84 Fig. 4 Grand average MMN surface waveforms in microvolts (baseline =solid line, low dosage=dotted line, high dosage=dashed line; p values from post-hoc tests; n=9) Psychopharmacology (2008) 199:77–88 S-ketamine DMT low dose < baseline (p=0.013) high dose < baseline (p=0.025) Fz n.s. activity of the temporal (S1, S2, S3) and the frontal sources (S4). Regarding the frequency-deviant stimuli, the effect of S-ketamine was somewhat weaker: We found only a marginal MMN reduction at one temporal source (S3) and at the frontal source (S4). Nevertheless, the difference between frequency- and duration-deviant MMN did not reach statistical significance. The activity of the frontal source was only affected by S-ketamine and not by DMT. d u r a t i o n d e v i a n t f r e q u e n c y d e v i a n t Fz F3 F4 Fz F3 F4 high dose < baseline (p=0.046) low dose < baseline (p=0.028) high dose < baseline (p=0.013) low dose < baseline (p=0.045) n.s. n.s. F3 F4 Fz F3 F4 n.s. n.s. n.s. n.s. n.s. S-ketamine DMT