The Mind and the Brain (42 page)

It often takes real effort to maintain the appropriate focus, which is why it takes so much concentration to get into the proper exit lane at a complicated freeway interchange. But once you muster the appropriate focus, you can literally direct your brain to filter out the suppressive effects of distracting signals. Willfully directed attention can filter out unwanted information—another example of how directed mental force, generated by the effort of directed attention, can modulate neuronal function.

When it comes to determining what the brain will process, the mind (through the mechanism of selective attention) is at least as strong as the novelty or relevance of the stimulus itself. In fact, attention can even work its magic in the total absence of sensory stimuli. If an experimenter teaches a monkey to pay attention to a certain quadrant of a video screen, then single-cell recordings find that neurons responsible for that area will fire 30 to 40 percent more often than otherwise, even when there is no there there—even, that is, when that quadrant is empty. So here again we have the mental act of paying attention acting on the activity of brain
circuits, in this case turning them up before the appearance of a stimulus. fMRIs find that activity spikes in human brains, too, when volunteers wait expectantly for an object to appear in a portion of a video monitor. Even before an object appears, attention has already stacked the neuronal deck, activating the visual cortex and, even more strongly, the frontal and parietal lobes—the regions of the brain where attention seems to originate. As a result, when the stimulus finally shows up it evokes an even greater response in the visual cortex than if attention had not primed the brain. This, says Robert Desimone (who happens to also be Leslie Ungerleider’s husband), “is the most interesting finding. In attention without a visual stimulus, you get activation in the same cells that would respond to that stimulus, as if the cells are primed. You also get activation in the prefrontal cortex and parietal lobes. That seems like strong evidence that these lobes exert top-down control on what the sensory system processes.” To summarize, then, selective attention—reflecting willful activation of one circuit over another—can nudge the brain into processing one signal and not another.

Much of what neuroscientists have learned about attention lately has come from brain imaging. As in so many other areas of neurobiology, imaging beckons with the siren call of finding “the neural correlates of…”: that is, pinpointing activity in some part of the brain that corresponds to a mental activity. And although I am the last person to equate brain states, or areas of neuronal activity, with attention or any other mental act or experience, it is worth exploring the results of imaging for what they tell us about what is happening in the brain (and where it’s taking place) when we pay attention. Briefly, these imaging studies have shown that there is no single attention center in the brain. Rather, there are multiple distributed systems, including those in the prefrontal cortex (involved in task-related memory and planning), parietal cortex (bodily and environmental awareness), and anterior cingulate (motivation). Also activated are the underlying cerebellum and basal ganglia (habit formation and coordination of movement). That’s all very
nice, but it doesn’t really tell us much about how attention works (that’s the trouble with the neural-correlates approach). Fortunately some brain imaging studies have gone beyond this, to reveal some truly interesting things about attention.

In 1990, researchers led by Maurizio Corbetta at Washington University went beyond the monkey work to study attention in humans, showing that when you pay attention to something, the part of your brain that processes that something becomes more active. The scientists’ subjects watched a computer screen while an array of a dozen identical little boxes appeared for 400 milliseconds. After a 200-millisecond pause, another screen, also filled with geometric shapes, appeared. Half the time, the first and second frames were identical; half the time they differed in one feature or more, such as color or shape or motion of the elements. The volunteers were sometimes told to determine whether the two succeeding images differed at all, and sometimes told to determine whether the images differed specifically in color, in shape, or in motion. Looking for any old difference is an example of “divided attention,” in that subjects have to pay attention to more than a single attribute in their visual field, searching and scanning to find a difference. Focusing on a specific attribute, on the other hand, requires “selective attention.”

As you might expect, when the volunteers focused attention on a single attribute (“Are the colors of these objects different from the ones you just saw?”), they did much better at identifying how the second screen differed from the first than when they divided their attention among several attributes (“What’s different here?”). But then the study turned up what has become a key finding in the science of attention. Active, focused attention to a specific attribute such as color, they discovered, ramps up the activity of brain regions that process color. In other words, the parts of the brain that process color in an automatic, “hard-wired” way are significantly and specifically activated by the willful act of focusing on color. Activity in brain areas that passively process motion are
amplified when volunteers focus attention on motion; areas that passively process shape get ramped up when the volunteers focus on shape. Brain activity in a circuit that is physiologically dedicated to a particular task is markedly amplified by the mental act of focusing attention on the feature that the circuit is hard-wired to process. In addition, during the directing of such selective attention, the prefrontal cortex is activated. As we saw in Chapter 9, this is also the brain region implicated in volition or, as we are seeing, in directing and focusing attention’s beam.

