Effect of 2,5-dimethoxy-4-methylamphetamine (DOM), 4-ethyl-2,5-dimethoxyamphetamine (DOET), 4-butyl-2,5-dimethoxyamphetamine (DOBU), and R-(-)-4-iodo-2,5-dimethoxyamphetamine (R-(-)-DOI) on the head twitch response (HTR). Data are presented as group means ± SEM for the entire 30-min test session. The individual data points for each compound are presented in Table S1. *p < 0.05, **p < 0.01, significant difference from the vehicle control group (Dunnett’s test).