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Mushroom-bodies regulate habit-formation in Drosophila
AuthorBjörn Brembs
Author email bjoern©
Author website
DescriptionDifferent brain circuits mediate the acquisition of skills and habits (via operant learning) and the acquisition of facts (via classical learning). Most learning situations comprise interactions of skill and fact-learning (composite learning). So far these interactions have escaped thorough scrutiny. Drosophila at the torque meter provides one of the very few systems where the relationship of operant and classical predictors can be studied with sufficient rigor. Experiments with mutant and transgenic flies show that there is an interaction between predictive stimuli (classical component) and goal-directed actions (operant component) which makes composite conditioning more effective than the operant and classical components alone. Rutabaga (rut) mutants are impaired in learning about the (classical) stimuli but show intact (operant) behavior learning. This is the first evidence that operant and classical conditioning differ also at the molecular level. The interaction between operant and classical components is hierarchical such that an impaired classical component (in rut flies) suppresses retrieval and an intact classical component suppresses acquisition of the operant component. Experiments with transgenic flies demonstrate that this suppression of operant acquisition is mediated by the mushroom-bodies and serves to ensure that the classical memories can be generalized for access by other behaviors. Extended training can overcome this suppression and transforms goal-directed actions into habitual responses. In conclusion rut-dependent fact-learning is facilitated by goal-directed actions during composite conditioning. Acquisition of this rut-independent operant component is suppressed by the mushroom-bodies which prevents premature habit formation and thereby keeps the classical component flexible. Habit formation after extended training reveals the gate-keeping role of the mushroom-bodies allowing only well-rehearsed behaviors to consolidate into habits.
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