For decades, researchers have known that the primate brain is organized into “maps” corresponding to each sense: one of the body for touch, one of the visual world for sight, and one of tones for hearing. In adult primates, these maps are divided into distinct areas responding to different classes of stimuli in each of the senses. Whether this organization is innate or develops over time through experience has been a mystery.
A study published July 3 in eLife by HMS researchers Michael Arcaro and Margaret Livingstone suggests that both could be true.
The research team’s findings reveal that a primitive blueprint of brain organization is present in primates just a few days after birth and appears to gradually fill in with age and experience.
The findings, the scientists say, may explain some features of neurodevelopmental disorders such as autism, which often involve avoidance of certain visual stimuli.
Livingstone, the HMS Takeda Professor of Neurobiology, has long worked to decipher organization in the brain’s visual system. In the current work, she and Arcaro studied the visual system in four macaques as young as 10 days by monitoring their brain activity with functional magnetic resonance imaging as the animals viewed various images, including scenes and faces.
Arcaro, a postdoctoral fellow in Livingstone’s lab, analyzed all the data, even those gathered while the animals were asleep, and made a surprising finding: Even when the animals were asleep, multiple parts of the visual system would turn on in conjunction with one another, a finding that suggests a functional organization that connects all vision-associated areas.
The researchers worked with the same macaques for several years, scanning their brains periodically as the animals matured. The investigators found that the primitive maps remained but that they gradually filled in and became more sophisticated, capable of responding to stimuli that weren’t in the primates’ environment at birth.
The young macaques’ visual system activity already appeared to be organized for object recognition and visually guided actions. They also had so-called retinotopic organization, with different areas of the visual system becoming active in response to input from various parts of the retina’s visual field.
They were, however, missing certain hallmarks early in life that were prominent in the same monkeys as they aged. They lacked, for example, clusters of neurons in the fusiform gyrus—a structure linked to recognition, among other functions—corresponding to visual stimuli from faces. Such neuronal selectivity arose as the macaques reached about 200 days of age.
Given the close genetic relationship between macaques and people, the findings, the researchers say, have implications for humans. The fact that the visual system does not later incorporate and respond to stimuli such as faces, they add, could explain some of the aberrations seen in neurodevelopmental disorders, including an aversion to looking at faces, a common feature in people with autism-spectrum disorders.
The new findings also underscore the importance of correcting any visual deficits present at birth to ensure the proper development of higher visual and cognitive functions.
Image: 123RF/Linda Bucklin