Speech and Short-term Memory Functions in Dyslexia: a Combined MEG and EEG Study

1. Background

Developmental dyslexia is the most prevalent learning disorder impairing reading skill even in individuals having normal intelligence and full availability of education (Lyon et al., 2003). It is highly heritable, and several candidate genes contributing to dyslexia have been identified (Scerri & Schulte-Körne, 2010). A wide range of deficits have been associated with dyslexia, but according to the most prevalent theory its major cause is impaired phonological processing (Ramus, 2001). The MMN, the generators of which overlap with brain areas reported to have anatomical abnormalities in dyslexia, has supported this notion. Diminished MMN amplitudes have systematically been reported in dyslexics for certain speech and non-speech sound features (for a review Kujala & Näätänen, 2001). These effects were even found in children and infants having an inherited risk for dyslexia (Leppänen et al., 2002; Lovio et al., 2010). Furthermore, a recent study in our group has shown that dyslexic children have problems in forming memory traces for words (Kimppa et al., in prep.), whereas both the auditory system of normal-reading adults (Shtyrov et al., 2010) and children (Kimppa et al., in prep.) was shown to rapidly form representations for novel words.

It was recently suggested that phonological deficits in dyslexia can occur due to impairments in different steps of sound processing. According to this theory, dyslexic individuals can be divided into different subgroups. Ramus and colleagues (2013) suggested that dyslexic individuals show an impairment in phonological representations or at later processing stages as an impairment in phonological skills.

The current project will utilize combined electroencephalographic (EEG) and magnetoencephalographic (MEG) recordings and neuropsychological and perceptual testing in order to determine how impaired phonological neural representations vs. phonological skills contribute to dyslexia. Defined as time-locked changes to external stimuli in EEG and magnetoencephalogram MEG, event-related potentials (ERPs) and event-related fields (ERFs), respectively, could provide an objective index of information processing in the human brain. Both EEG and MEG methods offer a high temporal resolution. The benefit of MEG is offered by a more exact localization of the activated neural sources due to a diminished effect of distortions caused by the skull and tissue than in the process of EEG source localization. It will complement and add more specific information to dyslexia ERP research additional to the previous studies that were mostly conducted with EEG. For a summary of dyslexia studies conducted with MEG, see the review from Salmelin (2007).

Neural responses recorded with EEG and MEG have widely been used both in the service of basic research and clinically-oriented research. During the recent years, a cortical response called the mismatch negativity (MMN) has been in intensive use in investigating auditory perception and its deficits. MMN is an ERP component elicited by any change in some repetitive aspect of auditory stimulation, peaking at 100-200 ms from change onset and detectable both electrically (MMN) and magnetically (MMNm). It was suggested that MMN provides an index of the auditory sensory ("echoic") memory and automatic (involuntary) change-detection. It also reflects native-language specific speech-sound memory traces (Näätänen et al., 1997). MMN is elicited even when the subject is not attending the auditory stimuli. Therefore, it has been popular in investigating a variety of patient groups during the recent years (for reviews, see Näätänen et al., 2007; Kujala et al., 2007).