Vision plays a crucial role in detecting and avoiding predators for many animals, including mice. New research shows that to survive, it is essential for mice to have not just one, but two eyes. While this may seem not surprising, the precise mechanisms by which visual information from both eyes is processed in the brain to trigger survival responses has remained largely unknown in neuroscience.
A team led by neuroscientists Robin Broersen (Erasmus MC/Australian National University [ANU]) and Greg Stuart (Monash University/ANU) sheds light on this issue for the first time in the scientific journal Current Biology.
The importance of binocular vision
When exposed to danger, all animals, including ourselves, respond by exhibiting the so-called ‘freeze-or-flight’ response. We either remain still and hope the danger will pass without noticing us, or we run away. Mice are no different. When a mouse detects an aerial predator such as a bird of prey, it will either freeze or escape to a safe location.
‘In our experiment, we simulated a bird of prey attack and studied how mice responded’, explains Robin Broersen, first author of the paper. ‘We observed that mice with two eyes opted for the flight response more often, while mice with only one eye were more likely to freeze. Additionally, mice with two eyes escaped more quickly and effectively. This suggests that binocular vision is crucial for assessing threats and triggering the most effective survival response.’
Crossroad
The researchers then determined how the visual information from the eyes is integrated in the brain. They discovered a brain region called the superior colliculus, which coordinates the freeze-or-flight response in mice, plays a key role in this process.
Broersen explains: “We examined what happens in the neurons of the superior colliculus when we stimulate a mouse’s eyes with flashes of light or visual patterns. We measured electrical signals in neurons of the superior colliculus, which are smaller than the diameter of a human hair. We discovered that visual information from the eye travels to the superior colliculus via multiple ‘highways.’ The superior colliculus acts as a crossroad, where different visual pathways converge. We now also have a better understanding of how the superior colliculus integrates information from different sources.”
Turning brain regions on or off with light
An important technique used in this study is called optogenetics. This method involves introducing special proteins into neurons, allowing specific neuronal pathways to be turned on or off by exposing them to light. By deactivating groups of neurons in different brain region while stimulating the eyes, and then observing whether the response in superior colliculus neurons changes, the researchers determined the involvement of these brain regions in binocular vision. Conversely, by activating neurons in different brain regions they were able to determine how neurons are connected.
‘This last method allowed us to study one of these pathways in brain slices—specifically, the direct connections between the eye and the superior colliculus’, says Felix Thomas, co-author of the study. ‘By activating these connections with light, we were able to measure which neurons in the superior colliculus are directly connected to the eye and which neurons receive their information through an indirect pathway.’
The researchers also found that the superior colliculus does not follow simple arithmetic rules. Thomas explains: ‘Contrary to expectations, the simple rule of ‘1+1=2’ does not apply to superior colliculus neurons. The response of neurons when stimulating both eyes simultaneously was less than what you would expect if you simply added up the responses from stimulating each eye individually. We believe this happens because some neurons inhibit others, rather than activating each other. However, the exact structure of this neural network still needs further investigation.’
This study reveals how the brain processes visual information from both eyes in a part of the brain critical for responding to danger, the researchers conclude. ‘How exactly this works turns out to be a complex process, part of which we have now clarified’, says Broersen.
Mice versus humans
Humans also have a superior colliculus, but relative to the entire brain, it is much smaller than in mice. Broersen states: ‘Despite differences between species, the superior colliculus still plays a significant role in humans. People with damage to the visual cortex—who are therefore blind—can sometimes still perceive moving objects, a phenomenon known as ‘blindsight’. The superior colliculus is thought to be crucial in this process. Additionally, we know that this region contributes to directing eye and head/body movements toward salient objects in the visual field.’
Thomas adds: ‘Our findings provide important insights into how the brain combines visual information to make rapid decisions. This study brings science one step closer to unravelling the complex workings of the brain and how sensory information is translated into behavior. As a result, it lays the foundation for the development of new applications and therapies.’
An artistic interpretation of the research. This image is composed of several brain slices with fluorescent neurons, photographed using a confocal microscope. These slices were edited and arranged in the shape of an eye, reflecting the researchers’ work. | © Yas Crawford, 2021. The Eye I. Yas Crawford is Fine Art Photographer and Visual Artist, who has exhibited internationally and won multiple international awards, including the 2021 Art of Neuroscience Award. Website Instagram LinkedIn
