Tuesday, November 27, 2012
The universal reader
This is the pre-edited version of my latest, necessarily much-curtailed news story for Nature.
New study suggests the brain circuits involved in reading are the same the world over
For Westerners used to an alphabetic writing system, learning to read Chinese characters can feel as though it is calling on wholly new mental resources. But it isn’t, according to a new study that uses functional magnetic-resonance imaging (fMRI) to examine people’s brain activity while they read. The results suggest that the neural apparatus involved in reading might be common to all cultures, despite their very different writing systems, and that culture simply fine-tunes this.
Stanislas Dehaene of the National Institute of Health and Medical Research in Gif-sur-Yvette, France, and his coworkers say that reading involves two neural subsystems: one that recognizes the shape of the words on the page, and the other that decodes the physical motor gestures used to make the marks.
In their tests of French and Chinese subjects, they found that both groups use both systems while reading their native language, but with different emphases that reflect the different systems of writing. They describe their findings today in the Proceedings of the National Academy of Sciences USA .
“Rather than focusing on ear and eye in reading, the authors rightly point out that hand and eye are critical players”, says Uta Frith, a cognitive neuroscientist at University College London. “This could lead into novel directions – for instance, it might provide answers why many dyslexics also have very poor handwriting and not just poor spelling.”
Understanding how the brain decodes symbols during reading might not only offer clues into the origin of learning impairments such as dyslexia, but also inform learning strategies for general literacy and how these might be attuned to children or adults.
It has been unclear whether the brain networks responsible for reading are universal or culturally distinct. Some previous studies have suggested that alphabetic (such as French) and logographic (such as Chinese, where single characters represent entire words) writing systems might engage different networks.
There is evidence that all cultures use a shape-recognition region in the brain’s posterior left hemisphere, including in particular a so-called visual word-forming area (VWFA). But some research has implied that Chinese readers also use other brain networks that are unimportant for Western readers – perhaps because the Chinese logographic system places great emphasis on the order and direction of the strokes that make up a character, thereby engaging a ‘motor memory’ for writing gestures.
Dehaene and colleagues suspected that such motor aspects of reading are universal. Some educators have long advocated this: the Montessori method, for example, uses sandpaper letters that children can trace with their fingers to reinforce the gestural aspects of letter recognition. Motor processing is evidently universal for writing, involving a brain region known as Exner’s area, and the researchers postulated that this is activated in reading too, to interpret the gestures assumed to have gone into making the marks.
To examine what the brain is up to during reading, Dehaene and colleagues used fMRI to monitor brain activity in French and Chinese subjects reading words and characters in their own language in cursive script. They asked the subjects to recognize the words and recorded their response times.
However, unbeknown to the subjects, their responses were being manipulated in subtle ways by a process called ‘priming’. Before the word itself was presented on a screen, the subjects saw other words or symbols flashed up in just 50 milliseconds – too short a time, in general, for them to be registered consciously.
These subliminal images prepared the brain for the target word. If one of them was identical to the target word itself, subjects recognized the true target more quickly. The ‘masked’ images could also show ‘nonsense’ words written with the strokes progressing in the usual (forward) direction, or as the reverse (backward) of the usual gestural direction. Moreover, the targets could be shown either as static images or dynamically unfolding as though being written – both forwards and backwards. Finally, the target could also be distorted, for example with the letters unnaturally bunched up or the strokes slightly displaced.
The researchers used these manipulations both to match the amount of stimulus given to the subjects for the very different scripts of French and Chinese, and to try to isolate the different brain functions involved in reading. For example, spatial distortion of characters disrupts the VWFA involved in shape recognition, while words appearing dynamically stimulates Exner’s area (the motor network), but this network gets thrown if the words seem to be being written with backwards gestures. In each case, such disruptions slow the response time.
Dehaene and colleagues found that the same neural networks – the VWFA and Exner’s area – were indeed activated in both French and Chinese subjects, and could be isolated using the different priming schemes. But there were cultural differences too: for example, static distortion of the target slowed down recognition for the French subjects more than the Chinese, while the effects of gestural direction were stronger for the Chinese.
The researchers suspect that the gestural system probably plays a stronger role while the VWFA has not fully matured – that is, in young children, supporting the idea that reinforcement via the motor system can assist reading. “So far the motor decoding side has been rather neglected in reading education,” says Frith.
“It is conceivable that you find individuals where one system is functioning much better than the other”, she adds. “This may be a source of reading problems not yet explored. In the past I have studied people who can read very well but who can't spell. Perhaps the spelling aspect is more dependent on kinetic memories?”
However, psycholinguist Li-Hai Tan at the University of Hong Kong questions how far these results can be generalized to non-cursive printed text. “Previous studies using printed non-cursive alphabetic words in general have not reported activity in the gesture recognition system of the brain”, he says. “However, this gesture system has been found in fMRI studies with non-cursive Chinese characters. The motor system plays an important role in Chinese children's memory of characters, whether cursive or not.”
The universality of the ‘reading network’, say Dehaene and colleagues, also supports suggestions that culturally specific activities do not engage new parts of the brain but merely fine-tune pre-existing circuits. “Reading thus gets a free ride on ancient brain systems, and some reading systems are more user-friendly for the brain”, says Frith.
1. Nakamura, K. et al., Proc. Natl Acad. Sci. USA doi:10.1073/pnas.1217749109 (2012).