 더 깊숙이 추적하며 우리의 욕망과 믿음이 어떻게 행동으로 변하는지(팔을 들어 올리는 것부터 금요일 밤 누군가에게 저녁 식사를 제안하는 것까지) 알아보았다. 오늘날 마오즈는 이 신경 연결망이 어떻게 기능하는지(어느 정도, 어쩌면) 답을 찾으려는 시도의 핵심 인물이다. 그의 연구는 정통 신경과학 연구들을 뒤집고 재해석했으며, 자유 의지 문제에 대한 순수 과학적 측면과 철학적 측면을 하나로 묶었다. 무엇보다도 그는 이 논쟁에서 새로운 쟁점을 밝혀내는 데 성공했다.
기계와 마술
자유 의지의 개념은 단순해 보이지만, 보편적으로 받아들여지는 정의는 없다. 직관적인 생각 중 하나는 우리 스스로 결정을 내리고 의도적으로 행동할 수 있는 능력, 즉 우리가 우리 삶을 통제한다는 것이다. 하지만 물리학자들은 우주가 미리 정해진 길을 따라가는 결정론적인지, 그런 우주에서도 인간의 선택이 일어날 수 있는지 질문할 수 있다. 마오즈는 그것은 물리학자들의 질문이라고 말한다. 신경과학자들이 할 수 있는 일은 사람들이 결정을 내릴 때 뇌에서 무슨 일이 일어나는지 알아내는 것이다. "그게 바로 우리가 하려는 일입니다. 우리의 소망, 욕망, 믿음이 어떻게 행동으로 변하는지 이해하는 것이죠."라고 그는 말한다.
마오즈가 박사 학위를 마친 2008년경, 이 문제에 대한 신경과학적 연구는 수십 년째 진행되고 있었다. 1960년대의 한 기초 연구는 사람이 (겉보기에 스스로 결정해서 하는) 손 움직임이 뇌에서 '준비 전위(Readiness potential)'라는 전기 신호가 나타난 뒤에 일어난다는 것을 보여주었다. 이 결과를 바탕으로 1980년대 신경과학자 벤저민 리베트(Benjamin Libet)는 마오즈의 흥미를 처음으로 자극한 실험을 했다. 이 실험은 최근까지도 많은 사람들에게 자유 의지 개념의 종말을 알리는 것으로 해석되었다. 우리 뇌의 전기적 충격은 우리가 정말로 우리 운명의 설계자인지 여부를 밝히는 데 한계가 있다. "그는 사람들이 그냥 앉아 있다가 움직이고 싶을 때 손목을 이렇게 흔들게 했어요." 마오즈가 손목을 꺾으며 말했다. 그런 다음 리베트는 참가자들에게 손을 움직이고 싶다는 충동을 처음 느꼈을 때 화면의 회전하는 점이 어디에 있었는지 물었다. 그는 준비 전위가 손을 움직이기 전에만 나타나는 것이 아니라 움직이고 싶다는 충동을 느꼈다고 보고하기 전, 즉 리베트의 해석에 따르면 자신이 움직일 것이라는 걸 알기 전에 이미 나타났다는 사실을 발견했다. 그 이후의 연구들도 이 관찰 결과를 확인했으며, 참가자가 의식적인 결정을 내렸다고 보고하기 1~2초, 그리고 fMRI 결과에 따르면 최대 10초 전에 준비 전위가 나타난다는 것을 보여주었다. 마오즈는 "이는 우리가 본질적으로 자율주행차의 승객이라는 것을 시사합니다. 그
Uri Maoz loved doing his human research, back when he was getting his PhD. He was studying a very specific topic in computational neuroscience: how the brain instructs our arms to move and how our gray matter in turn perceives that motion. Then his professor asked him to deliver an undergrad lecture. Maoz assumed his boss was going to tell him exactly what to do, or at least throw some PowerPoint slides his way. But no. Maoz had free rein to teach anything, as long as it was relevant to the students. “I could have gone to human brain augmentation,” he says. “Cyborgs or whatever.” Yet that admittedly fun and borderline sci-fi topic wasn’t what popped, unbidden, into his mind. His idea, he recalls with excitement: “What neuroscience has to say about the question of free will!” How—or whether—humans make decisions (like, say, about what to discuss in an undergrad lecture) had been on his mind since he’d read an article in his early twenties suggesting that … maybe they didn’t. This question might naturally beget others: Had he even had a choice about whether to read that article in the first place? How would he ever know if he was responsible for making decisions in his life or if he just had the illusion of control? “After that, there was no turning back,” says Maoz, now a professor at Chapman University, in California. He finished his PhD work in human movement, but afterward he scooted further up the neural chain to find out how desires and beliefs turn into actions—from raising an arm to choosing someone to ask out to dinner on a Friday night. Today, Maoz is a central figure in the attempt to (sort of, maybe) answer how that neural chain functions. His research has since overturned and reinterpreted canonical neuroscience studies and united the straight-scientific and philosophical sides of the free-will question. More than anything, though, he’s succeeded in uncovering new wrinkles in the debate. Machines and magic tricks The concept of free will seems straightforward, but it doesn’t have a universally accepted definition. One intuitive notion is that it’s the ability to make our own decisions and take our own actions on purpose—that we control our lives. But physicists might ask if the universe is deterministic, following a preordained path, and if human choices can still happen in such a universe. That’s a question for them, Maoz says. What neuroscientists can do is figure out what’s going on in the brain when people make decisions. “And that’s what we’re trying to do: to understand how our wishes, desires, beliefs, turn into actions,” he says. By the time Maoz had finished his PhD, in 2008, neuroscientific research into the question had been going on for decades. One foundational study from the 1960s showed that a hand movement—something a person seemingly decides to do—was preceded by the appearance in the brain of an electrical signal called the “readiness potential.” Building on that result, in the 1980s a neuroscientist named Benjamin Libet did the experiment that had first piqued Maoz’s interest in the topic—one that many, until recently, interpreted as a death knell for the concept of free will. An electrical impulse in our brains can shed only so much light on whether we truly are the architects of our own fates. “He just had people sit there, and