Olfactory adaptation and odour likeness

The sense of smell is soon blunted; the perfume of a spray of honeysuckle can be powerful and enchanting at the first sniff, but as endeavour is made by repeated sniffs to prolong the enchantment, the smell becomes progressively weaker and less and less satisfying. As a measure of compensation, unpleasant odours also become less and less noticeable. Adaptation is a characteristic of our sensory equipment, and if a stimulus for any one of the senses is kept constant the level of sensation wanes. Adaptation can ensue rapidly, often measurably in a fraction ofa second and noticeably in a few seconds, and recovery from adaptation may be fast or slow according as the initial stimulus was weak or strong (Adrian, 1928). When adaptation is severe, olfactory 'fatigue' supervenes and it becomes impossible to smell a smell clearly and sometimes impossible to smell it at all for a time. Recovery from such fatigue is usually fairly rapid, unless exposure is habitual as it may be with some process workers. It is unlikely that olfactory fatigue is due to peripheral adaptation. Smell differs from the other senses in that it is discontinuous, there being a rest period between successive inspirations and in this rest time the nerves may recover from their refractory state. Olfactory fatigue is more likely to be due to adaptation of some more deep-seated part of the nervous system than the receptors and the fibres leading from them.