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The second Drosophila talk at this summer school of the International Society for Neurochemistry covered both associative and nonassociative learning. Makis started out by outlining the problem all animals face of finding out which component of the sensory stream is relevant and which isn't, a problem related to selective attention.

The paradigm within which Makis addresses these problems is that of habituation: presenting a stimulus until it doesn't elicit a response anymore. How does an habituated stimulus interact with associative conditioning with that stimulus?

His experiment works as follows: he repeatedly shocks animals with mild electric shocks until the animals don't avoid the shock any more. This habituation takes about 10-12 shock presentations. After ten minutes the animals recover spontaneously from the habituation (avoidance back to normal). Dishabituating the animals with a strong aversive odor (benzaldehyde) also restores full avoidance to the shock. Similar habituation can be achieved with aversive odors and strong electric shock for dishabituation.

During the first phase of this habituation training, no habituation occurs. Habituation only starts after a certain number of presentations. Makis explained this phase of 'protection from habituation' as a time when the brain is assessing the relevance of the stimuli. This phase exists so that the stimulus retains its value to make associations possible. Screening for brain regions involved in premature habituation, he found lines expressing in the mushroom bodies. Silencing the mushroom body alpha and beta-lobes leads to premature habituation.

The learning mutant rutabaga does not show habituation in Makis' experiments, it is resistant to habituation. Other learning mutants, however, show premature habituation. Using RNAi experiments knocking down 14-3-3 genes in various brain areas, he found that these genes prevent premature habituation in the odor pathway and the shock pathway. Any of these two manipulations also affect associative conditioning. Interestingly, treating the prematurely habituating animals (learning gene knocked down in all neurons) with Ritalin rescues shock habituation and associative learning but not odor habituation.

In the final part of his presentation he showed yet more evidence of the interactions between habituation and associative learning using a number of other genes.
Posted on Thursday 25 August 2011 - 15:18:51 comment: 0
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