Objectives: The present study is aimed to disclose the detailed spatiotemporal profiles of the somatosensory information processing in ASD using magnetoencephalography (MEG). Specifically, we focused on the following 3 components of somatosensory evoked fields (SEFs); 1) the primary response which reflects the initial response of the somatosensory cortex, 2) mismatch response (MMR) which is known to represent the automatic discrimination processes (Näätänen, R., et al., 1978), and 3) a component related to selective attention.
Methods: The participants are 8 male high-functioning ASD (aged 21 to 27) and 8 male typically developing (TD) individuals (aged 23 to 29 yrs). There were no differences in IQ between the two groups. None of the participants had subjective skin sensation abnormalities. Written informed consent was obtained from all of the participants prior to the experiment. An air-puff-derived tactile stimulator (Nakamura, A., et al., 1998), which provides a light, superficial pressure stimulus to the skin surface, was used for the stimulation. Using an oddball paradigm, the right index finger and middle finger were randomly stimulated with probabilities of 80% (standard) and 20% (deviant), respectively. The experiment consisted of following two blocks; a) ignore condition (subjects were instructed to watch a silent movie and ignore the tactile stimuli) and b) attend condition (instructed to count the number of the deviant stimuli mentally). SEFs to these stimuli were recorded using a 74-channel MEG system (4D Neuroimaging, San Diego). Source estimations were done by calculating current source density (CSD) maps using the L2 minimum-norm technique. After the spatial normalization of the individual CSD maps, statistical tests were performed using the statistical parametric mapping (SPM). Finally, time courses of the electrical activity around the somatosensory cortex were visualized by region of interest (ROI) analysis.
Results: 1) All of the subjects elicited clear primary responses in the latencies around 40ms. There were no differences between ASD and TD. 2) MMRs were clearly elicited in both groups in the latencies around 80ms. However, ASD showed prolonged MMN activity which persisted until 170ms, while TD showed no MMN activity in the latencies after 140ms. 3) The effects of selective attention were also different between the two groups. TD showed significant activation in the left intraparietal sulcus during the attend condition compared with the ignore condition in the latencies around 80ms. However, ASD did not show such attention effects.
Conclusions: The results indicate that the somatosensory information processing in ASD, even without any subjective symptoms, is different from that in TD. The prolonged MMRs suggest that ASD individuals are hyperactive to a deviant stimulus which is different from stored memory trace. Further, ASD appears to have impairments in controlling selective attention.
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