ATTENTION & PERCEPTION
(Macknik & Martinez-Conde (2020)
How Neural Constraints Set Us Free to Create
By Stephen L. Macknik, Susana Martinez-Conde
(2020. American Scientist. July-Aug. P 196-???)
Perceptual limits drive innovation
Try the following experiment: Hold your magazine or screen at arm’s length, with your elbow straight, and use your thumb to cover a few letters on the middle of the page. Then focus your gaze on your thumbnail, and without moving your eyes, try reading any word to the left or the right of your thumbnail. You’ll find that it is impossible. Your eyes see fine details only with a tiny part of your central retina called the fovea, which covers about 0.1 percent of your visual field. Everywhere else, you are legally blind—your brain just doesn’t let you know it. Three to four times a second, your eyes make unconscious jumps, or saccades, that orient your fovea in different, successive directions. Your visual circuits stitch these bits together into a false experience of an all-encompassing, high-resolution view of the world.
left to right: Martinez-Conde and Macknik Labs, from an illusion by Akiyoshi Kitaoka; Jorge Otero-Millan; © Mask of Love, Gianni A. Sarcone, www.giannisarcone.com ; Jorge Otero-Millan
Another major blind spot is produced by the optic disc, the region where your optic nerve exits your eye to connect your retina to your brain. To make yourself aware of this one, extend both arms in front of you and make an L shape with your thumbs and index fingers while keeping the other fingers curled inside your fists. Hold your elbows straight and touch both thumbs together. Now close your left eye and look at your left index fingertip. Without shifting your gaze, pay attention to the tip of your right index finger. You will see that it has disappeared, and that it seems as if you can see what is behind the finger. What you are actually observing is your brain using information about the textures and colors around your finger to fill in the visual void that is floating in front of you.
One might suppose that these gaps in our visual perception create corresponding limitations in our ability to think creatively about the world. After all, how can we apply our imagination to things we aren’t even aware of? Research by our labs and others suggests quite the opposite, however. We are forced to be highly creative with our information inflow precisely because of how impoverished our sensory inputs are. Our survival as a species has long depended on a brain that can connect the dots not only in visual perception, but more broadly in our overall cognitive and memory processes. We all benefit from that evolutionary legacy.
The creativity associated with your perceptual shortcomings is responsible for the strange beauty of the optical illusions shown on these pages—images that appear to expand, bulge, wiggle, flicker, or alternate meanings as your brain’s compensation mechanisms try to make sense of them. Our limitations show up in especially explicit, entertaining ways in magic tricks. For more than a decade, our research group has therefore conducted experiments with magic-based illusions, using them to learn more about when and why our senses edit the reality of events around us.
We have found that our imperfect perceptual systems are actually a huge advantage. The constraints that these systems impose on us give us tremendous flexibility in interpretation. People often ask us what they can do to make themselves more resistant to illusions, to take in more of the big picture, to be better at multitasking. Our answer is: You wouldn’t want that. Those would be terrible things to do! Being able to focus on one thing while blocking out distractions is crucial to creativity. Science, art, and culture all depend on our ability to exploit our confines and to block out anything outside the task at hand. It’s how thinking happens.
Magic in the Mind
Human perception of the world is constrained not only by our sensory systems’ architecture, but also by our cognitive and attentional systems. These restrictions lie at the core of every magic performance you’ve ever watched, making the conjuring arts a fertile area for study. Our academic interest in magic did not begin until 2005, however, when we were asked to cochair an upcoming meeting of the Association for the Scientific Study of Consciousness (ASSC), which was to be hosted in Las Vegas.
We both lived in Phoenix at the time and were taking repeated trips to Las Vegas to preview the venues for the meeting. Our research concentrates on the neural bases of our visual experience. At the time we pursued that theme mostly by looking at the interaction between eye movements and the building blocks of vision. But while we were touring the Vegas strip, it struck us that we were surrounded by some of the world’s most elaborate experiments in visual cognition: magic shows.
