Neurobiology of dyslexia

Areas of the brain involved in reading

Many studies have looked at the parts of the brain that are activated in typical readers (and weak readers).

Neuro-imaging methods have revealed three brain regions involved in reading that develop as children gain word-level reading skills (see Figure 1):

  • Left frontal lobe (inferior frontal gyrus (IFG) & precentral gyrus (PG)stores information about the sounds in words and sequencing of these sounds. This area is active when reading aloud or silently.  The precentral gyrus is involved in articulation.
  • Left temporoparietal region (superior temporal gyrus (STG), supramarginal gyrus (SMG), angular gyrus (ANG) – This region is considered the “decoding” center of the reading network, linking letters and sounds within words, as well as linking to meaning.
  • Left occipitotemporal (OT) region (incl fusiform gyrus, inferior temporal gyrus, middle temporal gyrus (MTG) – recognizes familiar letters and words, and processes ‘sight’ words and meanings.
Reading pathways in the brain
Figure 1. Reading centres and pathways in the brain. With permission from Dr. Devin Kearns. 2019.

Reading pathways in the brain

These three areas of the brain are involved in TWO reading pathways:

  • Dorsal pathway (shown in red in Figure 1) is used by good readers to decode
    unknown words.
  • Ventral pathway (shown in green in Figure 1) is used by good readers to
    read familiar words that have been stored in long term memory.  Reading words via this pathway is faster and more automatic.

It is thought that beginner readers first use the dorsal pathway (decoding) when they are learning to read.  In early reading stages, most words must be ‘decoded’, that is, the letters in the word must be linked to the sounds they represent.  As words become familiar and stored in long term memory, they can be read more quickly through the ventral pathway; reading becomes much more automatic.  At this stage, it is very difficult to look at a familiar word and not read it – the process is so automatic.    Words that are read ‘automatically’ are often called ‘sight words’.

Orthographic mapping

This process where a word’s spelling and meaning gets stored into permanent, long-term memory as a ‘sight word’ is termed “orthographic mapping”.   Research has shown that orthographic mapping is enabled by phonological awareness and grapheme-phoneme knowledge (Ehri 2014; Yoncheva et al., 2015), and that instruction in letter-sound mapping also supports the self-teaching of unfamiliar but decodable words (Yoncheva et al., 2015).  Read more about orthographic mapping on the Science of reading page.

Dyslexia and the neurobiology of reading

Functional and structural differences have been found in parts of the brain used for reading in people with dyslexia compared to normal readers and these differences have been found prior to learning to read (Kearns et al., 2019, Norton et al, 2015; Ozernov-Palchik et al., 2016).  Differences in connectivity efficiency between the areas of the brain in reading have also been reported ( Finn et al., 2014; Lou et al., 2018; Raschle et al., 2011; Saygin et al, 2013; Saygin et al., 2016; Tschentscher et al., 2019; Vandermosten et al., 2016, Zhou et al., 2015).

For example, some people with dyslexia do not have the same activation in the brain in the same areas that are activated in good readers.  Some people may have good activation in the dorsal pathway (they can decode accurately) but the ventral pathway is not efficient, and they struggle with automaticity.

Dyslexia runs in families and several candidate genes for dyslexia susceptibility have been identified (Ozernov-Palchik et al., 2016).

There is evidence from neurological research that reading (especially decoding) difficulties can often be remediated with appropriate reading instruction. Recently, studies have shown that effective remediation/instruction is associated with increased activation or normalization of regions that typically show reduced or absent activation in dyslexia (Barquero et al, 2014; Gabrieli, 2016).

Finally, neuroimaging studies (along with many behavioural studies) support the use of foundational word-recognition instruction (Kearns et al., 2019) as provided by structured literacy instruction.