Demystifying the Mind
This feature is the second installment in a three-part series on the scientific struggle to explain consciousness. To read the previous installment and see what’s in the next issue, click here.
In one of science’s most iconic moments, Isaac Newton’s eye caught the red glint of an apple as it plunged toward the ground. He heard the leaves rustle in the light breeze and felt the warmth of the tea he was drinking at the time.
These sensory inputs streamed into his brain, where they met his vast stores of knowledge, his internal musings, his peculiar brand of curiosity and perhaps even a fond recollection of escaping the ground’s hold while climbing a tree as a boy. All at once, sights, sounds, emotions and memories converged to form a whole, rich experience in the garden that day.
It was this fortuitous experience — perfectly ripe for a big idea — that (legend has it) caused Newton to wonder why the apple fell not sideways or even upward, but straight down. Inspiration struck, ushering in a new understanding of gravity.
Newton gets the glory for figuring out that the same mysterious force pulls planets toward the sun and apples toward Earth, but how he did it hinges on an even deeper mystery: How his brain created a single, seamless experience from a chaotic flux of internal and external messages.
And that mystery isn’t confined to brains like Newton’s. In all conscious people, the brain somehow gives meaning to the external environment, allowing for thought, self-reflection and discovery. “It’s not that conscious experience is one little interesting phenomenon,” says neuroscientist Ralph Adolphs of Caltech. “It’s literally the whole world.”
Understanding how a rich inner experience emerges from fragments of data is a gargantuan task, akin to understanding the entire U.S. economy by mapping how all of its money flows. But rather than looking at the whole financial picture, insights can be gained from tracking a single dollar by its serial number as it jumps from wallet to cash register to bank.
Similarly, scientists are attempting to clarify the path that leads to consciousness by following a single, bite-sized piece of information — the redness of an apple, for instance — as it moves into a person’s inner mind.
Recent research into the visual system suggests that a sight simply passing through the requisite vision channels in the brain isn’t enough for an experience to form. Studies that delicately divorce awareness from the related, but distinct, process of attention call into question the role of one of the key stops on the vision pipeline in creating conscious experience.
Other experiments that create the sensation of touch or hearing through sight alone hint at the way in which different kinds of inputs come together. So far, scientists haven’t followed enough individual paths to get a full picture. But they are hot on the trail, finding clues to how the brain builds conscious experience.
The mind’s eye
One of the best-understood systems in the brain is the complex network of nerve cells and structures that allow a person to see. Imprints on cells in the eye’s retina get shuttled to the thalamus, to the back of the brain and then up the ranks to increasingly specialized cells where color, motion, location and identity of objects are discerned.
After decades of research, today’s map of the vision system looks like a bowl of spaghetti thrown on the floor, with long, elegant lines connected by knotty tangles. But there’s an underlying method in this ocular madness: Information appears to flow in a prescribed direction.
After planting a vision in a person’s retina, scientists can then watch how one image moves through the brain. By asking viewers when they become aware of the vision, researchers may pinpoint where along the pipeline it pops into consciousness.
One of the most fiercely debated stops for visual information is a small patch of wrinkles at the very back of the brain called the primary visual cortex, or V1. By virtue of its prestigious locale on the cortex — the sophisticated outer shell of the brain where thoughts are formed — V1 seems to many like a reasonable place for visual consciousness to arise.
But some researchers argue that V1 is too simple: Instead of the final authority, V1 may be a relay station that conveys the message to higher-ups, more specialized brain regions that have their say in what’s conscious and what’s not. “V1 is kind of the battlefield,” says Masataka Watanabe of the Max Planck Institute for Biological Cybernetics in Tübingen, Germany.
A key way to study V1 is to make an object visible to the retina while keeping it outside the mind’s awareness. The eyes may see the object just fine, but the brain will completely miss it.