The following year, another team of neuroscientists confirmed that attention exerts real, physical effects. This time, they looked not for increased neuronal activity but for something that often goes along with it: blood flow. After all, blood carries oxygen to neurons just as it does to every other cell in the body. Just as a muscle engaged in strenuous aerobic activity is a glutton for oxygen, so a neuron that’s firing away needs a voluminous supply of the stuff. In the 1991 experiment, some subjects were instructed to pay attention to vibrations applied to the tips of their fingers, while others were not. The researchers found that, in the subjects paying attention to the vibrations, activation in the somatosensory cortex region representing the fingertips increased 13 percent compared to activation in subjects receiving the identical stimulation but not paying attention. It was another early hint that paying attention to some attribute of the sensed world—colors, movements, shapes, faces, feels, or anything else—affects the regions of the brain that passively process that stimulus. Attention, then, is not some fuzzy, ethereal concept. It acts back on the physical structure and activity of the brain.

Attending to one sense, such as vision, does not simply kick up the activity in the region of the brain in charge of that sense. It also reduces activity in regions responsible for other senses. If you are really concentrating on the little black lines and curves on this white page, you are less likely to feel someone brush against you, or to hear someone speaking in the background. When you watch a
ballet, if you’re focusing on the choreography, you don’t hear the music so well. If you’re deep in conversation at a noisy party and your partner in dialogue has a deep baritone voice, it is probable that those parts of your auditory cortex that are tuned to low frequency will get an extra activation boost; at the same time, regions of the auditory cortex that process sopranos are likely turned down, with the result that you may literally not hear (that is, be conscious of) a high-pitched voice across the room. Attention, as the neuroscientist Ian Robertson of Trinity College Dublin says, “can sculpt brain activity by turning up or down the rate at which particular sets of synapses fire. And since we know that firing a set of synapses again and again makes [them] grow…stronger, it follows that attention is an important ingredient” for neuroplasticity, a point we will return to later. For now, it is enough simply to emphasize that paying attention to a particular mode of sensation increases cerebral activity in the brain region that registers that sensation. More generally, the way an individual willfully focuses attention has systematic effects on brain function, amplifying activity in particular brain circuits.

A growing body of evidence demonstrates that mindfulness itself may be a key factor in the activating process. In one fascinating experiment, Dick Passingham of Oxford University and colleagues at London’s Institute of Neurology compared the brain activity of a young man as he tried to figure out a mystery sequence on a keypad, to the brain activity after he had mastered it. All the man was told was that he had to figure out which sequence of eight keys was correct. He did it by trial and error: when he pressed an incorrect key, a low-pitched tone sounded, much as hearing a sour note tells you that you have hit the wrong key on a piano. When he pressed a correct one, a high-pitched tone sounded. Now he both had to remember the correct key and figure out the next one, and the next six after that. Throughout his trial-and-error ordeal, PET scans showed, the man’s brain was ablaze with activity. In particu
lar, the prefrontal cortex, parietal cortex, anterior cingulate, caudate, and cerebellum were very active; all are involved in planning, thinking, and moving.

When the young man finally worked out the correct sequence, he was instructed to keep tapping it out until he could do so effortlessly and without error. After an hour, though he was beginning to rebel at the boredom of it all, his fingers could fly over the keypad as if on automatic pilot. In fact, they were: he could tap out the sequence flawlessly while verbally repeating strings of six digits, or even while generating lists of verbs. The effortless automaticity was reflected in a marked change in his brain: according to the PET scan, the man’s brain had shut off the lights in numerous regions as if they were offices at quitting time. Although his brain was still remembering the eight keys in order, and signaling the fingers how to move, the mental and cerebral activity behind that output had diminished dramatically. Only motor regions, which command the fingers to move, remained active.

