The psychophysical space

Upon sniffing, three major tasks are performed by the brain: a qualitative classiffication of the incoming odorant, a quantitative estimation of its strength, and a hedonic decision about its acceptability. The first two are objective tasks (measuring molecule types and concentrations), while the last one is more subjective and will not be dealt with here.

Olfactory classification of a pure chemical or a mixture is a rather elaborate task. Unlike vision, audition and even gustation, olfaction is multidimensional, and is believed to involve dozens, if not hundreds of quality descriptors. Quantitative assessment of these qualities poses real challenges to research in olfactory psychophysics. Methods have been developed to assign descriptors to an odor, and to give relative weights of dominance to the different descriptors. The entire procedure is normally carried out by a human panel of experts who are familiar with the technique, and who are capable of distinguishing the different descriptors with a high degree of accuracy. As appealing as this might sound, it is quite difficult to obtain coherent results with profiling, since exact verbal descriptions of odor perception are too demanding. Human subjects often find it difficult to describe odor quality verbally, an observation supported by the fact that most natural languages have a poor vocabulary for odors, and these are sometimes described using words borrowed from other sensory modalities(e.g., cool, green).

Alternatives to the profiling technique use panels to accomplish simpler, thus perhaps more reliable, tasks, such as various ways of sorting a group of odors, comparing pairs or triples of odors, pointing out exceptions within groups of odors, etc. Some techniques collect enough statistics from the panels to be able to create a distance matrix that quantitatively expresses the level of dissimilarity between pairs of odors. Various kinds of multidimensional scaling (MDS) algorithms can then be applied to the data, resulting in a vector representation of the odors.

Whatever quantitative quality assessment technique is used, an odorant (o; c) is eventually represented by the odorant vector dP (o; c). We use the symbol l to denote the dimensionality of the resulting odor space, which we call the psychophysical space. If one uses odor profiling, then l is normally in the range 20-200, and the i'th element of dP (o; c) is the human panel's opinion regarding the weight of the i'th descriptor. If one uses MDS, l is typically much lower (< 10), and the elements of dP (o; c) do not have precisely describable meaning. We should emphasize that dP (o; c) is concentration dependent, since the perception of an odorant might change with concentration.

We might say that while (o; c) represents the chemical o in concentration c, the odorant vector dP (o; c) represents the human perception of this odorant, or simply its odor. From this perspective, the psychophysical space is the one on which we should focus, since the odor communication system is designed to directly work within it.

There are profound inter-relations between the psychophysical space and the sensory space. The brain itself is the tool that maps the r-dimensional odorant vectors dB (o; c) into their corresponding l-dimensional odorant vectors d P (o; c). Ignoring dynamical phenomena, such as adaptation, this mapping is considered robust, in the sense that identical inputs dB (o; c), evoke approximately the same outputs d P (o; c). This suggests a way to “fool" the human brain: if a certain odorant with a smell d P(o; c) elicits a neuronal response dB (o; c), then the same smell would be perceived if we succeed in developing a mixture of palette odorants that elicits the same neuronal response. The problem is that gathering data on the

behavior of the olfactory neurons is hard, and not much information is currently available. Moreover, the effect of mixtures on neuronal response has not yet been completely unravelled, making the prediction of the effect of mixture perception impossible. For this reason we would like to avoid the necessity of working with the odorant vectors dB (o; c), which leads to working with sniffers and human panels, as we shall see.