Scientists can do this by showing one picture to one eye and a different picture to the other. Because it’s impossible to integrate the two images into a single vision, people toggle back and forth. Input to each eye’s retina holds steady, while perception — whether an image pops into awareness — flips back and forth. Scientists can measure brain activity to track this perceptual switch.
As expected, cells in each retina react to the information the same way, regardless of which image the person perceives. “This eyeball doesn’t really care if the brain behind it is conscious or not,” says neuroscientist Christof Koch of Caltech and the Allen Institute for Brain Science in Seattle.
After a quick stop-off at the thalamus, the info heads to V1, where the story gets complicated. Functional MRI scans, which measure big changes in blood flow, have found that activity in V1 tracks with toggling perceptions. But other data call V1’s gatekeeper role into question. For instance, a technique that uses electrodes to measure the behavior of nerve cells found that V1 behavior did not change as perception switched.
Such conflicting findings stymied scientists until two new studies carefully separated out the confounding effects of a contaminator: attention. Attention is the focusing of the mind on a particular subject, often described as the mind’s spotlight.
“If you really want to understand consciousness, then you need to separate the effects of attention,” says neuroscientist Naotsugu Tsuchiya of Monash University in Australia.
Separating the two effects wasn’t an issue until recently, because people believed them to be the same thing. After all, the two often go together: Focusing on a tart, crunchy bite of a Granny Smith makes the experience more tangible, enhancing your awareness of it. And when attention is diverted, obvious things escape detection — a slip called inattentional blindness. The most famous example comes from a study in which observers are asked to count the number of times a basketball is passed between people. Engrossed in the ball-watching, many viewers are oblivious to a man in a gorilla suit who ambles into the middle of the scene, beats his chest and ambles out.
But just because consciousness and attention are often linked doesn’t mean that they are the same thing. A gripping demonstration comes from a recent experiment’s subliminal spiders. Participants who were terrified of spiders watched a screen as pictures of a spider or an outdoor scene flashed for 20 milliseconds — a split second that many scientists think is too fast to detect. None of the subjects could report what was flashed, indicating that the people who saw the spiders were not conscious of them.
Later, the participants were asked to walk into a room and touch a real tarantula. People who were exposed to the spider pictures got closer to the real spider than those who saw the natural scenes. The subliminal spiders desensitized people and reduced their fear, even though the participants were oblivious, psychologist Joel Weinberger of Adelphi University in Garden City, N.Y., and colleagues reported last year in Consciousness and Cognition. Though the pictures flashed too quickly to break into consciousness, the mind took note.
Most scientists accept that attention can occur in the absence of awareness. But evidence for the opposite idea, that conscious awareness can exist without attention, has been less clear. Some studies suggest that a person can report the gist of a scene — describing whether it’s a library or a garden, for example — even when a huge chunk of attention is siphoned away by a demanding task.
A recent study by neuroscientist Jeroen van Boxtel of UCLA and colleagues set out to cleanly separate consciousness and attention. Van Boxtel and colleagues study afterimages, the phenomenon at work when a person who has stared at a green square for a minute shifts the eyes to a white screen and a ghostly pink square floats where the green square had been. In the study, participants either devoted their full attention to a part of a screen where a half-black, half-white circle would cause an afterimage, or had their attention distracted by a counting job. At the same time, the team manipulated the participants’ awareness of the afterimage-inducing circle by showing it to only one eye while flashing a checkerboard pattern to the other eye. When the mind toggles to this dazzling pattern, the circle remains outside of consciousness.
In this way, participants could attend to something they didn’t see (attention without consciousness) and see something they didn’t attend to (consciousness without attention).
At the end of the experiment, once all the signals on the screen were turned off, the volunteers indicated how long the aftereffect lasted. Surprisingly, attention and awareness had different effects, the team reported in 2010 in the Proceedings of the National Academy of Sciences: The more attention a person deployed, the shorter the afterimage lasted. The more conscious a person was of the stimulus, the longer the afterimage lasted.
