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With which part of the brain do we smell? (special guest author edition)

  
  
  

We are honoured to present you a special blog edition written by our guest author Dr. Johannes Frasnelli.

Dr. Frasnelli specialises in odor perception. He conducts research in the field of neurophysiology of smell and taste as well as therapy in loss of the chemical senses.

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We have known for a long time that the occipital cortex is the brain region which is active if we are watching something; it is located in the very back of our brain. If we listen to music, on the other hand, the temporal region of the brain is active; this region is located just beneath the ear (interestingly, the left temporal region responds to sounds from the right ear and the right temporal region to sounds from the left ear). However, for a long time it was unclear with which part of the brain we smell, or, to put it into scientific terms, in which part of the brain olfactory information is processed. Over the last 30 years new imaging methods have become available which allow us to have a look at the brain at work, without harming the brain, such as functional magnetic resonance imaging (fMRI) and positron emission tomography (PET).

Brain MRI olfaction

Before fMRI and PET were available, researchers had to dissect the brains of dead bodies and follow the nerve bundles to see with which region the olfactory bulb was connected. But since PET and fMRI entered the stage we have gained much more insight into how our sense of smell works. The three most important brain regions are:

(1.)  the orbitofrontal (from Latin: between eye-socket and forehead) cortex is located just above and behind our eyes;

File:OFC.JPG

Source: Paul Wicks
 

(2.)  the insula (from Latin: island) is located deep beneath our ears;

File:Insula animation.gif

Source: Wikimedia

(3.)  the piriform (from Latin: pear shape-like) cortex is located just between the two other brain areas.

In addition, the olfactory brain includes smaller, but still very important brain regions (e.g., the anterior olfactory nucleus, the olfactory tubercle, the amygdala, and the entorhinal cortex). There are several interesting features related to the anatomy of the olfactory parts of the brain:

First, the olfactory regions do not only serve for smelling, but are also used, if we experience emotions and when we are memorizing events. This is the reason why odors can evoke very strong associations and memories of situations and place from a long time ago. Probably everyone knows an anecdote where he smelled a food or a perfume, and was brought back to early childhood and remembers exactly the circumstances of when he smelled that odor.

There is a second special characteristic of the sense of smell. For all the other senses (seeing, hearing, tasting, touching), the information from the sensory organs (the eyes, the ears, the skin, the tongue) travels through a brains structure called the thalamus (from Greek: room). The thalamus is something like a gate to our consciousness. If we focus on one sense (say, on vision while reading a very interesting book), we can blind out information from other senses (and not hear that someone was talking to us). This is done by the thalamus, who decides what we are aware of and what not. Since smell information is independent of the thalamus, we cannot blind out olfaction: we either smell an odor, or we do not smell it.

This particularity of not traveling through the thalamus is also responsible for another characteristic of the sense of smell. In all other sensory systems, a moderate (and for some even a light) sensation is enough to interrupt sleep: if we are asleep, a light, a sound or someone touching us is enough to wake us up. Unlike the other senses, a smell is not enough to wake us up; therefore we all have to install smoke detectors at home. If there is a fire, we would only wake up, if the smoke is so strong that it becomes stinging (and therefore is like a touch sensation). This may then be too late.

Learn how does works an electronic nose compared to the human nose:

Get the White PaperElectronic Nose Principles

 

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About the author: Dr. Johannes Frasnelli Ph.D.

Dr. J. Frasnelli is a graduate of the Medical Schools of the University of Vienna (Austria; 2001; Dr. med. univ.) and the Technical University of Dresden (Germany; 2009; Priv.-Doz.). Since 2006 he work in Montreal, first as an Academic Trainee at the Montreal Neurological Institute, since 2008 as a Postdoctoral Fellow at the Department of Psychology at the Université de Montréal. He currently hold a fellowship of the FRSQ. Dr. Frasnelli research interest is the neurophysiology of smell and taste as well as therapy in loss of the chemical senses.

Contact information: johannes.frasnelii@umontreal.ca

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