 In lower vertebrates (lampreys and teleost fishes), mitral cell (principal olfactory neurons) axons project exclusively to the right hemisphere of the habenula in an asymmetric manner.It is reported that the dorsal habenulae (DHb) are functionally asymmetric with predominantly odor responses in the right hemisphere.It was also shown that DHb neurons are spontaneously active even in the absence of olfactory stimulation.