Welcome to the ninth installment ofMind Matters is Sciam.com's "seminar blog" on the sciences of mind and brain. Each week, top researchers describe their disciplines' most significant new findings -- and what they, as fellow researchers, find most exciting, maddening, significant, odd, or otherwise noteworthy in the research driving their fields. Blog visitors can participate. We hope you'll join us.
Momentary Lapses of Attention
Pay attention ... if you can. A recent study shows how areas in the prefrontal cortex (red) and parietal lobes (orange) focus -- and sometimes fail to focus -- the attention of sensory areas (green) to the task at hand. Illustration courtesy Nature Neuroscience
Introductionby David Dobbs, Editor, Mind Matters
Attention Must Be Paidby Trey Hedden and John Gabrieli Gabrieli Lab, Massachusetts Institute of Technology
Pay attention! It's the first requirement in school and necessary for most of what we learn and accomplish in the rest of life. Attention -- directing awareness to a chosen stimulus or task -- is a key part of what many neuroscientists and psychologists call "executive control" or "cognitive control" -- the ability to focus our thoughts and behavior on what we've decided is important. We tend to succeed or fail at most things largely according to how well we can concentrate on them. Of course, we all pay attention better at some times than others. Whether solving a work problem, playing tennis or talking with friends at dinner, we can be keenly focused one minute and distracted the next, producing an oversight that costs the boss money, the double fault that loses a game or the faux pas that ruins dessert. Such lapses can rise from trying to multitask, daydreaming of vacations past or future, attempting to remember what we're supposed to pick up for dinner, or attending to any of other countless distractions any day in life presents. Network Problems What is the brain doing when this lapse happens? To answer that, it helps to first review the current understanding about which areas of the brain are involved in focusing awareness. A large body of work has established that attentional control processes (a.k.a. executive functions) are rooted in a network that connects the prefrontal and parietal lobes, at the front and top of your brain, respectively, with occipital regions at the brain's rear, where most visual, auditory and other stimuli are first processed (see illustration at top). This network is thought to send top-down modulatory signals from the prefrontal cortex and parietal areas to bias the occipital regions' processing of sensory information in favor of task-relevant information. To find out what happens during attentional lapses, a team of researchers led by Daniel H. Weissman used functional magnetic resonance imaging (fMRI) to try to identify how these brain areas behave when attention is paid and when it flags. They describe their work in "The Neural Bases of Momentary Lapses in Attention." They measured localized blood flow (and thus, presumably, brain activity) with the scanner as their test subjects tried to solve simple perceptual puzzles, then correlated that activity with the "reaction time" the participants took to respond to each trial -- the reaction time being taken as a measure of their distraction or attention. The puzzle was of the simple-but-difficult type that neuro-testers love. Participants were presented with a "global/local task" in which a large (global) letter -- H or S, in this case -- is made up of smaller (local) letters, also either H or S. The subjects were asked to identify, by pushing either an S button or an H button, either the large (global) letter presented or, in other trials, the small (local) letter. Sometimes the global and local letters matched (for example, a large H made up of small Hs), creating a so-called "congruent" stimulus. At other times the global and local letters conflicted (for example, a large H composed of small Ss), creating a non-congruent stimulus. The subjects thus had to focus attention on either the large or the small letters, often while inhibiting conflicting responses to incongruent stimuli. Simple enough (sort of), as long as you're paying attention.
The global/local task, designed to demand attention: In the study by Weissman and colleagues, subjects had to identify the appropriate letter denoted either by global (large) letters or local (small) letters in various configurations. Scanning people while they did the task revealed new dynamics in how our brains modulate attention.
