smell and the brain pt1

by michael holloway bsc, msc

Whilst studying the effects of sleep upon long term memory during a lovely summer in 2014 in Sheffield my research crossed paths with that of the science of smell. The sense of smell is rather unique amongst the senses. How quickly an aroma that evokes a memory can change a mood and how fast one might run in the event that we detect an invisible approaching fire!

Well it turns out that smell is processed in an interesting way in the brain. Of course many of you already know this. It's fairly obvious when you think about it but smell is a sense we often take for granted. More than any other sense the smell brain has far more potential to affect behaviour and even to alter the course of your very future!

You see the parts of the epithelium, white matter and cortex that mediate smell are largely independent from processing of conscious awareness. The olfactory bulb does not send information directly to the primary sensory processor, the thalamus, as it does with every other sense. Instead information from olfactory receptors go to the cortex and thalamus in parallel.

In effect we have two smell brains, as many animals with olfaction do. One smell brain is very old and is thought to mediate molecules that live in the pheromone family although robust evidence for humans pheromones is limited. The response from this pathway is fast and short. The second pathway goes through the thalamus and mediates every other type of smell. These two systems however work in tandem.

The stream that goes to the thalamus (and then to the cortex) becomes our conscious awareness of the smells in our environment. However, the stream that went straight to the cortex is much more mysterious, interesting and probably neuropsychologically functional. This means it is possible that a smell can influence our behaviour before the conscious brain has had time to figure out what just happened! I'll pick up on what this means in part 2.