Have you ever paid attention to the process of paying attention? Right now, I'm attending fully to the words I'm placing on this page. I'm a lousy typist, so I'm looking at the keys, ignoring the stack of papers on my workspace in front of me, the family pictures on my desk, and the dirty carpet beneath my feet that needs cleaning when I get around to it. The thing is, the dirt of the carpet has just not registered in my brain as important enough to . . . well . . . pay attention to.
Paying attention to paying attention has set me to wondering what paying attention actually means. We pretty much know when we are attentive and when we are not (although we don't always admit to the latter), but what is actually going on in our brains? What neuronal mechanisms direct our senses to home in on one set of inputs to the exclusion of all others?
Neuroscientists sometimes borrow vocabulary from sound engineers to describe how attention "pumps up the volume" of task-relevant nerve impulses and "dampens the vibes" of all others. So, for example, to check for typographical errors in what I've written here, my brain's attention-focusing mechanisms have to strengthen the visual signals of the black letters on their white background while simultaneously weakening signals from my coffee cup, tape recorder, carved wooden turtle (souvenir from Vietnam), statue of Hathor (souvenir from Egypt), and other objects that compete for my attention and space in my visual field. (Oops, I looked down again, and that dirty carpet is still there. It is beginning to cry out for my attention.) Much like a sound engineer who improves a recorded musical signal by diminishing the "noise" that accompanies it, my brain's attention mechanisms have to subtract irrelevant impulses while at the same time amplifying those that mean the most to me at the moment.
Blocking out external distracters is one thing, but the brain has an additional source of noise. In a brain that is constantly active with its own agenda (have you ever tried NOT thinking?), the noise-to-signal ratio can be high. Because lots of nerve impulses continuously dart here, there, and everywhere in the brain, cutting down on the noise is no simple task. To find out how the brain achieves it, researchers at the Salk Institute recently recorded activity in the V4 brain region of macaques. The V4 is one of the brain's visual-processing centers; it lies in the occipital lobe of the cortex, at the back of the head in both macaques and people.
When the macaques saw the same stimulus repeated several times, the neurons of the V4 responded in highly variable ways. Much of the variability arose from continuous low-frequency fluctuations in the rate of firing of neurons in the V4. When the animals paid attention to a particular stimulus, however, the fluctuations in firing rate diminished. So great was the dampening effect that the researchers think noise-to-signal reduction may be a far greater component of attention than signal strengthening.
Does this mean that brain noise is just junk and our brains would operate better without it? Probably not. A seemingly unrelated piece of research just crossed my desk and . . . yes . . . it caught my attention. Researchers at Washington University of St. Louis have collaborated with a team at the University of Chieti, Italy, to look at what happens to the brain's "white noise" when we learn something new.
The researchers used functional connectivity magnetic resonance imaging to scan the spontaneous brain activity of 14 volunteers who sat quietly, presumably not paying attention to much of anything. (I'm guessing the carpet beneath their feet was clean.) Next, the volunteers' brains were scanned as they went through a training period, learning to look for an upside-down "T" on a computer display. Later, the brains were again scanned while their owners sat idly once more.
While the people were learning the visual task, two brain regions were particularly active: (1) parts of the visual cortex and (2) regions of the frontal and parietal lobes known to direct visual attention. Before the visual training, activity of the neurons in those two brain parts showed little association; that is, activity in one area didn't predict activity in the other. After training, however, an odd kind of "anti-correlation" appeared. When one region showed greater spontaneous activity, the other showed less. Furthermore, the better the volunteer learned the task, the greater this "one up-one down" association.
The researchers think that changing the noise pattern in the brain may be essential to forming a "memory trace." The trace makes linking attention and sensory processing easier the next time performance on a learned task is called for. "It's as though these two brain systems are learning to get out of each other's way," says Maurizio Corbetta, a neurologist at the Washington University of St. Louis. "After learning, the brain can identify the targets at a glance in a way that requires less direct attention and thus less interaction between the regions involved in the task.
"Recent studies have shown that in the absence of any overt behavior, and even during sleep or anesthesia, the brain's spontaneous activity is not random, but organized in patterns of correlated activity that occur in anatomically and functionally connected regions," Corbetta continues. "The reasons behind the spontaneous activity patterns remain mysterious, but we have now shown that learning causes small changes in those patterns, and that these changes are behaviorally important."
Which brings me back to that dirty carpet. My brain has learned to ignore it . . . and I guess one more day of neuronal "white noise" is probably a normal memory trace in my brain. Maybe tomorrow I'll turn my attention to carpet cleaning . . . or maybe not.
Sources:
Mitchell et al. "Spatial Attention Decorrelates Intrinsic Activity Fluctuations in Macaque Area V4," Neuron (September 24, 2009).
Lewis CM, Baldassarre A, Committeri G, Romani GL, Corbetta M. "Learning Sculpts the Spontaneous Activity of the Resting Human Brain," Proceedings of the National Academy of Sciences, published online the week of Oct. 5, 2009.
"Scans Show Learning 'Sculpts' the Brain's Connections," Medical News Today, October 9, 2009.
