Time Course and Neural Network for Comparing Written and Spoken Words: A MEG and DTI Study

Abstract

The present study was designed to investigate the spatiotemporal signature as well as the neural network of brain activation
associated with word recognition. Twenty participants were studied with a whole head magnetoencephalography (MEG) system and
a 3T magnetic resonance imaging (MRI) scanner. The word stimuli consisted of 100 matched words (the visually and acoustically
presented words were the same) and 20 mismatched words (the visually and acoustically presented words were different). The
time course of brain activation was analyzed with a virtual sensor technique at source levels. The spatial location and connection
were estimated with wavelet-based beamformer and diffusion tensor imaging (DTI) tractography. To our knowledge, this is the
first study focusing on the time course and neural substrates involved in comparing spoken and written words with both MEG
and DTI. The visually and acoustically presented words evoked three responses named as PM1, PM2 and PM3, respectively. The
results of magnetic source imaging showed that the PM1 was localized to the primary auditory and visual cortices. The PM2
and PM3 were localized to Wernicke’s area and Broca’s area. Interestingly, virtual sensor waveforms showed a clear response
between PM1 and PM2 from the left temporal-occipital junction. This component was consistently identified around 162 ± 8.9
ms (M160) in 20 participants (100%, 20/20). MEG source-guided DTI tractography revealed neural fibers in this junction area.
The results suggested that the M160 was one part of the neural network of auditory and visual word recognition. The results
of the present study yield highly convincing evidence that the left temporal-occipital junction plays an important role in
comparing visually and acoustically presented words.

KeywordsWord recognition-Magnetoencephalography-Virtual sensor-Auditory/Visual evoked magnetic fields-Diffusion tensor imaging