The Mind and the Brain (39 page)

That idea was to find a way to pinpoint the moment when a person became aware of the conscious desire to act. In experiments he reported in 1982 and 1985, Libet asked volunteers to decide to flick or flex their wrist whenever they chose. These movements were to be performed, as Libet put it, “capriciously, free of any external limitations or restrictions.” Devices on the subjects’ scalps detected the readiness potential that marks neuronal events associated with preparation for movement. Libet found that this readiness potential began, on average, 550 milliseconds before the activation of the muscles moving the wrist. But not all readiness potentials were followed by movements. “The brain was evidently beginning the volitional process in this voluntary act well before the activation of the muscle that produced the movement,” Libet noted in 1999. That is, the readiness potential he was detecting appeared too long before muscle activation to correspond directly with a motor command to the muscle.

What, then, was this odd cerebral signal, which seemed to be acting as a sort of advance scout blazing a trail for the motor command? Libet had instructed his subjects to move the wrist any time they had an urge to do so. His next—and key—question became, When does the conscious intention to execute some movement
arise? According to the traditional view of will as something that initiates action, this sense of volition would have to appear before the onset of the readiness potential, or at worst coincidently with it; otherwise the neuronal train would have left the station, as it were, before the will could get into the act. In that case, will would be pretty wimpish, merely assenting to an action that was already under way. But 550 milliseconds is, neuronally speaking, an eternity: “An appearance of conscious will 550 msec or more before the act seemed intuitively unlikely,” Libet thought, preceding the act by way too long an interval to make sense. Was it possible, instead, that conscious will
followed
the onset of the readiness potential? If so, “that would have a fundamental impact on how we could view free will.”

In his next experiments, Libet therefore tried to establish when will showed up. At first, measuring the onset of will “seemed to me an impossible goal,” he recalls. But after giving the matter some thought, he decided to ask subjects, sitting in chairs, to note the position of the second hand on a clock at precisely the moment when they first became aware of the intention to move. Because he was dealing in intervals of less than a second, Libet knew that an ordinary sweep second hand would not suffice. He needed something faster. He came up with the idea of using a spot of light on the face of an oscilloscope. The spot swept around like a second hand, but twenty-five times faster. Each marked-off “second” on the oscilloscope’s face therefore amounted to 40 milliseconds. Although this might seem to present a stiff challenge to anyone trying to pinpoint the position of the spot of light, in a dry run Libet found that subjects (including him) were pretty accurate in their readings: when he gave them a weak electrical jolt to the skin and asked them what time the spotlight indicated, the subjects got it right to within 50 milliseconds. “We were ready to go,” he says.

Following Libet’s instructions, all of the five subjects flicked their wrist whenever the spirit (or something) moved them. They also reported where the oscilloscope spot was when they first became
aware of the will to move. Libet compared that self-report with concurrent measurements of the onset of the readiness potential. The results of forty trials—which have since been replicated by other researchers—are straightforward to relate, if difficult to interpret. The readiness potential again appeared roughly 550 milliseconds before the muscle moved. Awareness of the decision to act occurred about 100 to 200 milliseconds before the muscle moved. Simple subtraction gives a fascinating result: the slowly building readiness potential appears some 350 milliseconds before the subject becomes consciously aware of his decision to move. This observation, which held for all of the five subjects in each of the six sessions of forty trials, made it seem for all the world as if the initial cerebral activity (the readiness potential) associated with a willed act was unconscious. The readiness potential precedes a voluntary act by some 550 milliseconds. Consciousness of the intention to move appears some 100 to 200 milliseconds before the muscle is activated—and about 350 milliseconds after the onset of the readiness potential.

