Despite the nursery rhyme about three blind mice, mouse sight is surprisingly sensitive. Studying how mice see has helped researchers uncover unprecedented details about how individual brain cells communicate and work together to create a mental image of the visual world.
I’m a neuroscientist who studies how brain cells drive visual perception and how these processes can go awry in conditions like autism. My lab “listens” to the electrical activity of neurons in the outermost part of the brain called the cerebral cortex, a large part of which processes visual information. Lesions to the visual cortex can cause blindness and other visual deficits, even when the eyes themselves are not injured.
Understanding the activity of individual neurons—and how they work together as the brain actively uses and processes information—is a long-standing goal of neuroscience. Researchers have come much closer to achieving this goal thanks to new technologies aimed at the mouse visual system. And these findings will help scientists better understand how people’s visual systems work.
The mind in the blink of an eye
For a long time, researchers thought that vision in mice was slow and unclear. But it turns out that neurons in the visual cortex in mice—like those in humans, monkeys, cats, and ferrets—require specific visual features to trigger activity and are especially selective under conditions of alertness and wakefulness.
My colleagues, I, and others have discovered that mice are especially sensitive to visual stimuli right in front of them. This is surprising, because the mouse’s eyes look outward instead of forward. Forward-facing eyes, such as those of cats and primates, naturally have a larger forward focus area compared to outward-facing eyes.
This finding suggests that specialization of the visual system to highlight the frontal visual field appears to be shared between mice and humans. In the case of mice, visually focusing on what is right in front of them can help them be more sensitive to shadows or edges in front of them, avoiding threatening predators or better hunting and capturing insects for food.
It is important to highlight that the vision center is the one that most affects aging and many visual diseases in people. Since mice also rely heavily on this part of the visual field, they may be especially useful models for studying and treating visual impairment.
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A thousand voices drive complicated decisions
Technological advances greatly accelerated scientific understanding of vision and the brain. Researchers can now routinely record the activity of thousands of neurons at the same time and pair this data with real-time videos of a mouse’s face, pupil, and body movements. This method can show how behavior interacts with brain activity.
It’s like spending years listening to a grainy recording of a symphony with a single featured soloist, but now you have a pristine recording where you can hear each musician reading every finger movement note for note.
Using these improved methods, researchers like me study how specific types of neurons work together during complex visual behaviors. This involves analyzing how factors such as movement, alertness and the environment influence visual activity in the brain.
For example, my lab and I discovered that the speed of visual signaling is very sensitive to possible actions in the physical environment. If a mouse rests on a running disc, visual signals travel to the cortex faster than if the mouse sees the same images while resting on a stationary tube, even when the mouse is completely still in both conditions.
To connect electrical activity with visual perception, researchers must also ask the mouse what it thinks it sees. How have we done this?
Over the past decade, researchers have debunked long-standing myths about mouse learning and behavior. Like other rodents, mice are also surprisingly cunning and can learn to “tell” researchers about the visual events they perceive through their behavior.
For example, mice can learn to release a lever to indicate that they detected that a pattern was brightened or tilted. They can turn a Lego wheel left or right to move a visual stimulus to the center of a screen like in a video game, and they can stop running on a wheel and licking a water trough when they detect that the visual scene has suddenly changed.
Mice can also use visual cues to focus their visual processing on specific parts of the visual field. As a result, they can respond more quickly and accurately to visual stimuli that appear in those regions. For example, my team and I found that a faint visual image in the peripheral visual field is difficult for mice to detect. But once they notice it—and tell us it by licking a gutter—their subsequent responses are faster and more precise.
These improvements come at a cost: if the image appears unexpectedly elsewhere, the mice are slower and less likely to respond to it. These findings are similar to those found in studies on spatial attention in people.
My lab has also discovered that certain types of inhibitory neurons—brain cells that prevent the spread of activity—strongly control the intensity of visual signals. When we activated certain inhibitory neurons in the visual cortex of mice, we could effectively “erase” their perception of an image.
These types of experiments also reveal that the boundaries between perception and action in the brain are much less separate than previously thought. This means that visual neurons will respond differently to the same image in ways that depend on behavioral circumstances; For example, visual responses differ if the image will be successfully detected, if it appears while the mouse is moving, or if it appears when the mouse is thirsty or hydrated.
Understanding how different factors shape the rapid response of cortical neurons to visual images will require advances in computational tools that can separate the contribution of these behavioral signals from visual ones. Researchers also need technologies that can isolate how specific types of brain cells transport and communicate these signals.
Data clouds surrounding the globe
This surge in research into the mouse visual system has led to a significant increase in the amount of data that scientists can not only collect in a single experiment, but also share publicly with each other.
Leading national and international research centers focused on unraveling the circuitry of the mouse visual system have led the introduction of new optical, electrical and biological tools to measure large numbers of visual neurons in action. Additionally, they make all data publicly available, inspiring similar efforts around the world. This collaboration accelerates researchers’ ability to analyze data, replicate findings, and make new discoveries.
Technological advances in data collection and sharing can make the culture of scientific discovery more efficient and transparent, a key goal of data computing in the coming years.
If the last 10 years are anything to go by, I believe these discoveries are just the tip of the iceberg, and the powerful, not-so-blind mouse will play a major role in the continuing quest to understand the mysteries of the human brain.
*Bilal Haider is an associate professor of Biomedical Engineering at the Georgia Institute of Technology.
This article was originally published on The Conversation
