Brain Bases of Reading Fluency in Typical Reading and Impaired Fluency in Dyslexia

Although the neural systems supporting single word reading are well studied, there are limited direct comparisons between typical and dyslexic readers of the neural correlates of reading fluency. Reading fluency deficits are a persistent behavioral marker of dyslexia into adulthood. The current study identified the neural correlates of fluent reading in typical and dyslexic adult readers, using sentences presented in a word-by-word format in which single words were presented sequentially at fixed rates. Sentences were presented at slow, medium, and fast rates, and participants were asked to decide whether each sentence did or did not make sense semantically. As presentation rates increased, participants became less accurate and slower at making judgments, with comprehension accuracy decreasing disproportionately for dyslexic readers. In-scanner performance on the sentence task correlated significantly with standardized clinical measures of both reading fluency and phonological awareness. Both typical readers and readers with dyslexia exhibited widespread, bilateral increases in activation that corresponded to increases in presentation rate. Typical readers exhibited significantly larger gains in activation as a function of faster presentation rates than readers with dyslexia in several areas, including left prefrontal and left superior temporal regions associated with semantic retrieval and semantic and phonological representations. Group differences were more extensive when behavioral differences between conditions were equated across groups. These findings suggest a brain basis for impaired reading fluency in dyslexia, specifically a failure of brain regions involved in semantic retrieval and semantic and phonological representations to become fully engaged for comprehension at rapid reading rates

Activations for Fast > Medium > Slow contrasts for Typical Readers.

<p><i>p</i><.001, cluster level FDR corrected (T = 3.50); ET  = 10. N/A  =  Not applicable. L. =  Left hemisphere. R. =  Right hemisphere. Coordinates reported in Talairach space.</p

Sentence presentation rate differentially impacts brain activation by group.

<p>Fast > Medium > Slow parametric modulation (cluster level FDR corrected) for a) Typical Reader Group (p<.001) (top panel); b) Dyslexic Group (p<.001) (middle panel); c) Typical > Dyslexic Groups (p<.01) (bottom panel). Color bar indicates T-values.</p

Participant Scores for Typical Readers and Readers with Dyslexia.

<p>Mean ± SD; <i>p</i> values below .05 are statistically significant based on two-tailed t-tests. Note: Standard scores are based on a mean of 100 and a standard deviation of 15 (average range of 85-115) except for the CTOPP (based on mean of 10 and a standard deviation of 3; average range of 7-13) and NDRT (based on mean of 200 and a standard deviation of 25; average range of 175-225). SD  =  Standard deviation.</p

The Dyslexic Group showed reduced activation relative to the Typical Reader Group even when accuracy differences across conditions were equated between the groups.

<p>Greater activation for Fast > Slow contrast for Typical Readers versus Medium > Slow contrast for the Dyslexic Group (cluster level FDR corrected results displayed at p<.001). Color bar indicates T-values.</p

Activations for Typical Reader > Dyslexic Groups.

<p><i>p</i><.01, cluster level FDR corrected (T = 2.51); ET  = 10. N/A  =  Not applicable. L. =  Left hemisphere.</p

Activations for Fast > Medium > Slow contrasts for Readers with Dyslexia.

<p><i>p</i><.001, cluster level FDR corrected (T = 3.50); ET  = 10. N/A  =  Not applicable. L. =  Left hemisphere. R. =  Right hemisphere. Coordinates reported in Talairach space.</p