The effect of sedation on conscious processing: Computational analysis of the EEG response to auditory irregularity

Characterising the relationships between conscious and unconscious processes is one of the most important goals in cognitive neuroscience. Behavioural studies as well as neuroimaging techniques have been conducted to understand the nature of conscious perception in the brain.

Functional brain imaging and EEG (Electroencephalogram) methods allow for detailed exploration of neural and computational correlates of conscious and unconscious cognition. Using a high density EEG dataset, recorded from 129 electrodes over the scalp, we studied the neural responses of the brain to auditory stimuli. To this end, we employed an auditory oddball paradigm, called the local-global experiment. Bekinschtein et al (2009) designed this experiment to explore the neural dynamics at the early auditory cortex, associated with the MMN (mismatch negativity) component, generated by the local violation of auditory stimuli.

They also investigated a later novelty response, associated with the P3 (a late positive response) component, which was generated by the global violation of auditory stimuli. Their findings suggest that the global response, corresponding to working memory updating, independently from the local response, is a signature of conscious processing.

But our investigations shows that the local and global effects are not fully independent from one another. Therefore, we looked for other potential signatures of conscious processing. To do this, we studied 18 healthy participants who had been sedated. Using SPM (Statistical Parametric Mapping), which is a mass univariate approach, we analysed the sedation dataset in an omnibus statistical setting. We found an interaction between the local and global effects. In addition, we investigated the impact of sedation on both the early and late temporal components (i.e. the local and global effects), and their interaction. In addition to SPM analysis, we performed single-trial analysis. Unlike SPM analysis, which explores ERPs (average effect across replications) to assess significance, single-trial analysis looks for variation across replications, from one experimental level to another. More specifically, we looked at amplitude variation and temporal jitter when participants are sedated versus recovered. In the cases, when the null hypothesis is not rejected (i.e. there is no significant difference across different levels), we calculated Bayes factors to search for evidence in favour of the null hypothesis. With the exception of latency dispersion under dual (global and local) deviance, we could find no evidence for increased variability in single trial responses under sedation. This suggests the effects of reduced conscious level are systematic and can be summarised as an attenuation of dependency of (or interaction between) local and global processing.