Central processing in the mushroom bodies

Abstract

The mushroom bodies in the insect brain serve as a central information processing area. Here, focusing mainly on olfaction, we discuss functionally related roles the mushroom bodies play in signal gain control, response sparsening, the separation of similar signals (decorrelation), and learning and memory. In sum, the mushroom bodies assemble and format a context-appropriate representation of the insect's world.

Keywords: Drosophila; insect; locust; moth; olfaction; oscillation; sensory; synchrony.

Figure 1

Gain control in the Mushroom bodies. In several insect species, singular giant inhibitory neurons, or groups of smaller inhibitory neurons, have been shown to receive output from all Kenyon cells (KCs) and then feed it back as inhibition to all KCs. This mechanism maintains the activity of KCs within a narrow range.

Figure 2

Sparsening and decorrelation. Left: PNs (4 examples shown here) are spontaneously active and respond to odors with bursts of temporally-patterned spikes. Different odors (light gray bar at left, dark gray bar at right) elicit different patterns of activity. The responses of the PN population can be visualized as clouds of points (here, in a 2-dimensional space). Right: KCs, by contrast, are nearly silent at rest and respond to odors with great specificity, and with only a few spikes. Sparsening and decorrelation mechanisms separate the responses of KCs elicited by different odors, making them easy to distinguish.

Figure 3

Learning and memory. Left: In Drosophila, the processes of KCs extend from their bodies in the calyx down through the peduncle into the α, α’, β, β’, and γ lobes. These lobes appear to play different roles in the formation, storage, and recall of memories. Right: Short-term memory (stm), medium-term memory (mtm) and long-term memory (ltm) are processed in the γ, α’/β’, and α/β lobes, respectively. An interesting possibility is that these three forms of memory are processed in parallel (see text).