  We analyze the psychophysical responses of human observers to an ensemble of
monomolecular odorants. Each odorant is characterized by a set of 146
perceptual descriptors obtained from a database of odor character profiles.
Each odorant is therefore represented by a point in highly multidimensional
sensory space. In this work we study the arrangement of odorants in this
perceptual space. We argue that odorants densely sample a two-dimensional
curved surface embedded in the multidimensional sensory space. This surface can
account for more than half of the variance of the psychophysical data. We also
show that only 12 percent of experimental variance cannot be explained by
curved surfaces of substantially small dimensionality (<10). We suggest that
these curved manifolds represent the relevant spaces sampled by the human
olfactory system, thereby providing surrogates for olfactory sensory space. For
the case of 2D approximation, we relate the two parameters on the curved
surface to the physico-chemical parameters of odorant molecules. We show that
one of the dimensions is related to eigenvalues of the molecule connectivity
matrix, while the other is correlated with measures of the molecule polarity.
We discuss the behavioral significance of these findings.
