Precise map of primate foveolar cortex uncovers intricacies of brain’s visual system

Humans and other primates are innately able to perceive fine details, vibrant colors and focus their attention on specific elements of what they are seeing at a given time. This ability is supported by the foveola, a tiny section of the retina that is densely packed with photoreceptors.

Researchers at Zhejiang University and the Zhejiang Key Laboratory of Research and Transformation for Major Neurosurgical Diseases set out to precisely map how the foveola is represented in the primate visual cortex using high-spatial resolution imaging.

Their paper, published in Nature Neuroscience, shows that these representations are surprisingly complex, consisting of a central element surrounded by several rings of representations per brain hemisphere.

“In humans and nonhuman primates, the central 1° of vision is processed by the foveola, a retinal structure that comprises a high density of photoreceptors and is crucial for primate-specific high-acuity vision, color vision and gaze-directed visual attention,” wrote Meizhen Qian, Jianbao Wang and their colleagues in their paper.

“We developed high-spatial-resolution ultrahigh-field 7T functional magnetic resonance imaging (fMRI) methods for functional mapping of the foveolar visual cortex in awake monkeys.”

To study how information gathered by the foveola of primates is represented in the brain, the researchers used sophisticated and high-definition imaging techniques developed by their lab. They applied these techniques to the study of macaque monkeys, which were awake and performing visual tasks that they were previously trained on.

These tasks required the monkeys to fixate their gaze on specific visual stimuli, namely flashing blue or red spots that varied in size. As the animals completed this task, Qian, Wang and their colleagues used ultrahigh field 7T fMRI techniques to map how the stimuli that they were fixating their gaze and thus were being picked up by the foveola, were represented in the monkeys’ brain.

“In the ventral pathway (visual areas V1–V4 and the posterior inferior temporal cortex), viewing of a small foveolar spot elicits a ring of multiple (eight) foveolar representations per hemisphere,” wrote Qian, Wang and their colleagues.

“This ring surrounds an area called the ‘foveolar core,’ which is populated by millimeter-scale functional domains sensitive to fine stimuli and high spatial frequencies, consistent with foveolar visual acuity, color and achromatic information and motion.”

Notably, the researchers also found that the fMRI recordings they collected were consistent with data they collected using optical imaging techniques. The detailed map of foveolar representations that they derived offers new precious insight into the complex underpinnings of the primate visual system, particularly the processes underpinning visual attention and focused vision.

Overall, the researchers found that foveolar representations are formed in eight sites found in each hemisphere of the primate brain: four on the upper (i.e., dorsal) and four on the lower (i.e., ventral) part of the brain. These sites form rings around a newly uncovered brain region, which the team dubbed the “foveolar core.”

“This elaborate representation of central vision coupled with a previously unknown foveolar core area signifies a cortical specialization for primate foveation behaviors,” wrote Qian, Wang and their colleagues.

The findings of this recent imaging study could soon pave the way for additional research aimed at further examining the foveolar loci uncovered by the researchers.

These works could further enrich the current understanding of how the visual system represents visual stimuli in the brain, which could contribute to the development of alternative treatments for some visual impairments.

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