Spatiotemporal structure of olfactory inputs to the mushroom bodies.

A requirement to understand mushroom body (MB) function is to characterize the operations (or transformations) that they impose on incoming signals. Understanding the nature of these integrative operations requires an understanding of the inputs from other brain areas. By inputs we mean not only the anatomical pathways leading to the MBs, but also the dynamic structure of the inflow of sensory (and other) signals. Neurons are complex, capacitative, and generally nonlinear devices that transform barrages of neurochemical packets into electrical waveforms. Their modes of operation are intrinsically time dependent and therefore, their functions or roles in a circuit cannot be inferred only from structural data. Thanks to elegant anatomical, behavioral, genetic, and molecular (for review, see Crittenden et al. 1998; Hammer and Menzel 1998; Heisenberg 1998; Wolf et al. 1998) studies, there is convincing evidence that MB circuits are involved, at least in fruit flies and honeybees, in some forms of odor integration and learning. In vivo electrophysiological studies of MB neurons, however, are rare and mainly restricted to individual (or small populations of) so-called extrinsic neurons, that is, those whose processes link MBs with other brain areas (Schildberger 1983, 1984; Homberg 1984; Hammer 1993; Mauelshagen 1993; Li and Strausfeld 1997). Kaulen et al. (1984) examined extracellular potentials in the MBs of bees, using current source density analysis, and more recently, Laurent and Naraghi (1994) provided a description of stimulus-evoked activity in Kenyon cells (KCs), the intrinsic neurons of the MBs, using intracellular recordings. In this short review we will summarize the recent results from our laboratory in an attempt to provide a description of the spatiotemporal structure of olfactory inputs to the MBs and their intrinsic neurons. We will focus only on the encoding of odor quality. We will then speculate on the possible role of MB circuits for olfactory processing.