The role of local and global processing in the Muller-Lyer illusion

The role of local and global processing in the Muller-Lyer illusion

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The Müller-Lyer illusion has been tested, challenged and tested again many times.
There has only ever been one direct test of Day’s proposal (see Mundy, 2014), and that produced weak, but tantalising data: What if we could encourage our visual system to process more globally than it does normally? To always see the ‘whole’ instead of its parts? Assuming Day is correct, the illusion should be strengthened by greater attention to the whole figure – biasing the compromise we make when processing length. On the other hand, what if we were to encourage our visual system to process more locally? In that case, the illusion should be weakened by focussing perception to just single features, that is the line itself – ignoring the ‘whole’ and making our perception of length more accurate. Mundy (2014) found a particularly effective way to influence and modulate global and local processing through the use of Navon (1977) stimuli. The beauty of such stimuli relates to the fact that participants can respond to the large letter shape of such stimuli (global processing) or the smaller letters that make up the shape (local processing) whilst keeping the physical stimuli identical: