Psychopharmacology (2008) 199:77–88 81 Fig. 1 The generators of the MMN according to Waberski et al. (2001) AX-continuous performance task The dependent parameter missings (no response after “AX” combination) were also analyzed by means of ANOVAs, initially, separately for each drug with the factors condition (baseline, low dose, high dose) and ISI (0.8 s, 4 s.) and then with the factors drug (DMT, S-ketamine) and dosage (low, high). For the analysis of the false-alarm rates (response after “AY,” “BX,” or “BY” combination), we also performed two ANOVAs, separately for each drug, with the factors condition, ISI (0.8 or 4 s), and false-alarm type (“AY,” “BX,” or “BY”). Finally, we performed an ANOVA with the factors drug (DMT, S-ketamine), dosage (low, high), false-alarm type, and ISI. All analyses were performed using the SPSS software (version 12.0). Statistical significance was set at p < 0.05. Results Psychological effects The global intensity of psychological effects was similar for DMT and S-ketamine and was dose dependent. With the low dosage, most subjects displayed some degree of positive formal thought disorder such as loosening of associations and heightened distractibility, and some sub- jects reported alterations of visual perception and visual hallucinations. With the high dosage, all subjects had significant psychotomimetic effects including perceptional distortions and hallucinations, transient paranoid ideation, alterations of mood and drive, prominent formal thought disorder, and attention deficits. Phenomena resembling positive symptoms of schizophrenia, particularly positive formal thought disorder and inappropriate affect, were stronger after DMT intake. Phenomena resembling negative symptoms of schizophrenia such as hypomimia and psychomotor poverty, attention deficits, body perception disturbances, and catatonia-like motor phenomena were stronger after S-ketamine intake. The global scores of the Scale for the Assessment of Positive Symptoms (SAPS) and the Scale for the Assessment of Negative Symptoms (SANS) are presented in Fig. 2. Repeated-measures ANOVAs revealed significant main effects of drug and dose for the global SAPS (F = 20.76, p = 0.002 and F = 32.19, p < 0.0001) and the global SANS scores (F = 29.41, and F = 26.32, both p < 0.001). For details on the psycho- pathological effects, see Gouzoulis-Mayfrank et al. (2005). AX-CPT performance The descriptive statistics for missings (AX errors) and false alarms (BX, AY, and BY error rates) are presented in Fig. 3. There was a significant main effect of condition on the missings rate for both DMT (F[2, 7] = 8.26, p = 0.019) and S-ketamine (F[2, 7] = 10.22, p = 0.012). The post-hoc LSD tests revealed a significantly lower missings rate in the baseline condition compared to low DMT (p = 0.009), high DMT (p = 0.008), low S-ketamine (p = 0.021), and high S- ketamine (p = 0.002). There was no significant main effect of the factor ISI on the missing rate. Remarkably, the descriptive data showed a tendency for the expected effect of ISI for ketamine but not for DMT. The ANOVA with the factors drug and dosage revealed no significant effect of any factor on the missings rate. Regarding the false-alarm rates, there were significant main effects of the factors ISI and false-alarm type under both drugs and the interaction of the two factors was also significant (ISI: F[1, 8] = 13.24, p = 0.008 for DMT and F[1, Table 1 Location of MMN sources in Talairach coordinates (mm) Source S1 S2 S3 S4 x 49.2 −42.4 48.1 2.2 y 5.3 −18.0 −67.9 38.1 z −6.2 8.3 1.1 8.3