Tuesday July 31 2007
CEA
Reading - is it really cultural?
Neuron (2007) 55 :143-156.
CEA
To gain a better understanding of the brain mechanisms involved in deciphering words, letters…
A research team from the Cognitive Neuroimaging Laboratory (joint CEA-Inserm 562 unit) at NeuroSpin (DSV/ I²BM/NeuroSpin) working with Paris 6 University and the Paris hospitals has revealed that a region in the visual cortex of the left hemisphere, which is known to be essential to word-form recognition, is far from being a homogeneous structure. In fact, it presents a high degree of functionally and spatially hierarchical organization.
When we read a word, the brain goes through two core steps:
1. the visual system identifies the letters and their order
2. while the language system channels access to the senses, to pronunciation, etc.
The research team employed fMRI (functional magnetic resonance imaging) to study the visual stage of reading. It tested the claim that word-form analysis performed by the visual system is only a special case of the more general object-form analysis. In more detailed terms, data obtained from primate studies suggests that the visual system codes objects by hierarchically ranking successive neurons detecting increasingly complex fragments and possessing increasingly large receptors, and which are located progressively further towards the front of the occipitotemporal cortex.
When reapplied to reading, this data prompted the researchers to work on the hypothesis that the neurons in the occipitotemporal visual cortex (when running forward from the rear) detect increasingly complex word fragments: letter fragments, letters, pairs of letters (bigrams), groups of 4 letters (quadrigrams), and upwards. They tested this idea by presenting subjects with letter strings comprised of commonly-occurring or rarer letters, commonly-occurring or rarer bigrams, commonly-occurring or rare quadrigrams, and real, complete words. As expected, when you progress through the visual system of the left hemisphere, the area activations depended on the presence of increasingly ‘long’ frequently-occurring fragments (letters > bigrams > quadrigrams).
In more general terms, this means that as we are learning to read, the visual cortex of the left hemisphere ‘learns’ statistical regularities in letter combinations – regularities that are wholly specific to the writing system and language concerned. This represents specific brain fingerprinting of the culturally-acquired skill of reading.
Gradient Images Illustrating the Spatial Layout of Sensitivity
of the Occipitotemporal Cortex to Stimulus Manipulation