Though some scientists are still skeptical, these results and others like them appear to separate attention and consciousness, meaning each may have its own role in the brain. “The attentional spotlight picks out aspects of the environment and highlights them,” says van Boxtel. In contrast, consciousness may be a synthesizer that merges bits of information into a broader picture.
Recent studies designed specifically to clarify V1’s job show that when attention is removed from the equation, V1 behavior is no longer tied to consciousness.
In an fMRI study reported in the Nov. 11 Science, participants were either made aware of a striped pattern on a screen or not. Then, they either focused attention to the pattern or away from it. Like the study on aftereffects, this setup allowed scientists to separate awareness from attention. The fMRI signal in V1 didn’t change as the stripes became visible or invisible if attention held steady. But when attention shifted, so did V1 activity, Watanabe’s team reported.
More evidence comes from a recent monkey study, presented by neuroscientist Alexander Maier of Vanderbilt University in Nashville, Tenn., and colleagues last year at the annual meeting of the Society for Neuroscience. In this experiment, electrodes measured V1 nerve cell activity as monkeys either perceived or didn’t perceive an object, while either paying or not paying attention.
With fMRI studies and electrode studies in agreement, the results suggest that visual signals from the outside world don’t break into consciousness as soon as they reach V1. The same may be true for corresponding early cortical stops for auditory, touch and other kinds of sensory input.
“The separation of attention and consciousness is a beautiful example to show that you can make progress on this difficult mind-body problem,” Koch says. What’s more, the split calls for a careful examination of earlier studies to make sure attention wasn’t behind any results that were attributed to consciousness, he and Tsuchiya write in the February Trends in Cognitive Sciences.
Messages from the top
But the new findings don’t mean V1 isn’t important for consciousness. Signals leaving V1 flow on to increasingly complex areas: places in the cortex where memories, skills and thoughts reside. When these signals then travel back to the simpler cortical regions, conscious experience emerges, proposes cognitive neuroscientist Kaspar Meyer of the University of Southern California in Los Angeles.
Perhaps the best evidence for Meyer’s idea comes from his work on how a small bit of information that stimulates one sense can generate a sensation elsewhere. While watching video clips of someone handling a ball of yarn, volunteers’ brains filled in a sort of “mind’s touch,” re-creating the sensation of the soft yarn, Meyer and colleagues reported last year in Cerebral Cortex.
A similar effect showed up for vision and hearing. Watching silent movies that are suggestive of a noise — a crowing rooster or a finger hitting a piano key — spurred a person to have the experience of hearing that noise, the team reported in 2010. What’s more, videos of sounds that were more evocative to people (such as a howling dog to dog-lovers) had a stronger effect on the brain.
These findings suggest that the path from the outside world to the brain’s inner experience is not a straight line. Scientists don’t yet know all the stops along that path, or which are most important (although candidates for such stops have been identified, such as a cortical wrinkle called the superior temporal sulcus).
What is now clear is that the brain is not a stimulus-driven robot that directly translates the outer world into a conscious experience. “What we’re conscious of is what the brain makes us be conscious of,” Meyer says. “While the images we experience may be influenced to a certain degree by information that’s incoming, we need to get away from the idea that they reflect exactly what’s out there.”
In the absence of incoming signals, bits of memories tucked away can be enough for a brain to get started with. That ability is on display every time someone imagines anything or dreams. A person sitting in a silent dark room can vividly picture mom’s face, even if she is thousands of miles away. And dreams can be incredibly vivid even when they aren’t linked to sensory inputs. “That tells us that your brain, in the absence of any outside input, can generate this movie that appears totally realistic to you,” Meyer says.
In the Jan. 27 Science, Meyer proposed that, more generally, all conscious experiences could be thought of as what Nobel laureate and neuroscientist Gerald Edelman calls a “remembered present.” From tiny slivers of sensations, scraps of memories and flashes of emotion, the mind makes something much bigger. In the blink of an eye, the brain creates the entire world.