Audio-visual speech perception in adult readers with dyslexia: an fMRI study.

Developmental dyslexia is a specific deficit in reading and spelling that often persists into adulthood. In the present study, we used slow event-related fMRI and independent component analysis to identify brain networks involved in perception of audio-visual speech in a group of adult readers with dyslexia (RD) and a group of fluent readers (FR). Participants saw a video of a female speaker saying a disyllabic word. In the congruent condition, audio and video input were identical whereas in the incongruent condition, the two inputs differed. Participants had to respond to occasionally occurring animal names. The independent components analysis (ICA) identified several components that were differently modulated in FR and RD. Two of these components including fusiform gyrus and occipital gyrus showed less activation in RD compared to FR possibly indicating a deficit to extract face information that is needed to integrate auditory and visual information in natural speech perception. A further component centered on the superior temporal sulcus (STS) also exhibited less activation in RD compared to FR. This finding is corroborated in the univariate analysis that shows less activation in STS for RD compared to FR. These findings suggest a general impairment in recruitment of audiovisual processing areas in dyslexia during the perception of natural speech.

Audiovisual speech integration in the superior temporal region is dysfunctional in dyslexia.

Dyslexia is an impairment of reading and spelling that affects both children and adults even after many years of schooling. Dyslexic readers have deficits in the integration of auditory and visual inputs but the neural mechanisms of the deficits are still unclear. This fMRI study examined the neural processing of auditorily presented German numbers 0-9 and videos of lip movements of a German native speaker voicing numbers 0-9 in unimodal (auditory or visual) and bimodal (always congruent) conditions in dyslexic readers and their matched fluent readers. We confirmed results of previous studies that the superior temporal gyrus/sulcus plays a critical role in audiovisual speech integration: fluent readers showed greater superior temporal activations for combined audiovisual stimuli than auditory-/visual-only stimuli. Importantly, such an enhancement effect was absent in dyslexic readers. Moreover, the auditory network (bilateral superior temporal regions plus medial PFC) was dynamically modulated during audiovisual integration in fluent, but not in dyslexic readers. These results suggest that superior temporal dysfunction may underly poor audiovisual speech integration in readers with dyslexia.

Implicit learning is intact in adult developmental dyslexic readers: evidence from the serial reaction time task and artificial grammar learning.

Previous research yielded equivocal results concerning implicit learning abilities of developmental dyslexic readers. These studies employed a sequence learning task that requires a motor response to each stimulus. However, implicit learning has been often studied using non-motor tasks. Thus, we investigated implicit learning capabilities of adult developmental dyslexic readers in two standard implicit learning paradigms differing in the involvement of the motor system, namely the serial response time task (SRTT) and artificial grammar learning (AGL).Twelve adult developmental dyslexic and twelve age- and sex- matched normal readers were tested. In the serial response time task (SRTT), participants are exposed to a structured display. Learning is measured by comparing response time (RT) to the structured sequence with RT to a random display. In the artificial grammar learning task (AGL), letter strings following a markovian finite state grammar are presented. In a subsequent test phase subjects have to judge new letter strings according to their grammaticality. Learning of the stimulus regularities was found in both tasks and for both groups of subjects. Furthermore, participants were unaware of the underlying stimulus construction principles. Dyslexic readers were unimpaired in SRTT as well as artificial grammar learning relative to normal readers. These findings show that implicit learning is intact in dyslexia. Intact implicit learning capabilities should be taken into account when designing training programs for developmental dyslexic readers.