Psychopharmacology (2008) 199:77–88 Fig. 5 Time course of MMN activity in nanoampere meters at the four sources for the duration- deviant stimuli (baseline=solid line, low dosage=dotted line, high dosage=dashed line; means; p values from post-hoc tests; n=9) 1 e c r u o s 2 e c r u o s 3 e c r u o s 4 e c r u o s S-ketamine DMT 85 low dose < baseline (p=0.007) high dose < baseline (p=0.011) low dose < baseline (p=0.009) low dose < baseline (p=0.042) high dose < baseline (p=0.01) not significant not significant not significant high dose < baseline (p=0.048) not significant S-Ketamine had no effect on the N1 amplitude; therefore, the reduction in MMN by S-ketamine was not caused by a general weakening of ERP activity. Our findings regarding the NMDA antagonist S-ketamine are in line with the observation that MMN deficits in schizophrenia are more pronounced to duration deviants than to frequency-deviant stimuli (Michie et al. 2000). A recent study also found a reduction in MMN to duration but not to frequency deviants in patients with schizophrenia and a short length of illness (Todd et al. 2008). Remarkably, in the same study, patients with a longer length of illness showed a stronger reduction to frequency compared to duration deviants. The authors interpreted their findings as a result of a pronounced age-related decline in duration-deviant