LONDON — Neuroscientists at King's College London studied zebrafish during the first five days of their lives to investigate whether visual features in the environment affect eye cell development. The researchers observed that zebrafish raised in environments with horizontal or vertical stripes developed differently shaped retinal neurons and altered visual behavior.

Zebrafish growing up surrounded by horizontal stripes developed neurons with different shapes and responses compared to fish growing up surrounded by vertical stripes. These changes in neuron shape and activity occurred within the retina. Neural activity sent from the retina to the brain was biased towards the stripe orientation the fish had encountered in their first days after birth.

Researchers placed half of the fish in environments with horizontal stripes and the other half in environments with vertical lines. They used a microscope to image neurons in the retina, finding that the two groups of fish had differently shaped neurons. The study found that the innate preference for parallel stripes is reduced in fish that grow up in a horizontally striped environment. Fish that grew up surrounded by vertical lines retained this innate preference.

Robert Hindges, Professor of Developmental Neurobiology, said, "Neither humans nor fish are born with a fully developed eye and brain. Many neurons are physically present but will be refined further - through a process known as plasticity - before adulthood." He added, "Classically, the retina has been portrayed as 'hardware' and refinement was previously thought to occur mostly inside highly adaptable parts of the brain such as the cerebral cortex. We were extremely surprised and excited to find experience-based plasticity at this level in a primary sensory organ – the retina." Hindges said, "What wasn't known was that these changes are being at least partially driven by changes in the earliest stage of visual processing: in the retina." He noted, "This has a striking potential to suggest that the places we grow up influence how we see the world. This is in keeping with studies in humans that have shown that people who grow up in different visual environments perceive optical illusions differently. People actually use horizontal and vertical features to make sense of more complex images like faces, where the nose is a strong vertical line and the brow is a clear horizontal line."

The researchers tested whether changes in the retina affected fish behavior after five days. This involved a virtual reality behavioral test, developed in collaboration with the University of Konstanz, which tracked fish movements as they chose between turning towards stripes parallel or perpendicular to their bodies. Zebrafish typically have an innate preference to swim towards stripes that are parallel to their body's orientation. Zebrafish that grew up in the horizontal striped environment failed to display this innate preference for parallel stripes.

Researchers used genetic manipulation to isolate the contribution of retinal plasticity to the observed behavior. Without the influence of plasticity in the retina, fish from both environments behaved the same. Phoebe Reynolds, a postdoctoral researcher, said, "We wanted to explore how these changes that are happening in the eye can impact the actual behavior of the fish. To do this, we developed a novel behavioral test in virtual reality where the fish can display a preference for lines of certain orientations. This let us test whether their innate preferences were affected by the environment they were raised in." Armin Bahl, a professor, said, "The design of the tracking setup and the behavioral paradigms were inspired by our observations of the structural and functional changes in the retina. Whether these features would also impact the ability to distinguish stripe patterns during behaviour was completely unclear. We were therefore very surprised that the conditions in which the animals were raised indeed had a major impact on performance." The study was published in the journal Neuron.