A Yale-led study uncovered unexpected communication between retinal visual pathways that were thought to operate separately.

Researchers at Yale School of Medicine (YSM) have discovered unexpected insights into how the eye handles visual information.

When we view a scene, the visual system quickly separates different features, including color, contrast, and motion, and processes them independently. This process, known as parallel visual processing, helps the brain rapidly interpret the world around us.

Scientists have long believed that this separation begins in the retina and remains intact as information moves through the visual system. However, a new study published in Neuron suggests the pathways are more interconnected than previously recognized. According to the researchers, this integration may improve the ability to detect weak visual signals, such as those encountered in low light.

“We found that while different channels can deliver their own features, they’re also interconnected by underlying electrical circuitry,” says Yao Xue, PhD, a postdoctoral fellow in the department of ophthalmology and visual science at YSM and the study’s first author.

Bipolar Cells Show Unexpected Signal Crosstalk

Vision starts with rods and cones, specialized retinal cells that detect light and pass information to neurons known as bipolar cells. Within these cells, visual information related to factors such as brightness, color, shape, and contrast is divided into more than a dozen parallel pathways.

When the researchers examined the synapses of bipolar cells, where cells communicate with one another, they found evidence that these pathways are not entirely separate.

Neurons communicate through two types of synapses: chemical and electrical. Chemical synapses rely on neurotransmitters that carry signals between cells. Electrical synapses, also called gap junctions, allow signals to pass directly through electrical currents. Bipolar cells are generally thought to communicate mainly through chemical synapses.

The researchers discovered that electrical synapses connected many of the bipolar cell pathways that had previously been considered separate in both mouse and human retinas. When they stimulated a single bipolar cell, the resulting neurotransmitter release was not confined to that cell’s pathway. Instead, signaling spread across a wider network, creating diffuse, cloud-like activity patterns that revealed extensive communication between different bipolar cell types.

“When we stimulated one bipolar cell, many bipolar cells released neurotransmitters,” says Z. Jimmy Zhou, PhD, Marvin L. Sears Professor of Ophthalmology and Visual Science and principal investigator.

BC6 Cells Act as a Visual Network Leader

The researchers also identified a specific bipolar cell type, known as BC6, that appeared to coordinate this activity. These cells produced strong signals that spread through the parallel pathways in an organized hierarchy. “People had assumed that the different types of bipolar cells were more or less autonomous,” Zhou says. “But we found a driver among all these cell types that creates this network with a hierarchy.”

While separate pathways allow bipolar cells to process different aspects of visual information efficiently, the electrical connections between them may provide an important advantage when signals are weak.

“If the signal is already very weak and is divided into several channels, there isn’t much left for each channel to process,” says Seunghoon Lee, PhD, a research scientist in the department of ophthalmology and visual science at YSM and co-corresponding author of the study. “The integration is particularly useful for detecting low-contrast signals or signals from very small objects.”

“And the cells aren’t cooperating in a random way,” adds Xue. “There’s a commander within them—BC6—that leads them in relaying signals to the downstream target.”

Innovative Retina Recording Techniques Reveal New Insights

To investigate the circuitry of bipolar cells, the team combined multiple experimental approaches. These included imaging techniques that tracked cellular activity and neurotransmitter signaling, along with methods that stimulated bipolar cells and recorded responses in neighboring cells.

Studying bipolar cells is challenging because they are located deep within the retina. In many previous studies, researchers sliced retinal tissue to gain access to these cells, a process that can disrupt normal circuitry. In this study, the team instead used a dual patch-clamp technique on fully intact mouse retinas. The method uses electrodes to stimulate specific bipolar cells and measure the responses of connected cells.

“No other lab in the world has been able to pull off these kinds of recordings systematically,” says Zhou. “It is a tour de force of Yao Xue’s PhD thesis work, pairing an innovative approach with exceptional electrophysiological skill.”

The researchers then repeated the experiments using human retinas obtained through the Department of Pathology’s Legacy Tissue Donation Program. According to the team, these are the first experiments of this type conducted in an intact human retina.

Implications for Brain Function and Eye Disease

Because the retina is part of the central nervous system, understanding how it processes visual information may provide broader insights into how neural circuits function throughout the brain. The findings could also help researchers better understand diseases that affect retinal function, including macular degeneration, glaucoma, and congenital night blindness.

The study also highlights the value of curiosity-driven research in uncovering fundamental biological mechanisms.

Our experiments didn’t begin with a specific hypothesis but revealed a fundamental processing mechanism in the visual system,” says Lee. “It’s an important reminder of how essential curiosity-driven research is to discovery.”