Passingham then took the experimental step that really caught my eye because of its implications for my own nascent theory of directed mental force. What happens in the brain, he asked, if the person carrying out an automatic task suddenly makes a special effort to pay attention to that task? The PET scan kicked out the answer. When the young man again focused on the now-automatic keypad movements, his prefrontal cortex and anterior cingulate jerked awake, becoming metabolically active once again. This is a finding of tremendous importance, for it shows that mindful awareness has an activating effect on the brain, lighting it up. The take-home message of Passingham’s studies is that willfully engaging in mindful awareness while performing an automatic task activates the action-monitoring circuitry of the prefrontal cortex. It is this activation that can transform us from automatons to members in good standing of the species
Homo sapiens
(from Latin
sapere
, “to be wise”). Given the strong evidence for the involvement of the pre
frontal cortex in the willful selection of self-initiated responses, the importance of knowing we can modulate the brain activity in that very area with a healthy dose of mindfulness can’t be overstated.

More evidence for the capacity of willfully directed attention to activate a specialized brain region has come from Nancy Kanwisher’s lab at MIT. She and others had already demonstrated that a specific brain area, located where the temporal and occipital lobes meet, is specialized for processing the appearance of faces. Kanwisher had named this the
fusiform face area
. Does the appearance of a face activate this area automatically, or can you modulate that activity through attention? To find out, Kanwisher’s team had eight volunteers view a screen that briefly displayed two faces and two houses simultaneously. Before the images appeared, the researchers told each volunteer to take note of the faces in some trials, or of the houses in others. All four images appeared each time but stayed on the screen for a mere one-fifth of a second. Then the volunteers had to determine whether the cued items (faces or houses) were a matching pair. They were able to do this accurately a little more than three-quarters of the time. The key finding: the brain’s specialized face-detecting area was significantly more activated when the subjects were actively looking at faces to see whether they matched than when the faces were only viewed passively because the houses were the cued target. In other words, although both the faces and the houses impinged on the retina and the rest of the visual system (including the fusiform face area), choosing actively to focus attention on the face instantly ramped up activity in the brain’s specialized face-recognition area. Its activity, that is, is not strictly automatic, “but depends instead on the allocation of voluntary attention,” as the MIT team stated it. Their subsequent work has shown that attention can also ramp up activity in the brain’s specialized area for recognizing places, including houses and buildings. And it’s not only attention to the outside world that reaches us through our senses that causes such increased activity. Similar activations occur when you conjure up an image in your mind’s eye.
Thus the willful act of forming a mental image of a familiar face or place with your eyes closed selectively activates the very same face or place area of the brain that seeing the face or place with your eyes does. “We are not passive recipients but active participants in our own process of perception,” Kanwisher summed up.

It is pretty clear, then, that attention can control the brain’s sensory processing. But it can do something else, too, something that we only hinted at in our discussion of neuroplasticity. It is a commonplace observation that our perceptions and actions do not take place in a vacuum. Rather, they occur on a stage set that has been concocted from the furniture of our minds. If your mind has been primed with the theory of
pointillism
(the use of tiny dots of primary colors to generate secondary colors), then you will see a Seurat painting in a very different way than if you are ignorant of his technique. Yet the photons of light reflecting off the Seurat and impinging on your retina, there to be conveyed as electrical impulses into your visual cortex, are identical to the photons striking the retina of a less knowledgeable viewer, as well as of one whose mind is distracted. The three viewers “see” very different paintings. Information reaches the brain from the outside world, yes—but in “an ever-changing context of internal representations,” as Mike Merzenich put it. Mental states matter. Every stimulus from the world outside impinges on a consciousness that is predisposed to accept it, or to ignore it. We can therefore go further: not only do mental states matter to the physical activity of the brain, but they can contribute to the final perception even more powerfully than the stimulus itself. Neuroscientists are (sometimes reluctantly) admitting mental states into their models for a simple reason: the induction of cortical plasticity discussed in the previous chapters is no more the simple and direct product of particular cortical stimuli than the perception of the Seurat painting is unequivocally determined by the objective pattern of photons emitted from its oil colors: quite the contrary.