For a neuroscientist, the trouble with cocktail parties is not that there are cocktails or that it’s a party (many neuroscientists love both). Instead, what we call “the cocktail party problem” is the mystery of how anyone can have a conversation at a cocktail party at all.
Consider a typical scene: You have a dozen or more lubricated and temporarily uninhibited adults telling loud, improbable stories at increasing volumes. Interlocutors guffaw and slap backs. Given the decibel level, it is a minor neural miracle that any of these revelers can hear and parse even one word emitted by their friends.
The alcohol does not help to solve this complicated problem, but it is not the main source of confusion. The issue is that there is simply too much going on at once. How can our brains filter out the noise and focus on specific information?
This problem is a central one for neuroscientists—and not just during cocktail parties. The entire world we live in is quite literally too much to take in. Yet the brain does gather all of this information somehow and sorts it in real time, usually seamlessly and correctly. The sounds and sights around you include at least as much noise as signal, but the conversation or object that interests you remains in clear focus—at least this is how you perceive it.
How does the brain accomplish this feat? One critical component is that our neural circuits actively suppress anything that is not task-relevant. Our brains pick their battles. They stomp out irrelevant information so that the good stuff has a better chance of rising to awareness. This process, colloquially called “attention,” is how the brain sorts the wheat from the chaff. You pick and choose what you want to see and hear, and your attention system filters out the noise—which is everything else.
In collaboration with the laboratories of the neuroscientists Jose-Manuel Alonso of the SUNY College of Optometry and Harvey Swadlow of the University of Connecticut, we discovered some of the initial circuits that mediate attention in the primary visual cortex of the brain. To do so, we observed neurons in this area, some of which encourage activity in other brain cells, so-called excitatory neurons, and others that tamp down activity, known as inhibitory neurons. We found that when someone attends to a specific location, the inhibitory neurons take action, suppressing responses to other visual locations. In short, the brain depends on inhibitory neurons to enable focus.
Even more interesting was our finding that the harder you concentrate, the greater the suppression. One fundamental role of cognition is to select what your brain goes on to process. It does that, at least in part, by blocking irrelevant information.
But that is not attention’s only role. As the neural activity associated with attention travels down throughout the visual system’s circuits, it can also affect how we perceive and interpret the color and brightness of objects. The illusions in this chapter illustrate some of the numerous perceptual consequences of our brain’s attentional mechanisms.
James Randi (aka the Amaz!ng Randi), on the left, performed at the 2010 Best Illusion of the Year Contest. Dan Simons, on the right, is posing in his signature gorilla costume.
BY DANIEL SIMONS
UNIVERSITY OF ILLINOIS, U.S.A.
2010 FINALIST
In a famous experiment done in 1999, Daniel Simons and Christopher Chabris, both then at Harvard University, asked subjects to watch two groups of people dribbling and passing a basketball among themselves. Three players wore white shirts; three wore black. The watchers were asked to count the number of passes by the players in white shirts. About halfway through the exercise, a gorilla—that is, a person in a gorilla suit—walked into the ball-passing scene, beat its chest while facing the camera, then walked out.
Simons and Chabris were shocked to discover that about half of the people counting passes failed to notice the gorilla. Their spectacular demonstration became an instant classic, spreading to conferences, university courses, and textbooks. It is an excellent example of inattentional blindness, a phenomenon in which the brain ignores information that is not relevant to its current task. The gorilla illusion is so well-known that Simons decided to create a variation for the 2010 illusion contest. He appeared at the gala dressed as a gorilla, flinging bananas to the audience before he took the stage. “You are all good vision scientists,” he said. “You know that when people are passing basketballs you should be looking for gorillas.” The audience roared with laughter at the inside joke.
People can only experience the invisible gorilla illusion once. After you know to look for a gorilla, you’ll never miss it again. Does knowledge of the impending occurrence of unexpected events help you detect other unexpected events? Simons’s latest demonstration, called the Monkey-Business Illusion, shows that the answer is no. People who know to look for a gorilla are of course more likely to spot the gorilla. But these same viewers will fail to notice other unanticipated happenings—and are even more likely to do so than viewers who are unfamiliar with the illusion. Again, the harder you pay attention during a task, the more powerfully your visual system suppresses distracting information. The more you watch out for the gorilla that you expect to appear, the more you will miss other changes that are unpredicted.
