Two minds about odors

Abstract

It has long been suggested that synchronous fast oscillations in central sensory systems facilitate formation of neural assemblies that represent sensory objects. This hypothesis derives from the olfactory system, where odor-evoked fast oscillations of 40 Hz and above in the local field potential of the mammalian olfactory bulb (OB) were first reported by Adrian (1). Similar oscillations (≈20 Hz) were observed half a century later in the insect mushroom body (MB) driven by antennal lobe (AL) synchrony (2). Two reports have shown empirical evidence that the relative power of fast oscillatory activity represents underlying neural synchrony and correlates in a general way with the ability of the animals (mice and honey bees) to discriminate chemically similar stimuli (3, 4). These studies together suggest that more oscillatory synchrony allows better discriminability and that less synchrony degrades discriminability of chemically similar odors. What these studies do not address is the advantage of not discriminating odors in some circumstances, of clustering odors by chemical or other features. In this issue of PNAS, a computational model by Sivan and Kopell (5) addresses odor clustering in the insect olfactory system. The authors ask how the same system can be very efficient at both sensory object clustering and fine discrimination of similar objects. This is indeed an important question, particularly when we consider the functional structure of olfactory systems in diverse animals. Sivan and Kopell show that the modular structure of the insect system is capable of accomplishing both of these tasks in parallel pathways, so that discriminating individual odors is anatomically separate from, but simultaneous to, odor clustering. The striking similarity of separately evolved olfactory systems in insects and mammals drives speculation that convergent evolution has yielded a good solution to the odor coding and discrimination problem (6, 7), and, by focusing on …