Some more about smell…

At World Tea Expo 2016 I presented the following graph about smell, which shows where in the brain our awareness of smell is put together:

Odorants come into the nose either through the nostrils (orthonasal) or through the back of the throat (retronasal). There they are captured by receptors in the endings (dendrites) of nerve cells poking through the olfactory epithelium (marked 4 in the diagram) at the roof of the nose. The bodies of these nerve cells (6) are interspersed among the cells of the olfactory epithelium, while a long extension of each (the axon) makes its way through small holes the skull (3) to reach the olfactory bulb (1). Ends of groups of axons gather together to form glomeruli (5), and together they contact mitral cells (2) in the olfactory bulb. 

Image:Brain human sagittal section.svg Image:Head lateral mouth anatomy.jpg by Patrick J. Lynch, medical illustrator

The ratio of axons to mitral cells is approximately 1000 to 1, so you can see that the mitral cells are in a position to calculate how much odorant is coming through: the greater the number of receptor cells activated the more likely it is that the mitral cell will be triggered and the stronger the signal sent to the  brain.

At the same time, the fact that the mitral cells each receive messages from at least a thousand receptor cells means that the mitral cells are in a position to simplify the message to be sent to the brain. in order to extract a specific message from the huge number of possible odorant signals the mitral cells interact among themselves and among other cells in the olfactory bulb, activating some and inhibiting others. The net effect is a “sparse” message that is then sent on to the pyriform cortex.

The pyriform cortex takes the message and, before contacting the next set of brain structures, assembles the input into “odor objects.” For example a group of smells may be characterized by the pyriform cortex as minty or citrusy. The more sensitive you are to a group of odors, the more likely you are to assemble them into these objects (configural representation), and the less likely you are to perceive them as separate elemental odors.*

The pyriform cortex next sends on its analysis to several different parts of the brain, as shown in the box in the diagram above. Ultimately the message arrives in the orbitofrontal cortex right above the eyes, where the odor’s identity is finally brought into awareness, and its importance and valence (either pleasant or unpleasant) evaluated.

As you can see in the diagram, the superior frontal cortex may also get into the act. Its activation depends on whether the the odor is hedrnically simple or complex. What does this mean? Some aromas are either pleasant or unpleasant, in other words hedrnically simple. By contrast some aromas are hedonically complex, because they have both pleasant and unpleasant components. The aroma of oolongs is one such type of aroma: it contains a number of pleasant odorants in the jasmine family, but it also contains a considerable amount of indole, which is also found (for example) in fecal material—stinky, in other words. When we encounter hedonically complex smells, the superior frontal cortex sends the message: sit up and take notice! 

BTW, that’s why the best perfumes aways contain a less-than-pleasant component (such as indole): it makes a person pay more attention to the person wearing the perfume!

* Thierry Thomas-Danguin et al. The perception of odor objects in everyday life: a review on the processing of odor mixtures. Frontiers in Psychology. June 2014 | Volume 5 | Article 504 ; doi: 10.3389/fpsyg.2014.00504 

** Fabian Grabenhorst, Edmund T. Rolls, Christian Margot. A hedonically complex odor mixture produces an attentional capture effect in the brain. NeuroImage 55 (2011) 832–843.