For example, if discrimination between two odors required a minimal separation between the neural representations of these odors, then significant bias should become apparent only at large ePN distances.

Indeed, plots of decision bias versus Euclidean or cosine selleck screening library distances between ePN activity vectors showed that the magnitude of bias was bounded by logistic functions of distance for both metrics (Figure 2D). Flies expressed little or no bias when the distance between the representations of two odors was small, achieved saturating levels of bias when distances were large, and tended to display intermediate bias in the transition region between plateaus (Figure 2D). The same logistic bound held irrespective of whether flies discriminated two odors or a single odor against air (Figure 2E). Some well-separated odor-odor pairs and many odor-air pairings elicited lower-than-expected levels of bias (Figures 2D and 2E). These cases underscore that the distance-discrimination function is an upper bound; performance necessarily falls short of this bound when flies lack pronounced innate preferences for the experimental odor(s). When odor valences were measured individually against air and subtracted in order to generate pairwise preference distances (Figure S2 and Table S2),

these preference distances generally predicted the sign of the behavioral bias, but not necessarily its magnitude (Figure S2). Indeed, our data set contains several examples of odors that generated large and opposite biases when tested B-Raf mutation individually

against air but masked each other completely when paired. Hexyl acetate is a strong attractant with a bias score of 46.6%, and 2-heptanone is a weak repellant with a bias score of –15.9%; when the two odors were tested against each other, the decision bias vanished (2.6%). Similarly, isopentyl acetate is a strong attractant with a bias score of 42.4%, and ethyl butyrate is a weak repellant with a bias score of –14.6%; when these odors were tested against Oxygenase each other, the bias score dropped to 2.1%. The two-step model of odor choice suggests a likely explanation: if flies fail to discriminate two odors, then they are unable to attach preference selectively no matter how pronounced the preferences for the individual odors. Consistent with this interpretation, the distances between the ePN activity vectors of these odor pairs map to the bottom plateau of the distance-discrimination function (Tables S1 and S2). If performance is determined by the distance between ePN activity vectors, then the consequences of experimental manipulations that alter this distance should be predicted by the distance-discrimination function. To test this notion, we reversibly blocked synaptic transmission in subsets of ePNs by expressing a dominant-negative, temperature-sensitive dynamin mutant (shits1) (Kitamoto, 2001).