Recently, in a review article in this journal, Vlček and colleagues described the putative role played by the glutamatergic system in obsessive-compulsive disorder (OCD) and how this might explain the effects of certain treatments. They describe a neuroanatomical model, which includes a specific role of the amygdala-hippocampus complex (AHC) and would complete the classic cortico-striatal-thalamo-cortical (CSTC) mechanism of OCD. The role of the AHC can perhaps be better understood when considering its ancient relationship to the rest of the forebrain of mammals. This leads to distinguishing between primary (lamprey-like), secondary (amphibian-like) and tertiary (mammal-like) parts of the forebrain including amygdaloid, ventral extrapyramidal and dorsal extrapyramidal systems, respectively. A specific role in OCD may be played by the habenula-projecting part of the pallidum, which evaluated the result of behaviour in human's earliest vertebrate ancestors. The addition of these primary relationship to the authors' description could be fruitful when planning the future research, as suggested by them.