Chapter 1. Is there a measurement system for odour quality?

Although humans can discriminate a huge number of odours, the mechanisms behind the perception of odours are not well understood. The theory of the present essay and our experiments were developed with the view that odours are homogeneous perceptions (as opposed to visual objects that are heterogeneous percepts). Other typical examples of homogeneous perceptions are colours and some emotional states. Thus, through perceptual learning, which is a prerequisite for perceiving, we become aware of and may recognise odours in the environment. Typically, odours are detected at very low concentrations as compared to chemical instruments, the “electronic nose” inclusive. Four principles of investigating the human world of odour qualities are discussed: (i) classification and sorting according to odour-quality resemblance, (ii) using names and verbal attributes for odour sources, (iii) using odour qualities for a representative set of reference odorants, and (iv) measuring odour quality by similarity scaling of pairs of odorous stimuli. Although a huge number of odours can be discriminated, the quality of odour mixtures is intermediary among the odour qualities of their components. This applies to single chemical compounds or mixtures thereof and for very broadband mixtures like indoor air. Our perceptual world simply consists of a massive amount of odour qualities potentially attributable to sources in the environment. Their organisation is, however, still a mystery.