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Depth Perception
21 May 2015

Depth Perception

How do we discern distance? Here’s an illusion where both white lines are equal – so why do our brains interpret the top line as longer? One theory suggests that, as babies, we learn that things in the distance appear smaller so, the brain compensates and interprets the line as longer. But while this theory appeals to common sense, it’s not evidence-based. The problem is how to obtain the evidence – most people start to understand perspective when they’re infants but testing an illusion on babies isn’t practical. Instead, researchers showed the illusion to older children who‘d just acquired full vision after being blind from birth. Unexpectedly these individuals were immediately susceptible to the illusion – they saw the top line as longer despite having no experience of perspective. So it’s how the brain processes images, not previous learning, that makes this trick work.

Written by Julie Webb

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BPoD stands for Biomedical Picture of the Day. Managed by the MRC Laboratory of Medical Sciences until Jul 2023, it is now run independently by a dedicated team of scientists and writers. The website aims to engage everyone, young and old, in the wonders of biology, and its influence on medicine. The ever-growing archive of more than 4000 research images documents over a decade of progress. Explore the collection and see what you discover. Images are kindly provided for inclusion on this website through the generosity of scientists across the globe.

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