There are different techniques for imaging. fMRI (functional magnetic resonance imaging) has been very popular since the early 1990s. It is non-invasive but involves the person under examination lying immobile in the apparatus and responding to various stimuli, for example reading, counting dots, speaking, etc.
The brain activity of the participant is also measured when s/he is not doing anything (resting activity). By subtracting the brain activity when participants perform the target task from the brain activity when participants are resting, one obtains the pattern of areas of the brain which are activated, through an increase in blood flow in the areas which are activated to perform the target task.
In the key studies, fMRI has provided conclusive proof that different areas of the brain are activated when subjects with and without dyslexia read a text.
In general, people with dyslexia show a strong symmetric pattern of activation, whilst people without dyslexia show activity which is conspicuously lateralized. In these participants, the processing of the written language mainly happens in the left hemisphere.
Another technique is called positron emission tomography (PET). It uses a radioactive tracer injected into the person under investigation which initiates a cascade of physical processes that can be imaged three-dimensionally matching the brain activity of specific regions.
In a study published in 2001, Eraldo Paulesu and colleagues showed that people with dyslexia in Italian, which has a less complicated orthography than English or French, performed better than English- and French-speaking people with dyslexia on reading tasks.
However, people with dyslexia in all three languages performed worse than people without dyslexia in reading and phonological tasks. They concluded that there is a universal neurocognitive basis for dyslexia and that differences in reading performance in different countries are due to different orthographies.
In the scans shown below, image A shows brain activation in a group of people without dyslexia, B groups of people with dyslexia and C the brain areas significantly more activated in readers without dyslexia compared to readers with dyslexia. (You can relate C to the images from Stanislas Dehaene presented in ‘Neuroscience’ (see below).)
Paulesu et al., ‘Dyslexia: Cultural Diversity and Biological Unity’, Science, Vol 291, 16 March 2001
reproduced with permission
A non-invasive technique is functional near-infrared spectroscopy (fNIRS). Mahdi Mahmoudzadeh and colleagues measured patterns of blood composition and hence neural metabolic activity from sensors placed over the skull of asleep babies born pre-term by 10 weeks. Their results showed that the immature brain, in which all the neurons had not fully migrated, was able to start deciphering the speech signal. They tested for the phoneme distinction ‘ba’ vs ‘ga’, and syllables pronounced by male and female voices. The right hemisphere responded to change of sex of the speaker and change of syllable but only the left hemisphere responded to change of phoneme.Mahmoudzadeh, M., Dehaene-Lambertz, G., Fournier, M., Kongolo, G., Goudjil, S., Dubois, J. Grebe, R., and Wallois, F. (2013) ‘Syllabic discrimination in premature human infants prior to complete formation of cortical layers’,Proceedings of the National Academy of Sciences, Vol 110