Effects of phonological contrast on auditory word discrimination in children with and without reading disability: a magnetoencephalography (MEG) study.

Poor readers perform worse than their normal reading peers on a variety of speech perception tasks, which may be linked to their phonological processing abilities. The purpose of the study was to compare the brain activation patterns of normal and impaired readers on speech perception to better understand the phonological basis in reading disability. Whole-head magnetoencephalography (MEG) was recorded as good and poor readers, 7-13 years of age, performed an auditory word discrimination task. We used an auditory oddball paradigm in which the 'deviant' stimuli (/bat/, /kat/, /rat/) differed in the degree of phonological contrast (one versus three features) from a repeated standard word (/pat/). Both good and poor readers responded more slowly to deviants that were phonologically similar compared to deviants that were phonologically dissimilar to the standard word. Source analysis of the MEG data using minimum norm estimation (MNE) showed that compared to good readers, poor readers had reduced left-hemisphere activation to the most demanding phonological condition reflecting their difficulties with phonological processing. Furthermore, unlike good readers, poor readers did not show differences in activation as a function of the degree of phonological contrast. These results are consistent with a phonological account of reading disability.

The influence of semantic processing on phonological decisions in children and adults: a magnetoencephalography (MEG) study.

PURPOSE: To examine the behavioral effects and neural activation patterns associated with implicit semantic processing influences on phonological judgments during reading in children and adults. METHOD: Whole-head magnetoencephalography (MEG) recordings were obtained from 2 groups, children (9-13 years) and adults, performing a homophone judgment task. The stimuli consisted of pairs of sequentially presented written words that were either homophones, synonym foils, or unrelated control words. RESULTS: The difference in the time taken to respond to synonym pairs relative to control pairs of stimuli, called the semantic interference effect (SIE), was, on average, 24 ms for adults and 86 ms for children. Source analysis of the MEG data using minimum-norm estimation (MNE) yielded less activation in the adults for the synonym condition compared with the control condition in right anterior temporal and inferior frontal cortex 300-500 ms after the onset of the 2nd word in a pair, suggestive of semantic priming as well as inhibition of the SIE. A similar priming effect was observed for the children in left-anterior temporal cortex. CONCLUSION: The observed group differences in the magnitude of the SIE and brain activation patterns may reflect developmental differences in the effects of semantic information on phonological decisions during word processing.

Auditory word perception in sentence context in reading-disabled children.

Reading difficulties seem to be related to a phonological deficit that has its origin in poor speech perception. As such, disabled readers may use contextual cues to compensate for their weak speech perception abilities. We compared good and poor readers, 7-13 years old, on auditory perception of words varying in phonological contrast, in congruent versus incongruent sentence contexts. Both groups did worse in the phonologically similar than in the phonologically dissimilar incongruent condition. Magnetoencephalography revealed differential activation between the groups as a function of phonological contrast in left superior temporal gyrus between 200 and 300 ms, suggesting that poor readers may have processed phonologically similar incongruent stimuli as congruent. The results are consistent with a phonological account of reading disability.

Head movements of children in MEG: quantification, effects on source estimation, and compensation.

Head movements during magnetoencephalography (MEG) recordings may lead to inaccurate localization of brain activity. This can be particularly problematic for studies with children. We quantified head movements in 8- to 12-year-old children performing a cognitive task and examined how the movements affected source estimation. Each child was presented auditory word stimuli in five 4-min runs. The mean change in the MEG sensor locations during the experiment ranged from 3 to 26mm across subjects. The variation in the head position was largest in the up-down direction. The mean localization error in equivalent current dipole (ECD) simulations was 12mm for runs with the most head movement, with the frontal cortex appearing to be most prone to errors due to head movements. In addition, we examined the effect of head movements on two types of source estimates, ECDs and minimum-norm estimates (MNE), for an auditory evoked response. Application of a recently introduced signal space separation (SSS) method to compensate for the head movements was found to increase the goodness-of-fit of the ECDs, reduce the spatial confidence intervals of the ECDs, and enhance the peak amplitude in the MNE. These results are indicative of the SSS method being able to compensate for the spatial smoothing of the signals caused by head movements. Overall, the results suggest that MEG source estimates are relatively robust against head movements in children, and that confounds due to head movements can be successfully dealt with in MEG studies of developmental cognition.