The New Yorker: Every moment, our brains are bombarded with information, from without and within. The eyes alone convey more than a hundred billion signals to the brain every second. The ears receive another avalanche of sounds. Then there are the fragments of thoughts, conscious and unconscious, that race from one neuron to the next. Much of this data seems 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 turn of a doorknob and ignore the drip of a leaky faucet? How do we become conscious of a certain stimulus, or indeed “conscious” at all?
For decades, 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 many modern psychologists and neurobiologists, 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 more than eighty billion neurons in the brain. At this level, the question is: How do neurons signal to one another and to a cognitive command center that they have something important to say?
“Years ago, we were satisfied to know which areas of the brain light up under various stimuli,” the neuroscientist Robert Desimone told me during a recent visit to his office. “Now we want to know mechanisms.” Desimone directs the McGovern Institute for Brain Research at the Massachusetts Institute of Technology; youthful and trim at the age of sixty-two, he was dressed casually, in a blue pinstripe shirt, and had 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 wending downward like roots in rich soil.
Earlier this year, in an article published in the journal Science, 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 being presented at two frequencies — a new face every two-thirds of a second, a new house 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 facial recognition, and the house images by a neighboring but separate group of neurons specializing in place recognition.
Most importantly, the neurons in the two regions behaved differently. When the subjects were told to concentrate on the faces and 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 point in the score. When the subjects concentrated instead on houses, the reverse happened. [Continue reading…]
