Depth perception, the ability to see the world in 3D, is a marvel of our visual system. “It’s the reason we feel embodied in the world around us rather than seeing a sequence of flat images,” explains Petr Znamenskiy, group leader of the Specification and Function of Neural Circuits Laboratory. 

This phenomenon poses a challenge for the brain: how to understand the world in 3D when images are projected onto the retina, a light-sensitive tissue at the back of the eye, in 2D. 

“Despite its complexity, depth perception is innate in many animals,” says Petr. “Classic psychology experiments in the 1960s have shown that human infants will avoid a simulated drop off a cliff at just 8-9 months old, as will chicks immediately after birth.”

Rats reared in the dark also avoid the drop, suggesting that the brain is primed for depth perception without needing a visual stimulus. In other words, we don’t learn how to perceive depth from experience; it’s already built in. 

Mapping depth perception in the brain

Petr is leading a new five-year project, funded by the European Research Council’s Consolidator Grant scheme, to understand how the brain is wired to sense depth perception without prior visual experience. 

Using an experimental set-up involving mice and virtual reality, his team are focusing on a feature of depth perception called motion parallax. This is where closer objects appear to move faster across a person’s field of vision than farther objects as the person moves. It’s the reason why station signs and people on the platform whizz by quickly when viewed from a moving train, but the hills in the distance can appear almost stationary. 

“In the VR experiment, mice navigate a virtual environment where visual stimuli are presented at different distances,” says Petr. “The mice have to use motion parallax to estimate depth and ‘report’ this by licking at one of two spouts.”

Petr’s team will use these experiments to measure the activity of different types of neurons in the mouse brain and correlate cell type to depth tuning: whether a neuron is tuned to respond to objects that are close by or far away. They can then artificially change the tuning of particular cell types to see if this influences the mice’s ability to navigate the environment.

“We’re going to tackle this question one cell at a time, correlating information about a neuron’s molecular identity and location in the tissue to what it does when the animals are exploring the environment,” says Petr. “We also aim to follow projections from each cell as they travel elsewhere in the brain, to understand where depth information is further processed.”

The ultimate goal for Petr is to understand a fundamental experience that is conserved throughout evolutionary history and allows full immersion in the world. 

Petr joins 349 mid-career researchers across Europe receiving Consolidator Grants this year, including Crick satellite group leader Charlotte Odendall. With funding from the EU’s Horizon Europe programme, these grants aim to support cutting-edge research in 25 EU member states and associated countries.