It's a big skull. No, wait, it's two people under an arch. Hold on, it's a skull again. Two very different images can be perceived in this trick picture and now we are one step closer to working out how the brain spontaneously flips between such views, with the discovery of what may be the relevant brain region.
The precise neural mechanism that provokes the brain to switch its view of a scene is unknown, but it is thought to play a major role in perception by acting as a sort of reality check, says Ryota Kanai of University College London. "We need a trigger to prompt possible different interpretations so that we don't get stuck with a potentially incorrect interpretation of the world."
To find out which part of the brain might be involved, Kanai and colleagues asked 52 volunteers to watch a video of a revolving sphere and press a button when the rotation of the sphere appeared to change direction. Crucially, the sphere was not changing direction; it could simply be perceived to be rotating in either direction. How long each rotation-direction was perceived for was recorded and an average "switch rate" assigned to each of the volunteers.
The team then used structural magnetic resonance imaging to search for active brain regions during this task. This pointed to the superior parietal lobes (SPL), two areas towards the back of the head known to control attention and process three-dimensional images. People whose cortex was thicker and better connected in this region had faster switch rates.
To test whether or not the SPL had a role in triggering the switch, the researchers stimulated each lobe with a magnetic field - effectively knocking out the function of that lobe - while the volunteers rewatched the sphere illusion. The team found that the switch rate slowed when either lobe was exposed to the magnetic field (Current Biology, DOI: 10.1016/j.cub.2010.07.027).
"One possibility is that the SPL is actually triggering the perceptual switches by sending a signal to 'reset' the illusion," says Kanai. "Or it could be that people with a large SPL are better at noticing other possible interpretations for the ambiguous sphere, which would also result in a faster switch rate."
Andrew Parker at the University of Oxford reckons the work is intriguing but adds that there is more to be done to confirm that the region is directly involved in provoking these perceptual changes.
Image: Mary Evans Picture Library
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