Tuesday, January 12, 2010
Imagine a cup. Now rotate it 180 degrees. What is it now? It’s still a cup. Surprising? Not really. We live in a three dimensional world and take for granted that something flipped around an axis doesn’t change into something else. This generalization across mirror images is called mirror invariance and holds true for most visual stimuli.
But what about the letter b? Draw it on something transparent, turn it around – and lo and behold, it changes into the letter d! Writing systems are one of the few domains where an object’s orientation matters. Our brains have trouble with this, and this difficulty becomes apparent when children learn to read and write. It is well documented that children often flip letters when writing, and sometimes even write entire words spontaneously flipped. It is only after years of practice that they get over this tendency.
Neuroscientist Stanislas Dehaene and colleagues recently found evidence of mirror invariance (and the lack thereof for words) in the brain.
They used a technique known as fMRI repetition suppression. The basic logic of the technique is this: brain regions responding to a repeated stimulus will show a decreased activation. This is thought to be due to neuron adaptation or fatigue. Therefore, by showing someone different visual stimuli and looking for repetition suppression, we can get an idea of which stimuli the brain categorizes as a the same (a repeat), and which are different.
In this particular study, reported in Neuroimage ( 29, (2010) 1837-1848), participants saw words and pictures that were preceded either by a normal or mirror reversed image of the same category. (The participants were asked to perform an unrelated size judgment task.) The researchers found that picture and word processing regions both show repetition suppression to repeated identical images. This was not surprising. It’s the basic repetition suppression effect.
Dehaene and colleagues then looked for areas that showed repetition suppression for words/pictures and their mirror images. They found a region in the left fusiform gyrus that showed mirror repetition suppression for pictures, but no such region that showed mirror repetition suppression for words. Therefore, they found evidence for brain regions that considered pictures equivalent to their mirror images, but no such regions for words.
Surprisingly, the region that showed the strongest mirror invariance effect for objects was an area of the left fusiform known as the Visual Word Form Area, an area that has been shown in many studies to be active during word processing. Therefore, it’s possible that the same exact brain region that processes pictures in a mirror invariant way knows to behave differently for words. With the low resolution of fmri, we can’t rule out the possibility that the word network and picture network are in fact separate, but it should be an interesting question to pursue.
Is this lack of mirror invariance specific to writing systems we’ve learned, or does it hold for any script-like stimulus? A follow up behavioral study suggests the latter. In this study, people saw words, tools, faces, and scripts. Each image was presented for a quick flash of 200ms each, and was preceded by another image of the same category, either normal orientation or flipped. Their task was to say whether the two stimuli were the same, where a mirror image was also considered the same.
As measured by reaction time, people were relatively quick to judge an object and its mirror reflection to be the same, but slower to judge two mirror image scripts to be the same. This difficulty held true even for unfamiliar or false scripts, suggesting that the brain generalized the importance of orientation to unfamiliar, unlearned scripts.
Ideas for future research? The obvious next step would be to do this with children. Also, does anyone know of any studies of word to object cross categorical priming? That would help answer the question of whether the word network and object networks are the same.
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