In natural environments, odours are dispersed through turbulent processes. The resulting spatiotemporal concentration fluctuations provide odour localisation cues [1]. But plume dynamics also differ according to their proximity to the ground, as illustrated by recent recordings of odour plumes released in free-stream (mid-air) and near-bed (ground-based) configurations in a wind tunnel [2]. We applied bout analysis to these plume recordings, a bio-inspired, event-based approach to process sensory information. A bout is defined as an event of rising gas concentration. Bout counts were previously shown to predict odour source proximity in a concentration invariant manner [1]. We found marked differences in the distribution of bout features for free-stream and near-bed release configurations. Bout counts decreased more rapidly in free-stream downwind from the source. Bout amplitudes decreased over distance for both plumes, with greater amplitudes and variation in the near-bed condition. Bout durations were, on average, greater in the free-stream than near-bed, but unaffected by source distance. For both configurations, the length and variance of inter-bout intervals (IBIs) increased downwind, with the free-stream showing a greater increase with distance. Crosswind from the plume midline counts decreased for both conditions, but variance increased, and bout durations were longer and sparser with longer IBIs. Our results indicate that temporal features of odour concentration not only encode source proximity but also the release configuration close to the ground or in mid-air. Further research should explore the utility of these cues in practical navigation settings.

References
[1] Schmuker et al. (2016) Sens Act B Chem, https://doi.org/10.1016/j.snb.2016.05.098
[2] Connor et al. (2018) Exp Fluids, https://doi.org/10.1007/s00348-018-2591-3

Funding
NSF Next Generation Networks for Neuroscience #2014217 (JPC, ACT) & MRC #MR/T046759/1 (MS, SS, MP)
EU H2020 HBP SGA3 #945539 (MS)