Elsevier

Current Opinion in Insect Science

Volume 6, December 2014, Pages 99-103
Current Opinion in Insect Science

Central processing in the mushroom bodies

https://doi.org/10.1016/j.cois.2014.10.009Get rights and content

Highlights

  • The mushroom bodies integrate and format sensory information.

  • Mushroom body information processing functions include gain control, response sparsening, decorrelation, and learning and memory.

  • The mushroom bodies assemble and format a representation of the insect's world.

The mushroom bodies in the insect brain serve as a central information processing area. Here, focusing mainly on olfaction, I 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.

Introduction

The mushroom bodies are striking in appearance, resembling bilaterally arranged cups brimming with tiny neurons, supported by stems that bend and branch in several directions dorsally and laterally. The tiny neurons, Kenyon cells, (KCs) send long thin processes down through the stems, which form distinct lobes. These prominent and complex structures, found in all but the earliest insects, are as interesting as they look  they serve a number of functions important for processing sensory information. In many insects, groups of KCs receive sensory information from visual, gustatory, and mechanosensory areas, and, perhaps most often studied, thick tracts of olfactory input from the antennal lobes [1, 2]. In honeybees and other insects, different populations of KCs appear to receive direct input from different sensory modalities, although some KCs may also be multimodal. The KCs also receive inhibitory and recurrent input, and neuromodulators such as dopamine that provide reward signals [3]. Together, these inputs endow the mushroom bodies with information processing powers that are gradually coming to light. Here, focusing mainly on olfaction, I discuss functionally related roles the mushroom bodies appear to play in signal gain control, response sparsening, the separation of similar signals (decorrelation), and learning and memory.

Section snippets

Gain control

Sensory stimuli can be weak or strong, and sensory systems must accommodate this dynamic range. In several insect species the mushroom body's KCs have been found to form feedback connections with powerful inhibitory neurons that may help contain responses to sensory stimulus within limits (Figure 1). The anatomy of feedback connectivity provides a hint that any increase in the output of KCs will be tamped down by inhibition that increases proportionally with the response of the KCs, and is

Sparsening and decorrelation

Among the inputs received by KCs are olfactory signals carried by projection neurons from the antennal lobe. Anatomical studies show that each olfactory KC receives input from multiple presynaptic projection neurons [6, 7], and electrophysiological recordings show that the projection neurons (PNs), which are spontaneously active in the absence of stimuli [8], respond to odors with voluble bursts of spikes. Given the sheer number of action potentials arriving at KCs, one might predict these

Learning and memory

The mushroom bodies have long been associated with learning and memory. In numerous insect species, the volume of the mushroom body calyx has been shown to increase with sensory experience, not just with age (see [24] for several examples). Retrograde amnesia following olfactory training was induced in honeybees by specifically cooling the mushroom bodies [25], and honeybees treated early in life to develop without full mushroom bodies behaved quite normally as adults, but were deficient in

Reading the output of KCs

The processing taking place within KCs exerts its effects upon follower cells. In Drosophila, most of the neurons following from KCs have been identified and mapped [38], and their contributions to mushroom body function no doubt will soon be revealed. To date, though, work in other insects has provided interesting clues about how the output of KCs is processed by follower neurons. Recent work in the locust shows that the precise timing of the very sparse spikes elicited by odors in KCs carries

Conclusions: what the mushroom bodies are for

The mushroom body serves as a central processing unit within the insect brain. It receives sensory input from multiple modalities as well as modulatory signals reflecting internal state and external reward conditions; these modulators alter the properties and responses of the mushroom body's KCs and other neurons. The mushroom body combines and reformats the information it receives, projecting its volubly spiking input into sparse and well-separated representations. It excels at detecting

Acknowledgement

Many thanks to Dr. Kazumichi Shimizu for providing helpful comments on the manuscript.

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