You know how you put on perfume just before you leave the house and then it kind of ‘wears out’ as the day progresses, only for a co-worker or a friend to compliment you on the ‘nice perfume you’re wearing’ later in the day?
But even if other people tell you that they notice your perfume, you remain positive that you didn’t get your money’s worth on that brand new cologne or perfume you bought. This is because your nose is making you believe that the fragrance didn’t last for more than 10 minutes. Why is this so? How come other people can smell the scent you’re wearing, but not you? This is not because the atoms of your perfume are diffused unto the environment but because your nose ‘goes numb’ after a few whiffs of that cologne.
When you breathe in a scent, the molecules of the fragrance are picked up by what we call olfactory receptors. And then the molecules are carried by olfactory sensory neurons to the olfactory bulb. In the olfactory bulb, the notes of the scent are registered. The output signals from the olfactory bulb are then carried by cells called mitral cells to the olfactory cortex of the brain where the scent is perceived and recognized as either foreign or familiar, threatening or safe.
Nose blindness is the term used to refer to the phenomenon where you are left completely oblivious to a scent which is easily noticed by others. This occurs when your brain categorises the scent as familiar and non-threatening.
In the process of scent recognition, where the mitral cells deliver the signals of a certain scent to the olfactory cortex in the brain, the mitral also delivers these signals to certain areas in the human limbic system. The limbic system is primarily responsible for hormonal secretions, emotional responses, and memory. Part of the limbic system also determines the fight or flight response of the body where either adrenaline or noradrenalin is secreted.
When this happens, the brain is able to process the scent and recognize it. If the scent is determined by the brain to not be dangerous, it simply dismisses the odour. When the brain gives this signal to the body, the body ignores the scent.
The ability of the human body to be sensitive to smells is conceptualised to be a survival measure. In primitive communities, this ability was probably used by our ancestors to determine if the food was still safe to eat. The smell of rotting animal flesh sent signals to the brain that the flesh had gone bad, and hence was not safe for eating.
In the same sense, we are naturally repelled by foul odours because our brain sends signals to the body that there is something in the environment that is not normal. It could also be that the hippocampus and the amygdala, both being a part of the limbic system, matched the scent you inhaled to a negative memory or a negative thought which makes us have an aversion to certain fragrances.
Taking into consideration the concept of smell sensitivity being used as a survival mechanism, the same principle is applied to ‘nose blindness’. It is simply because your body has regarded your perfume’s fragrance as something that is not to be worried about because it doesn’t trigger a negative response in the brain – so your body simply ignores the scent.
Although what is uncanny about the sense of smell is that you cannot choose what to smell, unlike the sense of hearing where you can hear selectively. The ears are capable of singling out a particular sound from a mix of different sounds, and ignore the other sounds we disregard as noise. We are able to focus our attention to one sound only. Unlike smells which almost demand to be sensed.