One of the things that distinguish the picture on the right from a real toy ballerina is the feeling of depth that is evoked by a real object. Motion parallax is one such depth cue, when parts of the moving ballerina that are closer to the observer are more blurred than the parts that are further away. This is more obvious when a landscape is seen from the window of a moving train, when the foreground is more blurred than the background. Combined with other cues, such as binocular disparity (where the image received by each eye is slightly different and this difference depends on the distance of the object casting the image), information from motion parallax helps us to decide which things are near and which are far away.

On page 636 of this issue, Andrew Welchman and his colleagues at the University of Birmingham used functional imaging while testing their observers' perception to determine how these depth cues are combined by the brain. Although it is known that people are very good at combining these cues, there are many ways that these cues might be used by the brain. Information from each cue might be extracted independently, in separate parts of the cortex, or this information might be fused into a single measure.

To test these different possibilities, the authors showed subjects moving or stationary dot patterns that evoked a feeling of depth because of binocular disparity, motion parallax or both. They then looked for brain areas that showed the characteristic signature of extracting information from combining both the cues, rather than extracting information from each independently. They found that the visual cortical area V3B/KO fit this characteristic profile.