Neuromagnetic speech discrimination responses are associated with reading-related skills in dyslexic and typical readers.

Poor neural speech discrimination has been connected to dyslexia, and may represent phonological processing deficits that are hypothesized to be the main cause for reading impairments. Thus far, neural speech discrimination impairments have rarely been investigated in adult dyslexics, and even less by examining sources of neuromagnetic responses. We compared neuromagnetic speech discrimination in dyslexic and typical readers with mismatch fields (MMF) and determined the associations between MMFs and reading-related skills. We expected weak and atypically lateralized MMFs in dyslexic readers, and positive associations between reading-related skills and MMF strength. MMFs were recorded to a repeating pseudoword /ta-ta/ with occasional changes in vowel identity, duration, or syllable frequency from 43 adults, 21 with confirmed dyslexia. Phonetic (vowel and duration) changes elicited left-lateralized MMFs in the auditory cortices. Contrary to our hypothesis, MMF source strengths or lateralization did not differ between groups. However, better verbal working memory was associated with stronger left-hemispheric MMFs to duration changes across groups, and better reading was associated with stronger right-hemispheric late MMFs across speech-sound changes in dyslexic readers. This suggests a link between neural speech processing and reading-related skills, in line with previous work. Furthermore, our findings suggest a right-hemispheric compensatory mechanism for language processing in dyslexia. The results obtained promote the use of MMFs in investigating reading-related brain processes.

Auditory cortical responses in humans with congenital unilateral conductive hearing loss.

We recorded auditory evoked magnetic fields from 6 patients with congenital unilateral conductive hearing disorder with a 122-channel whole-head neuromagnetometer. The stimuli were 50-ms 1-kHz tones delivered to the better ear at interstimulus intervals (ISIs) of 2 and 8 s at two different intensities (50 and 70 dB HL). As in normal-hearing subjects, the amplitudes of N100m, the 100-ms response, were larger in 5 patients and the latencies were shorter in 3 patients over the hemisphere contralateral to stimulation. However, in one patient N100m peaked already at 61 ms over the contralateral hemisphere and amplitudes were larger over the ipsilateral hemisphere, possibly reflecting reorganization of the auditory pathways. In 3 patients the latencies were shorter over the ipsilateral hemisphere. The effects of ISI and intensity were similar over both hemispheres and did not differ from those in controls. It seems that congenital unilateral conductive hearing loss does not necessarily lead to any gross disturbances in the human auditory cortex.