How to play around Olfactory Adaptation?

jueves 30 de abril, 2015

The Human Brain receives a plentiful and constant flow of sensory stimuli. In today’s society we can be hearing the noise of cars in the street, talking with a friend on the mobile phone, seeing five different advertising posters and a white cloud in the bright blue sky, feeling the fresh breeze in our face, perceiving the changes of body positions while running downhill, the touch of our clothes, the coffee after-taste and the nice perfume of a person passing-by.

Such a lot of information could overload our brain and prevent us to keep our normal activities. Fortunately, our brain has developed a couple of survival skills to protect itself from this excess of information: Feature Extraction and Sensory Adaptation. While in the current article we focus mainly on the sense of smell, such skills apply to all our senses: smell, touch, taste, sight, hearing…

Feature Extraction is about reducing the amount of sensory information by extracting some selected features from the environment, putting them together and reconstructing them in our consciousness. Thus, what we perceive is actually an interpretation of a simplified ‘reality’. In the case of smells, this effect is very important, since the airborne components we smell can mix with each other and change completely the perceived result. For example, a person that dislikes vanilla scent can focus on this note within a fragrance composition and reject the whole fragrance without paying attention to the floral, green and woody nuances of the scent.

Sensory Adaptation involves the decrease of repetitive and constant sensory input so as not to overload the brain with redundant information. Sensory adaptation is a general effect for all the senses but it is particularly important for smell because the attenuation can lead to complete absence of sensation. Olfactory adaptation can be understood as sensitivity change or a change in the sensation ‘zero point’. Olfactory adaptation is the temporary, normal inability to distinguish a particular odour after a prolonged exposure to that airborne compound.

Interestingly, even if our noses become desensitised to some stimuli, we remain able to respond to new smells that are ‘out of the ordinary’. For example, when we get adapted to the coffee odour in a café, and we do not perceive the coffee scent of the air anymore, we can still enjoy the scent of the muffin that the waiter brings.

Changing olfactory stimuli is a fascinating field for innovative ideas around counteracting olfactory adaptation. The Ambi-Pur product 3-volution is an example of a successful market product that provides scent perception ‘again and again’ by changing the scent stimulus every now and then. On top of it, this product provides a nice scent game, based on ‘scent collections’ that bring both harmony and contrast to the olfactory experience.

The typical example of olfactory adaptation is the following: a person enters a gently scented room (the smell must not be too strong), the olfactory receptors will signal the presence of the odour. If the person remains in the room, the olfactory receptors will signal the same odour over and over again. After a while the olfactory adaptation phenomenon will take place: the redundant olfactory input will be attenuated (the system adapts) and the smell will fade away. The person might then have the impression that there is no scent anymore, while the stimulus is actually still there.

Another remarkable point about olfactory adaptation is that the olfactory sensation will only reappear if the input does not remain constant. For example, if the person steps out of the room and returns, the input will no longer be constant and the sensation of the smell will return.

Olfactory Adaptation is a very complex phenomenon that must be carefully managed because it creates difficulties in Sensory Evaluation. It means that an odour has a tendency to ‘disappear’ while it is being observed and that sensitivity to the following stimuli will be distorted. When a person is required to assess a series of scents, as soon as they begin to smell the first one, the odour will become weaker. The next scent will also be affected should the required olfactory receptors be adapted by the volatiles from the first one. The perception of any odour depends on the adaptation state of the array of olfactory receptors required to perceive it.

Adaptation changes caused by the evaluation methods can be seen to be largely responsible for disagreements in the literature concerning threshold and intensity measurements. Such type of bias when using human senses as measurement instruments must be anticipated in the experimental design. People evaluating scents must allow their noses to take a rest (and recover from adaptation) between samples in order to avoid a bias in their perceptions. For example, if we smell a pine fragrance immediately after a green and minty one, we might perceive a strawberry note instead of the expected pine scent.

Some authors refer to Olfactory Adaptation as ‘Olfactory Fatigue’. Yet, the word ‘fatigue’ suggests a muscular not a sensory phenomenon. It would be risky to think of adaptation as ‘fatigue’. Whilst muscular fatigue requires a long time for recovery, olfactory adaptation can recover rapidly. Using fatigue as a model could lead to the design of unsuitable sensory testing designs.

Olfactory adaptation can sometimes be used to advantage in sensory testing procedures. For example, it can contribute to discriminate in sensory difference tests because after getting adapted to scent A we can perceive more clearly the differences from scent B.

At Open-Senses we are passionate about sensory innovation. If you share this passion, you can contact us at www.open-senses.com.

 

 

 

Related Literature:

  • Anthony A. Williams, Gillian M. Arnold, “The influence of presentation factors on the sensory assessment of beverages”, Food Quality and Preference (1991) 3, 2, 101.
  • M.J.M. Theunissen, I.A. Polet, J.H.A. Kroeze, H.N.J. Schifferstein, “Taste adaptation during the eating of sweetened yogurt”. Appetite (2000) Vol. 34, N. 1, 21-27.
  • Marjon J. M. Theunissen, Jan H. A. Kroeze, Hendrik N. J. Schifferstein, “Method of stimulation, mouth movements, concentration, and viscosity: Effects on the degree of taste adaptation”. Perception, & Psychophysics (2000). Vol. 62, N. 3, 607-614.
  • O’Mahony, M. “Sensory Adaptation”. Journal of Sensory Studies (1986) 237-258.
  • O’Mahony, M.; Wong, Say-Yin. “Time-intensity scaling with judges trained to use a calibrated scale: adaptation, salty and umami tastes.” Journal of Sensory Studies 3 (1989) 217-236.

 

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