Libet thus produced the first experimental support for the version of free will that Richard Gregory famously called “free won’t.” At first glance, the detection of a readiness potential before consciousness of the wish to act appears to bury free will: after all, cortical activity leading to a movement is well under way before the subject makes what he thinks is a conscious decision to act. The neuronal train has indeed left the station. If free will exists, it seems to be like a late passenger running beside the tracks and ineffectually calling, “Wait! Wait!” Yet Libet does not interpret his work as proving free will a convenient fiction. For one thing, the 150 or so milliseconds between the conscious appearance of will and the muscle movement “allow[s] enough time in which the conscious function might affect the final outcome of the volitional process,” he notes. Although his results have been widely and vigorously debated, one interpretation with significant experimental support is this: there exists conscious cerebral activity whose role may be
“blocking or vetoing the volitional process so that no actual motor action occurs,” as Libet wrote in 1998. “Veto of an urge to act is a common experience for individuals generally.” It is also, of course, the essence of mindfulness-based OCD treatment and reaffirms Sherrington’s insight that “to refrain from an act is no less an act than to commit one”: thus, “free won’t.”

Experiments published in 1983 clearly showed that subjects could choose not to perform a movement that was on the cusp of occurring (that is, that their brain was preparing to make) and that was preceded by a large readiness potential. In this view, although the physical sensation of an urge to move is initiated unconsciously, will can still control the outcome by vetoing the action. Later researchers, in fact, reported readiness potentials that precede a planned foot movement not by mere milliseconds but by almost two full seconds, leaving free won’t an even larger window of opportunity. “Conscious will could thus affect the outcome of the volitional process even though the latter was initiated by unconscious cerebral processes,” Libet says. “Conscious will might block or veto the process, so that no act occurs.” Everyone, Libet continues, has had the experience of “vetoing a spontaneous urge to perform some act. This often occurs when the urge to act involves some socially unacceptable consequence, like an urge to shout some obscenity at the professor.” Volunteers report something quite consistent with this view of the will as wielding veto power. Sometimes, they told Libet, a conscious urge to move seemed to bubble up from somewhere, but they suppressed it. Although the possibility of moving gets under way some 350 milliseconds before the subject experiences the will to move, that sense of will nevertheless kicks in 150 to 200 milliseconds before the muscle moves—and with it the power to call a halt to the proceedings. Libet’s findings suggest that free will operates not to initiate a voluntary act but to allow or suppress it. “We may view the unconscious initiatives for voluntary actions as ‘bubbling up’ in the brain,” he explains. “The conscious will then selects which of these initiatives may go for
ward to an action or which ones to veto and abort…. This kind of role for free will is actually in accord with religious and ethical strictures. These commonly advocate that you ‘control yourself.’ Most of the Ten Commandments are ‘do not’ orders.” And all five of the basic moral precepts of Buddhism are restraints: refraining from killing, from lying, from stealing, from sexual misconduct, from intoxicants. In the Buddha’s famous dictum, “Restraint everywhere is excellent.”

The evolution of Libet’s thoughts about his own experiments mirrors that of neuroscience as a whole about the reality of volition. Libet had long shied from associating his findings with free will. For years he refused even to include the words in his papers and resisted drawing any deeper conclusions from his results. At the 1994 “Toward a Scientific Basis of Consciousness” conference (Tucson I), Libet was asked whether his results could be used to support the existence of free will. “I’ve always been able to avoid that question,” he demurred. But in later years he embraced the notion that free will serves as the gatekeeper for thoughts bubbling up from the brain and did not duck the moral implications of that. “Our experimental work in voluntary action led to inferences about responsibility and free will,” he explained in late 2000. “Since the volitional process is initiated in the brain unconsciously, one cannot be held to feel guilty or sinful for simply having an urge or wish to do something asocial. But conscious control over the possible act is available, making people responsible for their actions. The unconscious initiation of a voluntary act provides direct evidence for the brain’s role in unconscious mental processes. I, as an experimental scientist, am led to suggest that true free will is a [more accurate scientific description] than determinism.”