From 2005 to 2007, we set up a series of meetings with magicians, learning more about their techniques and forging a common vocabulary for how to talk about perception and illusions. We decided to include a workshop with professional magicians at the 2007 meeting of the ASSC, providing a forum for conjurers to explain their own interpretations of how they exploit the gaps in human perception. Our colleague Daniel Dennett, a philosopher and cognitive psychologist at Tufts University, initiated a series of introductions that eventually led us to productive meetings with the magicians Teller (half of Penn & Teller) and Apollo Robbins (a theatrical pickpocket who refers to himself as “the gentleman thief”).
One notable study that emerged from these collaborations concerned a widely used sleight-of-hand technique known as the French drop, which was an integral part of Robbins’s performances. The French drop uses misdirection to fool the audience into believing that a coin or other small object has moved from one of the magician’s hands to the other, despite never leaving the original hand. Robbins noticed that if he moved the object on a curved decoy trajectory, the audience would follow it as he intended. If he used a straight motion, however, their attention would wander back and forth between the beginning and the end of the trajectory, spoiling the trick. This difference suggested a type of perceptual gap that had never been studied before.
In a 2011 collaboration, we recreated the fundamental steps of Robbins’s French drop in a laboratory setting, and presented them to seven test subjects. During the experiments, we monitored the subjects’ eye movements to track where they were focusing their attention. Our study provided the first empirical evidence for what Robbins had suspected from years of performance: People track their attention along the entire trajectory of a curved motion, whereas they skip from the beginning to the end of a straight motion.
We had previously noted a similar effect in the spatial domain in some visual illusions: Curves and corners generate stronger neural activity, and appear perceptually more salient, than do straight edges, perhaps because they are more informative and less predictable. From an evolutionary perspective, curvy or zigzagging motions might more closely match the ways predators and prey move than rectilinear trajectories do. Similarly, the boundaries of trees or rocks generally contain more bends and turns than straight edges.
The reason that tricks such as the French drop work is grounded in the way attention is controlled by the brain. Neuroscientists and magicians alike often talk about the spotlight of attention, the location that draws the observer’s interest. In reality, what the brain does is more like an anti-spotlight: The object we concentrate on becomes more noticeable not because our neural circuits boost our perception of it, but because they actively suppress everything around it. As a result, magicians need only ensure that audiences aim their attention to specific locations on the stage. Each spectator’s brain will then take care of suppressing everything else, including the secret method hiding behind the magical effect.
Our studies clarify the way that audiences misunderstand the concept of magical misdirection. You may think that the magician distracts you during a critical move or manipulation. The reality is both simpler and more complicated: Magicians do not strive to distract, but instead aim to direct your attention in specific ways. Indeed, the act of trying to figure out the method behind a trick leads you to unconsciously suppress the very information that might give you a clue. Your brain acts as the magician’s confederate, crafting an imaginative interpretation of events based on the limited information it is able to process. Magicians effectively hack your brain, tricking you into allocating precious neural resources to cogitate on an impossible task, while they get away with magical murder—all in plain sight. It’s a form of mental jujitsu.
The Myth of Multitasking
Research that we conducted in collaboration with Jose-Manuel Alonso at SUNY College of Optometry and Harvey Swadlow at the University of Connecticut offers more insights into how to tap into the creativity inherent in the limitations of the human senses. We ran perceptual tests on macaque monkeys and determined that the relative enhancement versus suppression in the attentional spotlight is mediated by two separate populations of neurons in the visual cortex.
Magicians mess with their spectators’ attentional spotlights by splitting their focus, forcing audiences to try to multitask—and, despite what many self-help gurus might tell you, multitasking is not possible. The neural mechanisms that enable perceptual enhancement (at the center of the attentional spotlight) and suppression (in the surrounding areas) prevent multitasking from happening. Our unique attentional focus cannot be divided without losing effectiveness.