Commentary: Greater addition of neurons to the olfactory bulb than to the cerebral cortex of eulipotyphlans but not rodents, afrotherians or primates

Citation: Willemet R (2015) Commentary: Greater addition of neurons to the olfactory bulb than to the cerebral cortex of eulipotyphlans but not rodents, afrotherians or primates. Commentary: Greater addition of neurons to the olfactory bulb than to the cerebral cortex of eulipotyphlans but not rodents, afrotherians or primates A commentary on Greater addition of neurons to the olfactory bulb than to the cerebral cortex of eulipotyphlans but not rodents, afrotherians or primates Mammalian brains differ in many dimensions and how to compare them is one of the fundamental questions of comparative neuroscience. In a recent paper related to this issue, Ribeiro et al. (2014) examined the number of neurons within the olfactory bulb in several species of mammals. Among other things, Ribeiro and collaborators estimated that the olfactory bulb in humans contains as many neurons as the olfactory bulb of the largest species of Eulipotyphla. Assuming that " total numbers of neurons are a valid proxy for total information processing capacity in each structure " the authors concluded that " given the large absolute number of neurons predicted to compose their olfactory bulb, compared to macrosmatic eulipotyphlans, humans (and other primates) should no longer be considered microsmatic. " It is first important to note that the concept of micro/macro-smia is problematic. Sensitivity, acuity, selectivity, and the number of odorants that can be processed simultaneously are certainly major dimensions of olfactory ability that cannot be accounted for by an unidimensional, discrete factor. Moreover, the concept is relative to other mammalian groups and thus does not represent an absolute index of a species' olfactory ability. Finally, many factors other than the size of the olfactory bulb determine an animal's capacity to smell (such as the structure of the respiratory/olfactory system, Smith et al., 2004). These fundamental issues aside, the question here is whether the fact that our olfactory bulb contains as many neurons as those of species known for their olfactory abilities can be taken as evidence that humans and other primates also have large olfactory abilities. The short answer to this question is probably no. There are taxon specific differences in the organization of most brain structures that makes comparisons based on a single variable problematic (Willemet, 2012). For example, primates olfactory bulbs seem to have a larger number of glomeruli for each functional olfactory receptor genes compared to rodents (Maresh et al., 2008; Moriya-Ito et al., 2015). A more specific …