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

Julie Webb

Writer

Julie began her career as a researcher in genetics. In 2000 she won a Wellcome Trust award for her work on a collaborative science drama, staged at the Edinburgh Festival. She now works as a freelance science writer and communicator.

• Original image by Darren Lewis
• Image reproduced from work by Tapan Gandhi and colleagues
• Copyright Elsevier 2015
• Research published in Current Biology, May 2015