Algorithms for Olfactory Search across Species

Abstract

Localizing the sources of stimuli is essential. Most organisms cannot eat, mate, or escape without knowing where the relevant stimuli originate. For many, if not most, animals, olfaction plays an essential role in search. While microorganismal chemotaxis is relatively well understood, in larger animals the algorithms and mechanisms of olfactory search remain mysterious. In this symposium, we will present recent advances in our understanding of olfactory search in flies and rodents. Despite their different sizes and behaviors, both species must solve similar problems, including meeting the challenges of turbulent airflow, sampling the environment to optimize olfactory information, and incorporating odor information into broader navigational systems.

Keywords: active sensing; memory; olfaction; olfactory navigation; olfactory search; turbulence.

Figure 1.

Odor environments vary depending on the distance from a substrate and the air velocity. A, A diffusive environment typical of search by substrate-bound organisms, such as fly larvae. B, Plume forming near a boundary typical of search by walking organisms. C, Plume forming in open air typical of flying organisms. B, C, Adapted with permission from Connor et al. (2018).

Figure 2.

Olfactory search entails active sampling behaviors. These actions optimize sensory input in olfactory scenes. A, Head casting behavior in fly larvae. The animal sweeps its head from side to side to effectively sample the odor gradient. B, Active sampling behavior in mice. By coordinating head movement with sniffing, mice scan turbulent plumes and guide their locomotion to olfactory targets. A, Adapted with permission from Gomez-Marin and Louis (2012).

Figure 3.

Long-distance olfactory search with a cognitive map. In rodents, olfactory search combines odor-guided and memory-guided navigation. In a new environment, the animal relies on odor cues to find a target. With experience in an environment, the animal can learn the probabilities of target locations and forage more efficiently.

Figure 4.

Long-distance olfactory search without a cognitive map. In flying flies, olfactory search behavior emerges from a sequence of chained reflexes. Odor encounter triggers an upwind surge, in which the animal flies parallel to the wind direction. Loss of the plume evokes a crosswind cast, in which the fly flies perpendicular to the direction of flow, which allows the fly to reenter the plume and trigger another upwind surge. As the fly gets close to the target, it becomes attracted to visual objects and executes a sequence of behaviors, including deceleration, leg extension, and landing.