A Visionary Leap: Researchers Restore Full Color Vision in Squirrel Monkeys

Danny Fossati, an aspiring military man, was excited to embark on a journey. He was headed to Northwestern University on a U.S. Navy ROTC scholarship. Eager to pursue his education and ambitions in the Navy, he arrived at the celebratory ceremony where he was awarded a $250,000 scholarship. Upon completing the required medical clearances, Fossati received some unexpected news: He had failed the color blindness test. Fossati was diagnosed with color vision deficiency, which makes it difficult to distinguish between certain colors. As a result, his plans to join the Navy were crushed and he lost his scholarship.

Individuals diagnosed with a type of color vision deficiency (CVD) experience challenges similar to Fossati’s. One deficiency type, red-green color deficiency, impairs the ability to distinguish between red and green hues, with the severity dependent on the specific type of deficiency. For someone with red-green color blindness, a red and green apple may both appear a murky yellow, making them hard to distinguish—unlike for those with normal color vision. These symptoms make everyday tasks such as driving, preparing food, and even securing jobs can be challenging.

The most prevalent form of color blindness is red-green color deficiency, which impacts nearly 300 million people globally, or roughly one in 12 men and one in 200 women, according to the Cleveland Clinic. The eye contains photoreceptors in the retina that enable us to see in dim light (rods) and color (cones). For full color vision, all three types of cones—red, green, and blue—must fully function. Each type of cone contains an opsin photopigment sensitive to different wavelengths of light. If one or two cones are impaired, certain colors become hard to differentiate, leading to red-green color blindness.

In 2009, University of Washington researchers cured red-green color blindness in adult male squirrel monkeys, a species in which all males are naturally red-green color deficient. Researchers tested six adult monkeys—four colorblind males and two control females—using a computer program that displayed a series of colorful clumps against a background of gray dots. The test confirmed that the male squirrel monkeys could not distinguish the red and green clumps from the background, whereas the female squirrel monkeys could.

Dr. Jay Neitz, a professor of ophthalmology and color vision researcher at the University of Washington, led a team that restored full color vision in squirrel monkeys. He was aware of a common belief that treatment must occur during a critical developmental window of adolescence when the brain is highly sensitive to environmental stimuli.

“Researchers would cover one eye of a kitten and monkey for some period of time, then they would uncover it. This process caused the eye to be blind even though there was nothing wrong with the eye,” Neitz explained. “The eye becomes blind because it needs proper visual information to develop. These experiments led to a strong belief that for the brain to develop normally and achieve typical vision, it needs to have a normal visual experience.”

Initially, Neitz anticipated that his team might be able to study baby monkeys during their critical developmental period, but logistic challenges led the team to focus their research on adult squirrel monkeys instead. Using gene therapy, they injected a human gene into the eyes of two colorblind monkeys. The researchers aimed to produce long-wavelength opsin in the monkey’s retinas—a protein essential for detecting red and green light.

Approximately 20 weeks after the injection, testing displayed significant improvements in the monkeys’ ability to distinguish between red and green clusters against a gray background during a computer test—evidence that the monkeys had gained trichromatic, or full color, vision.

Neitz’s team demonstrated that full-color vision can be successfully restored in adult primates, even beyond the critical period for visual development. The research holds potential for future treatments targeting congenital color vision deficiencies in humans.