The subjects made few mistakes regardless of the tasks' complexity. But although error rates were consistently low, the reaction times people needed to solve the tasks varied substantially, both among subjects and within a given subject's set of responses. The slower reaction times, the researchers assumed, occurred when a person was momentarily distracted by irrelevant thoughts or inattention. On this lapse the researchers focused, comparing brain activation shown in trials with relatively fast reaction times (deemed "focused attention trials") to the activation in trials with relatively slow reaction times ("lapsed attention trials"). They found interesting differences. For starters, the slower reaction times were associated with reduced activation in prefrontal regions -- not a terribly surprising finding, given the established importance of that region to executive control. More significant, however, is that this reduced activity generally occurred before the stimulus was presented. Focusing the mind just before a stimulus or task, then, seems vital to dealing with it effectively. The study further found that lapses were also associated with reduced activation in the sensory-processing areas at rear, as if those areas had not been properly alerted by the distracted prefrontal areas. The relative sluggishness of the hemodynamic (blood-flow) response measured by fMRI prevents firm causal conclusions about this relationship. But it's an intriguing suggestion, and whether and how prefrontal areas alert those at rear is a ripe question for further study using methods with finer temporal resolution, such as magnetoencephalography. So How to Fix? Having shown how we get distracted, the study also revealed two mechanisms that may help people regroup and focus. First, in the trials in which people seemed distracted beforehand, greater activity in the prefrontal and parietal regions once the task began -- extra attention, presumably -- did indeed compensate for the reduced pre-stimulus attention; this extra focal activity took time (producing slowed reaction times) but allowed the subjects to avoid mistakes. The study also uncovered a second compensatory mechanism, one that seemed to let a subject apply a lesson learned about a lapse on one trial to the focus of attention on the following trial. When subjects activated the right IFG and the right temporal-parietal junction (see illustration) in a slow-reaction trial, they tended to respond faster in the following trial. They didn't always activate these areas; but when they did, they responded faster on the next task. The right IFG and temporal-parietal junction, then, seem to help reorient attention after a lapse, enabling the volunteers to regroup and refocus. In other words, when you've lost focus on the tennis court and hit your first serve long, these are the areas you must call on to let you spin the next one down into the court. Finally, the study found some intriguing trade-offs between internally focused attention and outward focused attention, suggesting that some lapses may simply be a failure to switch from internal thoughts and feelings to external tasks at hand -- an idea that certainly matches well with subjective experience. Altogether, this study provides fascinating evidence that a lack of prefrontally mediated attentional focus, especially before a given task or stimulus, diminishes the quality of sensory processing of a target stimulus, and thus how well we attend. And even in the fairly simple attentional trade-offs and compensations this study explores, it shows clearly that multiple neural networks must compete for our attention. Further defining these competing networks and compensatory mechanisms should help us better understand not only the waxing and waning of attention that we all deal with from day to day, but also attentional deficits that accompany disorders and conditions ranging from attention deficit/hyperactivity disorder (ADHD) or depression to stroke or aging.
Trey Hedden and John Gabrieli are researchers at Gabrieli Lab for Cognitive and Affective Neuroscience at Massachusetts Institute of Technology, where they study the organization of thought, memory, and emotion in the human brain.
Separating Wheat from Chaff
by Sam Ling
New York University
There's a remarkably low limit to how much information our brain can process at a time. Recent calculations suggest that we can use only around 1 percent of our brain at any given moment. This limit in processing capacity is constantly at odds with the vast information in our environment, which is why attention is such an important cognitive mechanism. Attention plays the critical role of guiding our brain to selectively process relevant information while ignoring the irrelevant. How does it do so? A growing body of evidence suggests that attention can enhance some of the earliest inputs to the brain. For instance, it can actually affect how well we see by effectively "turning up" the visual contrast of attended stimuli and "turning down" the unattended stimuli. This sensory enhancement seems to be mediated by a complex network of brain regions that work to separate the informational wheat from the chaff. In general, this attentional system is very good at what it does. But what happens when our attentional system lapses? Weissman and colleagues trace such lapses, at least in one test scenario, to decreased activity in executive processing regions in the frontal lobe, which in turn leads to decreased activity in sensory processing regions, which in turn leads to poorer performance at an attentionally demanding task. Although it is well established that not attending to a task leads to poorer performance, this study goes further by suggesting the source of this impairment: instead of putting all resources into the task, sometimes our cognitive resources are overtaken by our own internal noise, such as daydreaming. Instead of attending to the task at hand, you ponder your grocery list or calculate how much longer you have to lie in the fMRI scanner pressing buttons. This finding suggests that your attentional system faces quite an uphill battle. It must deal not only with a torrent of irrelevant noise in your external environment but also with all the noise within your own head. This finding draws a strong link between specific brain activity and subsequent behavioral outcomes -- adding to a growing body of similar research linking specific brain dynamics directly to specific behaviors. By illuminating the cortical activity associated with attentional failures, Weissman and colleagues shed light on how the attentional control network operates and what cognitive processes the attentional system must compete with for the brain's resources.