23Several lines of evidence identify aberrant excitatory-inhibitory neural processes across 24 autism and schizophrenia spectrum disorders, particularly within the psychosocial domain. 25 Such neural processes include increased excitatory glutamate and reduced inhibitory GABA 26 concentrations, which may affect auditory pre-attentive processing as indexed by the 27 mismatch negativity (MMN); thus, an excitation-inhibition imbalance might lead to aberrant 28 MMN, which might in turn drive the relationship between the MMN and psychosocial 29 difficulties. This research has the potential to enhance the neurochemical understanding of 30 the relationship between electrophysiology (MMN) and behavioural/clinical measures 31 (psychosocial difficulties). 32 Thirty-eight adults (18 male, 18-40 years) completed the Schizotypal 33 Personality Questionnaire (SPQ) and Autism-Spectrum Quotient (AQ). Glutamate 34 and GABA concentrations in bilateral superior temporal cortex (STC) were quantified 35 using proton magnetic resonance spectroscopy (1H-MRS) while auditory MMN to a 36 duration deviant was measured with magnetoencephalography. Spearman 37 correlations probed the relationships between STC glutamate/GABA ratios, MMN 38 amplitude and latency, and AQ and SPQ dimensions. Mediation effects of 39 glutamate/GABA ratios on the relationship between MMN and AQ-SPQ dimensions 40 were probed using causal mediation analysis. 41 Only SPQ-interpersonal and AQ-communication were significantly correlated 42 with right hemisphere glutamate/GABA ratios and MMN latency (ps<.05), which 43 were themselves correlated (p=.038). Two mediation models were investigated, with 44 right MMN latency as predictor and SPQ-interpersonal and AQ-communication as 45 outcome variables. Right STC glutamate/GABA ratios significantly mediated the 46 relationship between MMN latency and SPQ-interpersonal scores (ß=86.6, p=.033), 47 but only partially mediated the relationship between MMN latency and AQ-48 communication scores (ß=21.0, p=.093). 49 These findings support the growing body of literature pointing toward an 50 excitation-inhibition imbalance that is central to psychosocial functioning across 51 multi-dimensional spectrum disorders, such as autism and schizophrenia, and 52 provides neurochemical indicators of the processes that underlie psychosocial 53 dysfunction. 54 3 55