As an outfielder tracks a soaring fly ball and sprints across the field, his gaze shifts several times a second to track the baseball's trajectory and hustle to its likely landing position. Through the wonders of visual perception, the player…
As an outfielder tracks a soaring fly ball and sprints across the field, his gaze shifts several times a second to track the baseball's trajectory and hustle to its likely landing position. Through the wonders of visual perception, the player never notices these tiny jumps. The ball's flight—ideally right into his open glove—appears steady and smooth, as does his own path to get under it. This is just one example of the visual system's sleight of hand, which neurobiologist Tirin Moore says allows us to see the world in a coherent way—a feat we take for granted. "The apparent stability of vision is entirely an illusion," Moore says. The illusion is created by neural mechanisms he studies in his laboratory at Stanford University. "Your representations of the visual world are not faithful representations of what is on the retina, because the visual system has to deal with the movement of the observer." Moore has always been fascinated with how the subjective nature of vision shapes our ideas and thoughts and how we behave; it's also fundamental to art. As a child he drew cartoon strips and later used a home movie camera to make stop-motion animations—a perfect example of creating illusions in our mind's eye. His secret ambition to become a filmmaker eventually lost out to the lure of neuroscience, in which he received a PhD degree from Princeton University. Moore seeks to understand, at the neural circuit level, how we extract visual information from the environment to make behavioral decisions. He has developed tools to monitor brain activity in monkeys as they perform visual tasks, which he hopes to use to identify the neural mechanisms that underlie visual attention—the critical ability to focus on something of interest while ignoring or filtering out irrelevant visual information. His landmark papers showed that neurons in the brain's prefrontal cortex that control eye movements also help focus attention. When you prepare to shift your gaze to track a target, the corresponding prefrontal neurons fire more strongly, stimulating circuits in the visual cortex that process visual information about the target and produce visual attention. "Just by deciding to move your eyes to look at a certain area in front of you, you can essentially raise the volume on the strength of signals coming from that part of space." As a result, those signals outweigh competing signals from other parts of the visual field that would be distracting; this creates the selective attention crucial to work, play, and survival. Interestingly, Moore, who became an HHMI early career scientist in 2009, has shown that merely planning to shift one's gaze to an object, but not actually doing so, increases the firing of the signals of interest. "When you prepare an eye movement to a location every few hundred milliseconds, you're mustering up information about the target to help plan the movement," he explains. "When you do that, you are necessarily filtering out information about the location—and that filtering really is attention." But this capacity is flawed in the estimated five percent of US children and adults diagnosed with attention deficit hyperactivity disorder (ADHD). They can't "turn up the gain" on relevant visual signals and filter out distractions. Such individuals have reduced activity of the neurotransmitter dopamine in the brain's frontal lobes, where Moore has identified the nerve circuitry for attention. In 2011, he reported that raising dopamine levels in the brains of macaque monkeys increased their tendency to look at visual targets and intensified neuronal responses to sensory stimulation in the brain's visual cortex. Stimulants like Ritalin and Adderall boost dopamine in the frontal lobes, but Moore calls these treatments for ADHD "blunt instruments" that may have problematic side effects. His continuing exploration of the role of dopamine circuitry, he believes, could lead to improved treatments not only for ADHD, but also for other disorders in which attention is impaired, including Parkinson's disease, autism, and age-related cognitive decline.