During sensory-guided navigation, animals refine their ongoing movement through a series of dynamic, iterative sensory-motor algorithms. In natural contexts, odors signal the location of resources and hazards, offering an ethologically relevant window on motivated sensory search. While odor responses has been studied intensively in head-fixed animals, little is known about the dynamic sensory signals that guide freely moving animals during active sampling of their environment or the sensory-motor strategies available to the animal at each stage of their search. Animals may navigate using comparison of signals across successive 'sniff' samples, using instantaneous 'stereo' comparison across hemispheres, or employ both under different conditions. To overcome the challenges of measuring bilateral odor responses in unrestrained animals, we developed new miniaturized microscopy tools for large-scale visualization of neural activity, and used them to image both hemispheres of the main olfactory bulb in mice exploring odor sources in an open arena. Sensory-evoked activity was detectable in discrete bursts occurring in a restricted area of ~10 cm surrounding the odor source. Increasing proximity to the source activated additional glomeruli, revealing that spatial information is encoded by progressive recruitment of receptors of varying affinity.
A subset of glomeruli exhibited a directional bias in activity for stimuli near the corresponding naris. Homologous pairs of glomeruli in either hemisphere were identified by their correlated signals in bilateral imaging. Subtracting left and right signals produced strongly biased directional tuning and predicted turning in a motivated foraging task. These data suggest that animals may employ multiple strategies to localize odor sources during free exploration, initially comparing the degree of glomerular recruitment across time during early approach phases, and ultimately reading out a bilateral direction code at close proximity.