The Mushroom Bodies as a Locus for Memory

Although CS and US information converges at both the antennal lobes and the mushroom body calyces, the mushroom bodies appear to be especially important in conditioning, and direct evidence confirms this (Hammer and Menzel 1995). Cooling the calyces of the mushroom bodies produces amnesia similar to that produced by cooling the whole animal (Erber et al. 1980). Mutations resulting in abnormal mushroom body structure cause a loss of conditioning to odors (Heisenberg et al. 1985), and so does destruction of the mushroom bodies (de Belle and Heisenberg 1994).

Associative learning of any kind requires a point of neural convergence between conditioned and unconditioned stimuli. Neurobiological studies of associative learning have begun to describe what occurs at these points of convergence. An important concept introduced by Donald Hebb (1949, 62) serves as a guide for this research: "When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased." In other words, structural changes in the nervous system result from one cell taking part in the firing of another. In the case of the honeybee proboscis extension response, Hebb's postulate leads us to ask what happens to mushroom body neurons when projections from the antennal lobe cause them to fire, and that firing is rapidly followed by further firing of these cells by octopamine release from the VUMmx1 axons. To find the answer to this question, we must now look inside the neurons that are activated in this way.

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