Functional neuroanatomy of impaired reading in dyslexia

In reading tasks, an underactivation of the left inferior occipitotemporal cortex in dyslexia seems to be the most consistent finding both in neurophysiological and hemodynamic studies. This marked difference appears at about 150 msec after word presentation when the brain enters the letter-string-specific (or, more generally, object-specific) processing stage. This early prelexical deficit in the letter-string-specific processing stage is apparently the immediate reason for the manifest difficulties in reading. Recent findings strongly suggest that in dyslexia the object-specific analysis stage is intact when faces are processed (Tarkiainen, Helenius, & Salmelin, 2003), thus pointing to a deficit that is rather specific to letter strings. The ability to read is a very new feature in human evolution. The letter-string-specific activation most probably develops with motivated exposure to written text. The important question is why this rapid visual recognition system that would immediately set letter strings apart from other objects does not develop normally in dyslexia. The contribution of functional neuroimaging in the attempt to understand the basis of impaired reading in dyslexia is discussed.