Temporal Reformatting of Odor Signals by Flow Environments

In broad terms, the goal of a sensory system is to allow an organism to gain information about the external world, taking into account the physical processes that intervene between the sources of the signals and the organism's receptors. In olfaction, these transformations may be particularly complex, as they include the fluid mechanics of odorant transport, which is often turbulent. Here we focus on this transformation, viewing it as an inescapable signal processing stage that occurs before sensory transduction. The typically passive nature of an odorant (i.e., that it is carried by the flow, but does not affect the flow) allows for a concise characterization of how flow transforms the temporal characteristics of odorant concentration at the source into its temporal characteristics downstream. Specifically, the power spectrum (but not the odor concentration time series itself) is transformed in a linear fashion: Spectral components at the source are filtered and mapped to other frequencies at a downstream sensor. We characterize the dominant processes in the mapping as (1) frequency filtering acting as a low-pass filter on the source signal, (2) frequency spreading that redistributes power about the source frequency, and (3) frequency production of an underlying spectrum regardless of input frequency. Each of these processes arises naturally from the multiscale nature of turbulent flow environments. This machinery provides a framework for comparison with active sensation, viewed as another form of signal processing that occurs prior to sensory transduction.