Challenges in magnetoencephalographic studies of naturalistic reading and speech

Knowledge on the cortical basis of reading and speech is mostly built on studies which restrict and simplify these tasks greatly. Yet, reading and speech in everyday life are very complex functions that require the intricate co-operation of several subprocesses. Consequently, to obtain data that is truly representative of the cortical correlates of reading and speech, they must also be studied using paradigms where participants can read and speak as freely and thus as naturally as possible. Furthermore, the length of the read texts or spoken utterances should not be unrealistically short. However, the use of naturalistic language paradigms especially in combination with magnetoencephalography sets challenges which must be met. The most significant of these are reading- and speech-related artefacts that confound the measurement data. Also necessary is a novel perspective on quantifying cortical effects in long continuous data, such as an emphasis on cortical rhythms. The studies in this thesis sought ways to investigate the cortical processes underlying naturalistic reading and to a smaller extent also naturalistic speech production and perception with magnetoencephalography. Study I compared the performance of two methodological pipelines in removing reading-related artefacts particularly via the effect that they have on the distribution of cortical rhythmic activity. The pipeline chosen as the more recommendable alternative was further used to remove artefacts in Studies II and III. In Study II, the spatiospectral distribution of cortical rhythmic activity during naturalistic reading of multi-page texts was examined. Contrasting previous results, Study II found more abundant cortical activity in the right hemisphere than the left in several frequency bands. Study III expanded the examination of the cortical correlates of naturalistic reading to cortico-cortical connectivity. This study showed that naturalistic reading is supported by a wide range of connections, especiallyin the beta and gamma bands. Study IV investigated the cortical mechanisms of naturalistic speech production and perception through the modulation of a set of key speech features. The modulations were associated with spatially and spectrally distinct activity patterns, which suggested that the cortical bases of the production and perception of naturalistic speech overlap particularly in the right hemisphere. The removal of speech production artefacts had an important role in the study. The results from these studies showed that while the classical views of the cortical basis of reading and speech largely apply to the processing of longer texts and utterances, the transition from short to long stimuli seems to also alter the activity patterns associated with these tasks. Particularly intriguing are the consistent and abundant right hemispheric effects in our results, which are in contrast with the established view of reading and speech as strongly left-lateralised processes.