Every moment, our brains are bombarded with information, from without and within. The eyes alone convey more than one hundred billion signals to the brain every second. The ears receive another avalanche of sounds. Then there are the internal fragments of thoughts, conscious and unconscious, racing from one neuron to the next. Much of this data is random and meaningless. Indeed, for us to function, much of it must be ignored. But clearly not all. How do our brains select the relevant data? How do we decide to pay attention to the beep of a smoke alarm and ignore the drip of a leaky faucet? How do we become conscious of a certain stimulus, or indeed “conscious” at all?
For decades, psychologists, philosophers, and scientists have debated the process by which we pay attention to things, based on cognitive models of the mind. But in the view of modern scientists, the “mind” is not some nonmaterial and exotic essence separate from the body. All questions about the mind must ultimately be answered by studies of physical cells, explained in terms of the detailed workings of the hundred billion neurons in the brain. At this level, the question is, How do a group of neurons signal to one another and to a cognitive command center that they have something important to say?
“Years ago,” the neuroscientist Robert Desimone told me during a recent visit in his office, “we were satisfied to know which areas of the brain light up under various stimuli. Now, we want to know mechanisms.” Desimone directs the McGovern Institute for Brain Research at the Massachusetts Institute of Technology. Youthful and trim at age sixty-two, he was casually dressed in a blue pin-striped shirt, with only the slightest gray in his hair. On the bookshelf of his tidy office were photographs of his two young children; on the wall was a large watercolor titled Neural Gardens, depicting a forest of tangled neurons, their spindly axons and dendrites winding downward like roots in rich soil.
In an article published in the journal Science in 2014, Desimone and his colleague Daniel Baldauf reported on an experiment that shed light on the physical mechanism of paying attention. The researchers presented a series of two kinds of images, faces and houses, to their subjects in rapid succession, like passing frames of a movie, and asked them to concentrate on the faces but disregard the houses (or vice versa). The images were “tagged” by flashing them at two different frequencies—a new face image every two-thirds of second and a new house image every half second. By monitoring the frequencies of the electrical activity of the subjects’ brains with magnetoencephalography (MEG) and functional magnetic resonance imaging (fMRI), Desimone and Baldauf could determine where in the brain the images were being directed.
The scientists found that even though the two sets of images were presented to the eye almost on top of each other, they were processed by different places in the brain: the face images by a particular region on the surface of the temporal lobe that is known to specialize in face recognition, and the house images by a neighboring but separate group of neurons specializing in place recognition.
Most importantly, Desimone and Baldauf found that the neurons in the two regions behaved differently. When the subjects were told to concentrate on the faces but to disregard the houses, the neurons in the face location fired in synchrony, like a group of people singing in unison, while the neurons in the house location fired like a group of people singing out of synch, each beginning at a random part of the song. And when the subjects concentrated on houses and disregarded the faces, the reverse happened. Furthermore, another part of the brain called the inferior frontal junction, a marble-sized region in the frontal lobe, seemed to orchestrate the chorus of the synchronized neurons, since it fired slightly ahead of them. Evidently, what we perceive as “paying attention” to something originates, at the cellular level, in the synchronized firing of a group of neurons, whose rhythmic electrical activity rises above the background chatter of the vast neuronal crowd. Or, as Desimone once put it, “This synchronized chanting allows the relevant information to be ‘heard’ more efficiently by other brain regions.”
A connection between attention and neuronal synchrony was first hypothesized by Ernst Niebur and Christof Koch twenty years ago. Desimone was one of the first scientists to prove it for particular cases, in 2001. A pioneer in the field, he is quick to mention other leaders, such as John Reynolds of the Salk Institute, who uses a combination of physics, neurophysiology, and computational neural modeling to study how simultaneous objects in the visual field, such as separate highlighted areas in an illuminated grid, compete with one another for attention. Meanwhile, Sabine Kastner of Princeton has recently begun comparing humans with monkeys in their attention to visual tasks; and Columbia’s Michael Goldberg has recently shown that, in the process of attention, a particular area of the brain called the lateral parietal area “sums up” visual signals and cognitive signals. In this growing field of neuroscience, Desimone has trained over thirty-five people himself.
I asked Desimone how the conductor of the neuronal chorus, in this case the inferior frontal junction, would know that a particular stimulus should be attended to. In his experiment, the subjects were told to focus their attention on either faces or houses, but what about an unexpected stimulus—say a charging lion or the sudden entrance of a potential romantic partner? “We don’t understand the answer to that yet,” said Desimone. And how do a bunch of random voices come into synchrony? Can they do so merely by exchanging notes with one another, or do they need an outside director? At the second question, Desimone broke out in a boyish grin and took six small metronomes from his briefcase. He placed them side by side on a wooden board, balanced on two empty lemon soda cans. Then he set the metronomes ticking, out of synch with one another. After a couple of minutes, they were all ticking in synchrony. They had communicated with one another and come into synch solely through the vibrations of the board, without any outside agency. Neurons, of course, use a different method of communication with one another: passing chemical messengers between the hundreds of root-like filaments radiating from each neuron. Desimone’s pendulums suggest that some neurons could come into synch on their own, without a conductor. But the question of which neuronal processes are self-organizing and which require a higher-level cognitive director isn’t yet understood.
As my visit came to an end, I asked Desimone about the seemingly strange experience of “consciousness,” to me the most profound and troubling aspect of human existence. How does a gooey mass of blood, bones, and gelatinous tissue become a sentient being? How does it become aware of itself as a thing separate from its surroundings? How does it develop a self, an ego, an “I”? Without hesitation, Desimone replied that the mystery of consciousness was overrated. “As we learn more about the detailed mechanisms in the brain,” he said, “the question of ‘What is consciousness?’ will fade away into irrelevancy and abstraction.” As Desimone sees it, consciousness is just a vague word for the mental experience of attending, which we are slowly dissecting in terms of the electrical and chemical activity of individual neurons. He threw out an analogy. Consider a careening automobile. A person might ask: Where inside that thing is its motion? But he would no longer ask that particular question after he understood the engine of the car, the manner in which gasoline is ignited by sparkplugs, the movement of cylinders and gears.
I am a scientist and a materialist myself, but I left Desimone’s office feeling somehow bereft. Although I cannot say exactly why, I do not want my thoughts, my emotions, and my sense of self reduced to the electrical tinglings of neurons.
I prefer that at least some parts of my being remain in the shadows of mystery.