Olfactory BrownCiconelle, Ana C.M., Brazil; Batscha, Jonathan, USA;Weiss, TaliDepartment of NeurobiologyAbstract In vision and audition exist, respectively, the phenomena of brown color and white noise. However, such a phenomenon is much more elusive in olfactory perception, as how physiochemical properties are translated into olfactory precepts is not so well understood. In order to create such an olfactory brown, two tests were made, one rating the similarity of different mixtures and another rating the composition of the same mixtures according to 26 verbal descriptors taken from Dravnieks atlas. Results have shown that mixtures of 15, 20, 25 and 30 components have a similar percept; however, they are significantly different from 4 component mixtures. Those results suggest that mixtures containing large numbers of different odors that span a greater perceptual olfactory space may generate a similar percept: olfactory brown. Introduction In studies of vision and audition, the physical stimuli can be systematically varied by wavelength and frequency, but there is no equivalent metric for olfaction. Furthermore, the mapping of various physical stimuli onto perception is well understood in vision and audition as being directly related to color and luminance or decibel level and pitch, respectively, but in olfaction rules governing the relationship between the physicochemical features of molecules and their perceived odor remain unknown. A significant obstacle to such understanding is the high dimensionality of features describing both percepts and molecules. The perceptual space is made of odorants that can be described by a large number of verbal descriptors. Dravnieks Atlas of Odor Character Profiles (1982) characterized 144 monomolecular odorants with 146 perceptual descriptors. Each odorant is therefore represented by a point in a highly multidimensional sensory space. One of the methods to reduce dimensionality in both odor percepts and physicochemical descriptors is PCA. It was found that the primary axis of perception is odor pleasantness, and that the primary axis of physicochemical properties reflected the primary axis of olfactory perception (Khan et al, 2007). Most of the research in olfaction has relied on studies using pure neat molecules as stimuli. The natural olfactory world, however, rarely contains such neat compounds. The world of odor is made of mixtures containing anywhere between several and several-thousand components. For example: a rose may contain up to ~400 different components (Ohloff and Demole, 1987). In this project we tried to test the assumption that mixtures containing large numbers of equally intense odorants may generate a similar percept. In vision, a mixture that combines all wavelengths in the visible spectrum at equal energy is perceived as "white, and a mixture of primary paint colors is perceived as brown. In fact, a careful mixture of just three primary paints red, blue, and yellow - is sufficient to generate a brown. In a previous study in this lab, 28 and 14 component mixtures were carefully chosen from Dravnieks list based on algorithm which divided the odorants so they will span the greatest possible perceptual olfactory space. In a three-alternative forced-choice discrimination test, discrimination accuracy between two different 28-component mixtures was equal to discrimination accuracy between two 14-component mixtures; both were