As the gorilla-suited Simons explained, there are several changes that most people overlook when they watch the Monkey-Business Illusion: the background of the image changes color from red to gold, and one of the three black-shirted players leaves the game by discreetly backing out of the scene. Simons had one final surprise: “Did any of you spot a pirate?” Simons asked the audience. The spectators groaned, rolled their eyes, and shook their heads at yet another impossible oversight. But the undetected pirate was not in the video. Simons pointed to stage right, where a spotlight beamed down on a pirate, previously unnoticed yet completely out in the open for all to see, holding a sword to the neck of the contest’s technical director (Steve). The illusion had spilled out of the video onto the stage!
Still frames from the illusion show (top left) the scene before the gorilla appears, (top right) the gorilla entering and one of the players in black backing out of the scene, (middle left) the gorilla thumping its chest, (middle right) the gorilla exiting, and (bottom) the scene after the gorilla has left. The color of the curtain has changed, and now only two black-shirted players remain.
BY PETER TSE
DARTMOUTH COLLEGE, U.S.A.
2005 FINALIST
At the 2005 illusion contest, Peter Tse presented one of the simplest but most important illusions ever discovered: three semitransparent overlapping circles. Look carefully at the blue dot in the center of the three intersecting disks while directing your attention to each of the three disks in turn. If you are paying attention to the bottom disk, for example, you will see that it looks brighter than the other two disks. The same is true when you turn your attention to one of the other disks. Before Tse discovered this illusion, neurophysiologists believed that people cast a spotlight of attention on a specific location, leaving the rest of the world in relative darkness. Tse showed that the spotlight concept was not just a useful metaphor. When we direct our attention to a specific object or area of an image, it appears more salient than the regions we don’t attend to. Even the apparent contrast of the focused area is higher than the rest of the picture! The neural mechanisms responsible for this phenomenon are the same that prevent you from seeing the gorilla when counting basketball passes in Simons and Chabris’s original demonstration. Our brain actively suppresses the parts of a scene that we don’t pay attention to, so that the regions that we do focus on appear more prominent in comparison.
BY PETER TSE
DARTMOUTH COLLEGE, U.S.A.
2012 FINALIST
Seven years after creating the Attention to Brightness Illusion, Tse discovered a related color effect. Three colored disks overlap in the center like a Venn diagram. If you fix your gaze on the central intersection and focus on one disk, that entire disk will appear to become uniform in color—for instance, the disk with the blue outline and outer region will appear uniformly blue. It will also seem to be floating transparently above the other disks, even though the colors are mixed in some regions—and the center of the composition is actually gray!
This illusion demonstrates the brain’s remarkable ability to see different things in the same scene, depending on your focus. For example, when you look at a pond, you may see clouds reflected on the surface, but with a subtle shift of attention you can instead find yourself observing the stones at the bottom. In the same way, as you shift your attention to a specific disk in Tse’s drawing, your brain suppresses the other disks and enhances the one you are focusing on. One reason our attentional systems may have evolved to work the way they do is to concentrate our limited neural resources—so we can make the best of them—on specific tasks. We simply don’t have the neural machinery to grasp all the sensory information around us in any depth, so our brains must pick and choose what to prioritize from one moment to the next. In that sense, our attentional focus is not a neural shortcoming but an enhancement. Human accomplishments such as science, technology, and even art would not exist but for people who were—and are—able to concentrate on minute areas of interest for extended periods of time, while taking no notice of most of the world around them.
BY PETER TSE
DARTMOUTH COLLEGE, U.S.A.
2010 FINALIST
In 2010, Tse showed that attention can also affect the perception of afterimages, the illusory visuals that linger after you look at a bright light or stare at a picture for a while. Focus your gaze on the center of the checkered pattern for one full minute, then shift your eyes to the empty rectangles at the right. You will see a colorful afterimage filling in the formerly empty frames. Now try paying attention to the vertical rectangle, and you will see an afterimage that matches it. Pay attention to the horizontal rectangle and you will see a different afterimage. You can go back and forth between the two afterimages simply by shifting your attention from one rectangle to the other.
Afterimages help scientists understand how neurons in our eyes and brains temporarily cease responding to an unchanging stimulus. The ghostly visions appear during the brief period before the neurons reset to their normal, responsive state. Neuroscientists know that retinal neurons play a role in the perception of afterimages, but it has been difficult to demonstrate the importance of neural processing at higher levels in the visual pathway from the eye to the brain. Tse’s illusion proves that afterimages can be strongly modulated by cognitive processes such as attention.
