The habenula.

The habenula is a tiny brain region the size of a pea in humans. This region is highly conserved across vertebrates and has been traditionally overlooked by neuroscientists. The name habenula is derived from the Latin word habena, meaning "little rein", because of its elongated shape. Originally its function was thought to be related to the regulation of the nearby pineal gland (which Rene Descartes described as the "principal seat of the soul"). More recent evidence, however, demonstrates that the habenula acts as a critical neuroanatomical hub that connects and regulates brain regions important for divergent motivational states and cognition. In this Primer, we will discuss the recent and converging evidence that points to the habenula as a key brain region for motivation and decision-making.