Figure 2: Response of the Or42a OSN stimulated by dynamic odor stimuli.

(A) Schematic diagram of in vivo extracellular recordings of the larval Or42a OSN stimulated by real odors [26]. (B) Stimulation of the Or42a OSN by symmetrical linear odor ramps with three different slopes: shallow (lilac), medium (magenta) and steep (violet). The odor ramps were produced by a microfluidics system using isoamyl acetate [26]. The duration of the upward phase is fixed to 4 s for the three ramps. The downward phase of the three ramps has the same “medium” slope, which explains differences in the time taken to complete the ramp. (Bottom) Peristimulus time histogram (PSTH) of the Or42a-OSN firing activity corresponding to the ramps shown in the top panel: the light to dark gray traces correspond to the shallow, the medium and the steep ramps, respectively. During the upward phase of the stimulus, the firing activity of the Or42a OSN reaches an asymptotic value that scales with the slope of the ramp. During the downward phase of the ramp, the OSN activity tends to follow the intensity time course, which highlights the nonlinear dimension of the Or42a OSN response. At the end of the ramp, the activity of Or42a OSN is inhibited below its basal level (dashed line). (C) Stimulation of the Or42a OSN by an exponential odor ramp. Consistent with the idea that the OSN responds to the slope of the stimulus during the upward phase of the ramp, the firing activity increases exponentially (the derivative of an exponential is an exponential). (D) Stimulation of the Or42a OSN by a “parabolic” odor ramp. During this ramp, the first derivative of the stimulus intensity decreases monotonically over time. And so does the OSN activity. Panels B-D are redrawn from [26].