Humans can have normal olfaction without apparent olfactory bulbs (seen in 0.6% of women, but not in men); this is associated with left-handedness

Human Olfaction without Apparent Olfactory Bulbs. Tali Weiss et al. Neuron, November 6 2019. https://doi.org/10.1016/j.neuron.2019.10.006

Highlights
•    Humans can have normal olfaction without apparent olfactory bulbs
•    Olfaction without apparent bulbs is seen in 0.6% of women, but not in men
•    Olfaction without apparent bulbs is associated with left-handedness

Summary: The olfactory bulbs (OBs) are the first site of odor representation in the mammalian brain, and their unique ultrastructure is considered a necessary substrate for spatiotemporal coding of smell. Given this, we were struck by the serendipitous observation at MRI of two otherwise healthy young left-handed women, yet with no apparent OBs. Standardized tests revealed normal odor awareness, detection, discrimination, identification, and representation. Functional MRI of these women’s brains revealed that odorant-induced activity in piriform cortex, the primary OB target, was similar in its extent to that of intact controls. Finally, review of a public brain-MRI database with 1,113 participants (606 women) also tested for olfactory performance, uncovered olfaction without anatomically defined OBs in ∼0.6% of women and ∼4.25% of left-handed women. Thus, humans can perform the basic facets of olfaction without canonical OBs, implying extreme plasticity in the functional neuroanatomy of this sensory system.