Figure 1.

Population Representations of Odors

(A) ORN (top) and ePN (bottom) representations of 1-octanol (left), ethyl acetate (center), and 2-heptanone (right). The odor responses of 24 ORN classes (expressing the odorant receptors indicated on top) were measured (Hallem and Carlson, 2006; Hallem et al., 2004). ePN responses were predicted from these measurements with the experimentally supported equation (Olsen et al., 2010):

$$R_{ePN}=R_{\max }\frac{{R_{\mathit{ORN}}}^{1.5}}{{R_{\mathit{ORN}}}^{1.5}+{\sigma }^{1.5}+{\left(m/190\sum R_{ORN}\right)}^{1.5}}$$

Here, $R_{ePN}$ is the firing rate of a particular class of ePN, $R_{ORN}$ is the firing rate of the cognate class of ORN, $R_{\max }$ is the maximal possible ePN firing rate, σ is a constant, and m is an inhibitory scaling factor. The following parameter values were used: $R_{\max }$ = 165 spikes per s, m = 10.63, and σ = 12 spikes per s (Luo et al., 2010; Olsen et al., 2010). The input-output relationship described by the equation is depicted graphically for the three odors. Note that the transformation differs between odors because inhibitory scaling depends on $\sum R_{ORN}$.

(B) Responses in ePN projections to the LH were evoked with 5 s pulses of odors and imaged by two-photon microscopy. Flies carried GH146-GAL4:UAS-GCaMP3 transgenes. Examples of individual responses to the six indicated odors (right) are contrasted with a common reference—the response to 1-octanol (left). The activity maps are pseudocolored according to the key on the right.

(C) Correlation distances between the experimentally determined response maps are linearly related to calculated Euclidean distances between ePN activity vectors (mean ± SEM, R² = 0.5334, p < 0.0001, n = 13 flies).

See also Figure S1.