whenever they feel like it, they would go like this ,” says Maoz, wiggling his wrist. Libet would then ask where a rotating dot was on a screen when they first had the urge to flick. He found that the readiness potential appeared not only before they moved their hand but before they reported having the urge to move—or, in Libet’s interpretation, before they knew they were going to move. Studies since have confirmed the observation and shown that the readiness potential appears a second or two—and maybe, fMRI implies, up to 10 seconds—before participants report making a conscious decision. “It suggests we are essentially passengers in a self-driving car,” says Maoz. “The unconscious biological machine does all the steering, but our conscious mind sits in the driver’s seat and takes the credit.” Maoz initially approached his own research with variations on Libet’s experiments. He worked with epilepsy patients who already had electrodes in their brains, for clinical purposes, and was able to predict which hand they would raise before they raised it. Still, some of the Libet-inspired studies people were doing nagged at him. “All these results were about completely arbitrary decisions. Raise your hand whenever you feel like it,” he says. “Why? No reason.” A decision like that is quite different from, say, choosing to break up with your partner. Try telling someone they weren’t in the driver’s seat for that . The field wasn’t looking at meaningful decisions, he says—the ones that actually set the course of lives. Maoz began pulling in philosophers to help guide his approach. They would challenge him to confront the semantic differences between things like intention, desire, and urge. Neuroscientists have tended to lump those concepts together, but philosophers tease them apart: Desire is a want that doesn’t necessarily progress toward an action; urge carries implications of immediacy and compulsion; and intention involves committing to a plan. (Maoz has come to focus specifically on intention—including, recently, the potential intentions of AI .) In 2017, he organized his first in a series of free-will conferences, drawing many autonomy-interested philosophers. “Thank you so much for coming,” he recalls saying at the opening of the meeting. “As if you had a choice.” One day, the crew took an excursion out on a lake. As the group munched on shrimp, someone joked that they hoped the boat didn’t sink, because everybody in the field would die. The comment didn’t make Maoz feel existential dread. Instead, he figured that if the whole field was already there, why not lasso them all into writing a research grant? “He just thinks what should be the next step and just has a very good ability to just make it happen,” says Liad Mudrik, a neuroscientist at Tel Aviv University and a frequent collaborator. That ability is special among scientists, says Chapman colleague Aaron Schurger, with whom Maoz co-directs the Laboratory for Understanding Consciousness, Intentions, and Decision-Making (LUCID, appropriately). “I really think that Uri is kind of at the nexus of this field right now because he’s really, really good at bringing people together around these big ideas,” he says. Donations and interruptions Maoz has recently been making progress on one of the big ideas that have consistently occupied his working hours: how trivial and significant decisions play out differently in the brain. In collaborations with Mudrik, he’s parsed the neural difference between picking and choosing—their terms for arbitrary decisions and those that change your life and tug on your emotions. Readiness potential? Their measurements didn’t clock it ahead of choices. In 2019, Maoz and a crew published a paper measuring the electrical activity in people’s brains as they pressed a key to choose one of two nonprofits to donate $1,000 to—for real, with actual dollars. Then the researchers compared that activity with what they saw when the same group pressed a key at random to donate $500 each to two nonprofits. The team saw the readiness potential in the arbitrary decision, but not for the $1,000 question. Libet’s result, they concluded, doesn’t apply to the important stuff, which means readiness potential might not actually be a sign that your brain is making a choice before you’re aware of it. “If Libet would have chosen to focus on deliberate decisions, then maybe the entire debate about neuroscience proving free will to be an illusion would have been spared from us,” Mudrik says. Maoz’s research has spurred others to reinterpret Libet’s work. It’s “enriched my thought process a great deal,” says Bianca Ivanof, a psychologist whose di