Sensory Systems/Olfactory System/Signal Processing

Only substances which come in contact with the olfactory epithelium can excite the olfactory receptors. The right table shows thresholds for some representative substances. These values give an impression of the huge sensitivity of the olfactory receptors.

It is remarkable that humans can recognize more than 10,000 different odors. Many odorant molecules differ only slightly in their chemical structure (e.g. stereoisomers) but can nevertheless be distinguished.

Signal Transduction
An interesting feature of the olfactory system is that a simple sense organ which apparently lacks a high degree of complexity can mediate discrimination of more than 10'000 different odors. On the one hand this is made possible by the huge number of different odorant receptor. The gene family of the olfactory receptor is in fact the largest family studied so far in mammals. On the other hand, the neural net of the olfactory system provides with its 1800 glomeruli a large two dimensional map in the olfactory bulb that is unique to each odorant. In addition, the extracellular field potential in each glomerulus oscillates, and the granule cells appear to regulate the frequency of the oscillation. The exact function of the oscillation is unknown, but it probably also helps to focus the olfactory signal reaching the cortex

Smell measurement
Olfaction consists of a set of transformations from physical space of odorant molecules (olfactory physicochemical space), through a neural space of information processing (olfactory neural space), into a perceptual space of smell (olfactory perceptual space). The rules of these transforms depend on obtaining valid metrics for each of those spaces.

Olfactory perceptual space
As the perceptual space represents the “input” of the smell measurement, its aim is to describe the odors in the most simple possible way. Odors are ordered so that their reciprocal distance in space confers them similarity. This means that the closer two odors are to each other in this space the more are they expected to be similar. This space is thus defined by so called perceptual axes characterized by some arbitrarily chosen “unit” odors.

Olfactory neural space
As suggested by its name the neural space is generated from neural responses. This gives rise to an extensive database of odorant-induced activity, which can be used to formulate an olfactory space where the concept of similarity serves as a guiding principle. Using this procedure different odorants are expected to be similar if they generate a similar neuronal response. This database can be navigated at the Glomerular Activity Response Archive.

Olfactory physicochemical space
The need to identify the molecular encryption of the biological interaction, makes the physicochemical space the most complex one of the olfactory space described so far. R. Haddad suggest that one possibility is to span this space would to represent each odorant by a very large number of molecular descriptors by use either a variance metric or a distance metric. In his first description single odorants may have many physicochemical features and one expects these features to present themselves at various probabilities within the world of molecules that have a smell. In such metric the orthogonal basis generated from the description of the odorant leads to represent each odorant by a single value. While in the second, the metric represents each odorant with a vector of 1664 values, on the basis of Euclidean distances between odorants in the 1664 physicochemical space. Whereas the first metric enabled the prediction of perceptual attributes, the second enabled the prediction of odorant-induced neuronal response patterns.