Neuromagnetic activation and oscillatory dynamics of stimulus-locked processing during naturalistic viewing

Naturalistic stimuli such as watching a movie while in the scanner provide an ecologically valid paradigm that has the potential of extracting valuable information on how the brain processes complex stimuli in realistic visual and auditory contexts. Naturalistic viewing is also easier to conduct with challenging participant groups including patients and children. Given the high temporal resolution of MEG, in the present study, we demonstrate how a short movie clip can be used to map distinguishable activation and connectivity dynamics underlying the processing of specific classes of visual stimuli such as face and hand manipulations, as well as contrasting activation dynamics for auditory words and non-words. MEG data were collected from 22 healthy volunteers (6 females, 3 left handed, mean age – 27.7 ± 5.28 years) during the presentation of naturalistic audiovisual stimuli. The MEG data were split into trials with the onset of the stimuli belonging to classes of interest (words, non-words, faces, hand manipulations). Based on the components of the averaged sensor ERFs time-locked to the visual and auditory stimulus onset, four and three time-windows, respectively, were defined to explore brain activation dynamics. Pseudo-Z, defined as the ratio of the source-projected time-locked power to the projected noise power for each vertex, was computed and used as a proxy of time-locked brain activation. Statistical testing using the mean-centered Partial Least Squares analysis indicated periods where a given visual or auditory stimuli had higher activation. Based on peak pseudo-Z differences between the visual conditions, time-frequency resolved analyses were performed to assess beta band desynchronization in motor-related areas, and inter-trial phase synchronization between face processing areas. Our results provide the first evidence that activation and connectivity dynamics in canonical brain regions associated with the processing of particular classes of visual and auditory stimuli can be reliably mapped using MEG during presentation of naturalistic stimuli. Given the strength of MEG for brain mapping in temporal and frequency domains, the use of naturalistic stimuli may open new techniques in analyzing brain dynamics during ecologically valid sensation and perception

Sex differences in coordinated brain activity in clinical child populations

A disruption of normal brain development during early stages of life has been associated with higher male vulnerability expressed by male preponderance among affected individuals and/or more severe impairments in males for developmental disorders. Although this phenomenon is frequently acknowledged by the scientific community, its neurophysiological underpinnings remain largely unclear. In this thesis I investigate male vulnerability in very preterm children and individuals with Autism Spectrum Disorder (ASD). Both clinical child populations entail early developmental adversity leading to behavioural and cognitive alterations, believed to be elicited, in part, by disrupted communication between brain areas. Therefore, I examine resting state whole-brain connectivity and its developmental changes in these clinical populations using fMRI and MEG and test the hypothesis of sex-specific connectivity differences between males and females resulting in male disadvantage. In the first study I investigate sex differences in interhemispheric homotopic connectivity and its developmental trajectories in participants with ASD as well as in typically developing individuals. Our findings demonstrate differences in developmental trajectories rather than connectivity. Both females and males with ASD deviate from typical female trajectories while expressing similar developmental trajectories to typical males. In the second study I examine local connectivity and its age-related changes using a similar cohort of participants. Group and sex differences are observed in both local connectivity and its developmental trajectories. Females with ASD are characterised by more robust alterations. Lastly, in the third study I test the hypothesis that male vulnerability in very preterm children can be detected as more pronounced alterations in inter-regional connectivity in boys compared to girls. Our results confirm this hypothesis suggesting that connectivity alterations might contribute to male disadvantage reflected in long-term behavioural and cognitive outcome. Overall, this thesis highlights that disruptions in brain connectivity and/or its developmental trajectories differ between males and females with altered early development supporting the existence of female protective features preventing females from developing pathological outcome

Classification of evoked responses to inverted faces reveals both spatial and temporal cortical response abnormalities in Autism spectrum disorder

The neurophysiology of face processing has been studied extensively in the context of social impairments associated with autism spectrum disorder (ASD), but the existing studies have concentrated mainly on univariate analyses of responses to upright faces, and, less frequently, inverted faces. The small number of existing studies on neurophysiological responses to inverted face in ASD have used univariate approaches, with divergent results. Here, we used a data-driven, classification-based, multivariate machine learning decoding approach to investigate the temporal and spatial properties of the neurophysiological evoked response for upright and inverted faces, relative to the neurophysiological evoked response for houses, a neutral stimulus. 21 (2 females) ASD and 29 (4 females) TD participants ages 7 to 19 took part in this study. Group level classification accuracies were obtained for each condition, using first the temporal domain of the evoked responses, and then the spatial distribution of the evoked responses on the cortical surface, each separately. We found that classification of responses to inverted neutral faces vs. houses was less accurate in ASD compared to TD, in both the temporal and spatial domains. In contrast, there were no group differences in the classification of evoked responses to upright neutral faces relative to houses. Using the classification in the temporal domain, lower decoding accuracies in ASD were found around 120 ms and 170 ms, corresponding the known components of the evoked responses to faces. Using the classification in the spatial domain, lower decoding accuracies in ASD were found in the right superior marginal gyrus (SMG), intra-parietal sulcus (IPS) and posterior superior temporal sulcus (pSTS), but not in core face processing areas. Importantly, individual classification accuracies from both the temporal and spatial classifiers correlated with ASD severity, confirming the relevance of the results to the ASD phenotype.Peer reviewe