In 2011, Leyla Isik, a brand-new PhD student, delivered a short presentation to her new colleagues in Tomaso Poggio’s lab in what is now the Center for Brains, Minds, and Machines. She outlined her upcoming research on peripheral vision with great enthusiasm and fully expected a positive response. But she was in for a disappointment when, minutes after the presentation, she was approached by the lab’s research assistant, Gadi Geiger.

“He was not happy with my little two-minute summary and decided that I needed to educate myself a lot on prior research,” says Isik. She quickly realized he was right. After reading the literature Geiger recommended, Isik rethought her research project, saw how much it was improved, and made a mental note to keep Geiger close at hand when designing future experiments.

Known to all simply as Gadi, Geiger takes time to coach students and postdocs as they design experiments, and he volunteers to be a test subject himself when he can. He’s also a good listener—and students often turn to him for advice on matters outside the lab. “He’s really been invaluable in all parts of my research and life in the lab,” says Isik, who earned her PhD in 2015. Poggio, the Eugene McDermott Professor in the Department of Brain and Cognitive Sciences and director of the Center for Brains, Minds, and Machines, describes Geiger as the wise and fatherly person in his lab.

Standing 5 foot 6—and usually fortified by espresso—Geiger also happens to be an internationally known neuroscientist and psychophysicist who investigates connections between physical stimuli and the sensory and perceptual responses they evoke. But while his work has been published in Science, Nature, and the New England Journal of Medicine and he’s the lab’s go-to guy for career advice, Geiger doesn’t hold a doctorate, or even a college diploma.

“Nowadays it’s not the same time. You are not going to be able to get anywhere without a degree,” Geiger says. “I’m sure that if I would have been a doctor, a professor—I don’t know what—I would have so much more money. I never was interested.”

When he enrolled at Hebrew University in Jerusalem in the mid-1960s, Geiger was drawn to biology, math, and physics. But pursuing all of them made it difficult to satisfy degree requirements. Planning his own curriculum, Geiger nonetheless took courses in all three disciplines while working as a technician in the physics lab’s solid-state division. The lab was then growing rubies and other crystals for energy projects—including the rubies being used to build what he was told would be the country’s first laser. But laser building was interrupted by reserve duty in the Israeli military, which ultimately led to fighting in the 1967 Arab-Israeli War. During his service, he was severely injured and had to take a one-year leave to start recovering.

Geiger had already harbored antiwar sentiments before his service. “When I came back from the war, I became fairly radicalized politically, to the point that they would not let me back to the lab because I did not have security clearance anymore,” he says. “I was immediately for talking to the Palestinians and all that to make peace. So all of this at this time was a no-no.”

Unable to work on lasers, Geiger took courses he found interesting and left Hebrew University in 1970. He spent about a year in Helsinki, studying all he could about optics and visual systems, and then headed to Germany, where he was offered a position with the Nobel Prize–winning chemist Manfred Eigen at the Max Planck Institute for Biophysical Chemistry. Before he could accept, a mutual friend suggested that Geiger meet with Werner Reichardt, who was studying visual information processing in flies at the Max Planck Institute for Biological Cybernetics. He, too, wanted to hire Geiger, and he created a new position to get around Geiger’s lack of degrees.

As the Reichardt lab’s first scientific technician, Geiger worked alongside Tomaso Poggio, then a research assistant, who’d joined the lab a few months after he did. The duo started publishing papers that examined how flies orient themselves visually and began to describe the neuron structure of the fly’s optic system. The work helped push the scientific community to use the fly as a model for understanding visual perception and eventually landed Geiger an invitation to speak at the 1981 Society for Neuroscience Conference in Los Angeles. While in the United States, he gave a talk at MIT and visited his friend Jerome Lettvin, an MIT professor of electrical and bioengineering and of communications physiology, who was known for his research on how frogs’ optic nerves communicate with their brains. (See “In a Frog’s Eye,” January/February 2013.)

Lettvin invited Geiger to come to MIT to work on a different organism—“the big fly, the human being,” Geiger says—and the pair turned their attention to studying central and peripheral vision. In one experiment, they observed that five of the 100 subjects they’d recruited had strong peripheral vision. Upon learning that all five also happened to be dyslexic, the researchers wondered if dyslexia was truly a problem with language processing in the brain. Perhaps, they thought, current thinking was wrong, and it might be a visual or perceptual condition instead. If the latter proved to be the case, it might be possible to develop more effective reading methods and teaching techniques geared toward a dyslexic person’s perceptual strengths.

Experiments they wrote about in a 1987 paper published in the New England Journal of Medicine suggested that dyslexic readers have wider peripheral vision but “noisier” focus in the center of their visual fields. Shortly thereafter, Geiger left MIT for research stints in Italy and New Jersey but continued to collaborate with Lettvin on work to better diagnose dyslexia. When he returned to MIT in 1994, the two also began developing reading techniques that encourage dyslexic readers to tap into their peripheral vision first and then move back to the center.

Several researchers objected to the premise that dyslexia could be a perceptual or visual condition, and Geiger says his work didn’t make waves in the scientific community. But neuroscientist John Gabrieli, director of the Athinoula A. Martinos Imaging Center at MIT’s McGovern Institute for Brain Research, says Geiger’s work has had “a substantial impact” on our understanding of the condition. “The field has focused on a deficit in phonological awareness (auditory language processing) as a key cause of dyslexia, but the idea about visual differences remains important and has gained wider attention in recent years,” he says.

Geiger’s willingness to go his own way has served him well. Institute Professor Emeritus and linguist Noam Chomsky considers Geiger a longtime friend and consults him on cognitive science. He describes him as “very independent-minded,” adding that Geiger “pursues his own course doggedly, effectively,” and “doesn’t pay much attention to what the conventions are.”

Geiger says that without the responsibility of writing papers or applying for grants, he’s better able to pursue his own scientific interests and guide students who want to follow theirs.

“Not being in the system, I think, gave him a huge amount of freedom and let him be a lot more what he wanted to be,” says Sayan Mukherjee, PhD ’01, a Poggio Lab alumnus who asked ­Geiger to be a groomsman in his wedding in 2011. Now a professor of statistical science, mathematics, and computer science at Duke University, Mukherjee adds that Geiger’s freedom is also reflected in his role as a mentor, in which he often focuses on a student’s life priorities. He says that Geiger urges students to do what they care about, often telling them that “if things are fashionable, you don’t necessarily do them unless you think they’re right.”

Now 76, Geiger is contemplating retirement. He’s been working part time, offering input on the lab’s biological research and acting, as Poggio puts it, as an “unofficial psychologist” and friend to its students.

“He puts a lot of things into perspective,” Poggio says. “Whether you agree or not is a different story, [but] it’s a nice perspective to have around.”