This may seem an enfeebled sort of free will, if it does not initiate actions but only censors them. And yet the common notion of free will assumes the possibility of acting otherwise in the same circumstances, of choosing not to perform actions that tempt us each and every day. By “the possibility of acting otherwise,” I mean not
that possibility as judged by an outside observer, one who might sneer that, well, you didn’t have to scream at me. I mean, instead, that the possibility of an alternative action is one that you feel as more than theoretical. It must be one that you consider, even if only briefly, before acting. As a matter of fact, William James believed that will seized the moment after the first thought about an intended action, but before the actual action. Consistent with his feeling that “volition is nothing but attention,” James argued that the ability to “emphasize, reinforce or protract” certain thoughts at the expense of others percolating into consciousness—an ability he identified with attention—manifests itself as will. So for James, too, will derives not from the freedom to initiate thoughts, but to focus on and select some while stifling, blocking—or vetoing—others. For Buddhist mindfulness practice, it is the moment of restraint that allows mindful awareness to take hold and deepen. The essence of directed mental force is first to stop the grinding machine-like automaticity of the urge to act. Only then can the wisdom of the prefrontal cortex be actively engaged.

Free will as gatekeeper raises a deeper question: how does the gatekeeper decide which thoughts to let pass and which to turn back, which to allow to be expressed and which to veto unceremoniously? Libet himself concedes that although his discovery of the 550-millisecond gap offers a hint of how free will operates, it does not address whether our willed actions are strictly determined by the prior history and state of the neurons in our brain, or whether the will to action is truly free—by which I mean not reducible to and predictable from material processes. The initiatives that bubble up in the brain are, he suspects, based on the person’s past—memories, experiences, the values transmitted by the larger society—as well as on present circumstances. If willed actions are strictly determined, and if the brain’s veto of possible actions is strictly determined by neural antecedents, then we are right back where we started, with (presumably) unconscious neural states’ calling all the shots. Such “free” will seems hardly worth having,
and we find ourselves once again in a purgatory where our brains or our genes control our actions as a puppeteer controls a marionette. But Libet insists that such is not the case. “I propose…that our conscious veto may not require or be the direct result of preceding unconscious processes,” he declared. “There is no logical imperative in any mind-brain theory, even identity theory, that requires specific neural activity to precede and determine the nature of a conscious control function. And there is no experimental evidence against the possibility that the control process may appear without development by prior unconscious processes.”

Libet turned eighty-five in 2001, and he had lost none of his fire. He seemed resigned, though, to remain a voice in the wilderness. “Most neuroscientists shy away from my argument invoking free will and a mental field that are not encompassed by existing physical law,” he says with a hint of a grin.

Both Buddhist and William James’s philosophy are quite consistent with this interpretation of volition. In Buddhism, the quality of awareness or attention determines the nature of the consciousness that arises, and thus the action (karma) that takes place. The only willful choice one has is the quality of attention one gives to a thought at any moment. Similarly, James believed that “th[e] strain of attention is the fundamental act of will.” And in the Four Steps, of course, to Refocus mindfully away from a destructive obsession
or compulsive urge and onto a benign object of attention is the core volitional act, as I will describe further in the next chapter.

Libet’s discovery of the 550-millisecond gap in the mid-1980s launched a thousand symposia and inspired a neuroscience of volition. Typically, considering how enamored brain scientists are of mapping the regions that correspond to mental events, they have had a field day (or decades) recording cerebral activity during willed acts. As early as 1977, for instance, researchers led by the Swedish physiologist David Ingvar had volunteers first automatically and rhythmically clench their hand, and then imagine doing the same act without moving their hand. Measuring cerebral blood flow, which serves as a proxy of neuronal activity, they found activation of motor cortex during automatic hand clenching. In addition, and quite markedly, the prefrontal cortex was activated during the willful mental activity. Many subsequent studies have similarly found that willed mental effort results in prefrontal cortex activation. In schizophrenics who show symptoms of a “sick will,” which is marked by autistic behavior and inactivity, the dorsolateral prefrontal cortex shows lower-than-normal activity. In depression, one symptom of which is a lack of initiative (will?), three decades of brain mapping have shown consistently low activity in the prefrontal cortex. This has led to the suspicion that the prefrontal cortex houses, at minimum, what Ingvar calls “action programs for willed acts.”