**1. Introduction**

Glomeruli in the brains of insects and vertebrates are the morphological and physiological structures where the primary processing of olfactory information takes place [1]. Glomeruli are housed in the olfactory centers of insects, the antennal lobes, and in the olfactory bulbs of vertebrates. Their widespread presence in different taxa has been interpreted to suggest common functionality. Experimental evidence based on recordings from principal output neurons in olfactory glomeruli of vertebrates and invertebrates supports this notion [2, 3]. The striking structural similarity, as well as the similarity of the responses to odor stimulation between neurons in the insect

antennal lobe and vertebrate olfactory bulb, suggests that glomerular microcircuits across taxa may share similar means of processing olfactory input [1, 4, 5]. Studies on glomerular circuitry address the central question of the functional organization of olfactory glomeruli.

The antennal lobes of the sphinx moth *M. sexta* have emerged as a model system to determine mechanisms underlying olfactory information processing in early olfactory centers such as the glomerular microcircuits. (1) In *M. sexta*, the antennal lobes house a male-specific olfactory subsystem. This subsystem is specialized to process information about the female sex pheromone [6]. Input and output relationships in this experimentally advantageous model system can be precisely defined. (2) Other glomeruli in the antennal lobes of *M. sexta* are clearly different in both function and morphology compared to the male-specific subsystem that comprises the macroglomerular complex (MGC). The MGC consists of three glomeruli, the toroid-1, toroid-2, and the cumulus [7, 8]. (3) The MGC receives input from antennal sensory neurons [9] that are specifically tuned to one of the two essential components of the female sex pheromone [10].

The glomeruli of the MGC process information about the two essential pheromone components of the female sex pheromone. The components of the odor stimulus released by the female have been determined in terms of the concentration and ratio of the pheromone components. The number of neurons projecting from the MGC to higher brain centers is relatively limited. About 30 to 40 projection neurons (PNs) innervate the MGC, and about 860 PNs innervate all the glomeruli in the AL [11]. Many local interneurons (LNs) and PNs in the ALs have been described both morphologically and physiologically [3, 8, 12, 13].

The goal of research on olfactory glomeruli is to understand the role(s) of individual glomeruli, for example, the glomeruli that constitute the MGC in olfaction, namely, the toroid-1, toroid-2, and the cumulus, by analyzing how the neural circuits associated with these glomeruli process pheromonal information. The functional organization of the MGC can be studied by means of single-unit intracellular recording, staining and laser scanning confocal microscopy, and more recently, imaging techniques, multi-unit recordings, and computational models [14–22]. This line of research attempts to address several topics: How do features of the stimulus determine pheromone-evoked response characteristics of MGC interneurons? How do MGC interneurons discern pheromone components in a complex odor blend? Can MGC– PNs resolve and encode the naturally intermittent temporal structure of pheromonal stimuli? Do the two essential pheromone components serve specific and different roles? Answers to these questions will help define the functional role of glomeruli in olfaction and will aid our understanding of how different features of an odor stimulus are processed in the brain.
