Active Sensing in Olfaction

Excerpt

A fundamental feature of sensory systems is that the animal can actively control the interaction between a stimulus and the sensory neurons detecting it. This active control is important because it allows an animal to sample regions of interest in space, to regulate stimulus intensity in order to maintain optimal receptor function, to extract features of interest from a complex stimulus, and to protect sensory neurons from damage due to excess exposure to strong or (in the case of chemoreception) toxic stimuli. Active sensation is especially prominent in olfaction; in vertebrates, for example, odorants cannot be detected without the movement of air or water into the nasal cavity, and vertebrates and invertebrates alike have impressively complex behavioral repertoires built around the process of sampling odorants. This chapter will focus on the importance of active sensing to olfactory system function. A key point is that active sensing is important not only in shaping how sensory neurons respond to a stimulus, but also in determining how incoming sensory information is processed at higher levels, modulated by behavioral state, and, ultimately, perceived by the animal. For example, active odorant sampling constrains the temporal structure of sensory input to the nervous system, a feature that probably has important consequences for how the postsynaptic networks that process olfactory information are designed and function. At the same time, sampling behavior is tightly linked to behavioral state, so that “top-down,” state-dependent modulation of sensory processing probably goes hand-in-hand with “bottom-up” changes in the nature of sensory input. Finally, in order to correctly process incoming information, sensory processing must be coordinated with the motor systems involved in stimulus sampling. This chapter will review how active sensing shapes olfactory system function at each of these levels. Because of the wide-ranging nature of the subject, treatment of individual topics is not exhaustive; the reader is referred to a number of excellent, more focused reviews at the end of the chapter. In addition, relevant chapters in this volume are cited where possible to minimize overlap of content.