An fMRI study of the differential effects of word presentation rates (reading acceleration) on dyslexic readers' brain activity patterns

Several lines of evidence have recently provided a clear indication that word reading rate can be considered as an independent variable which influences comprehension as well as accuracy in reading. Thus, not only is fluent reading a critical characteristic of skilled (automatic) reading, it has been shown that faster reading does not necessarily incur a cost in terms of accuracy. Indeed, readers of various levels of reading proficiency, as well as clearly impaired readers (dyslexics), if made to read faster than their normal (routine) reading rate, can increase their decoding accuracy and comprehension. Using block design, blood-(de)oxygenation-level-dependent (BOLD) functional magnetic resonance imaging we studied the differences in brain activation patterns induced by reading and script processing in adult dyslexics and normal reading controls as a function of two word presentation rates. Word presentation rates were set individually for each participant to correspond to his/her routine reading rate (slow) and to a correspondingly faster rate (fast). Three task conditions were tested: sentences (plausibility judgment), single words (concrete/abstract judgment), non-words (homophonic judgment). Comprehension and accuracy in the faster presentation rates were unimpaired in both groups. There were no significant differences between the activation patterns induced in both groups in 'slow' reading of sentences and single words, but 'fast' reading was related to higher activations in visual areas in the normal readers. However, in the slow non-words condition the dyslexics were characterized by activations in the Lt IFG (Broca's area) and operculum, while the control readers clearly activated visual processing areas (extra-striate cortex). These differences in brain activation patterns were not found in the fast non-words condition. We propose that time-constrained (accelerated) script decoding may prompt the dyslexic brain to process graphemic information in a different manner compared to the one employed in unconstrained (routine) reading, in some conditions in a manner of processing much closer to the one employed by normal reading controls.