Whole-head magnetoencephalography (MEG) was employed to track noninvasively the cortical activation sequences during visual word recognition in 6 adult dyslexic and 8 control subjects examined at the Brain Research Unit, Helsinki University of Technology, Espoo; and the Departments of Psychology and Radiology, University of Helsinki, Helsinki, Finland. Significant differences between the two groups were found for the time window 0 to 200 msec after single word presentation in the left inferior temporo-occipital cortex, for 200 to 400 msec in the left temporal lobe, and for 0 to 400 msec in the left inferior frontal lobe. Considerable interindividual variability was shown for spatiotemporal activation patterns. Dyslexics failed to activate the left inferior temporo-occipital cortex within 200 msec after word presentation. The left temporal lobe, including Wernicke’s area, a region associated with phonological aspects of language, was strongly involved in controls but not in dyslexics. Dyslexics activated, instead, the left inferior frontal lobe, involving Broca’s area, whereas activation of the right motor/premotor cortex, present in controls, was absent in dyslexics. Perception of words as specific units was impaired in dyslexies. 
COMMENT. An impaired perception of visual word processing of written words, resulting from dysfunction of auditory language areas in the left inferior temporoparietal area, appears to be a factor in the causation of dyslexia in some subjects. Early training in auditory language might help in the prevention of dyslexia.
Poeppel D and Rowley HA, Biomagnetic Imaging Laboratory, University of California, San Francisco, comment that the utility of magnetic source imaging (MSI) lies in the combination of MEG with the anatomic images supplied by MRI, providing anatomic location of activity at a given time-sampling point. MSI may be used clinically for presurgical mapping in evaluation of patients with epilepsy and determination of hemispheric dominance. The cost of MEG installations and MSI systems limits their practical use at present.