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ABSTRACT:
We examine the effects of repetition and stimulus quality on the generalization of differentially conditioned stimuli across contexts.
Visual discrimination in Drosophila is a case of differential conditioning of pairs of stimuli (i.e., A+, B- or the reverse; patterns or colors). A second pair of stimuli (i.e., C/D; colors) is arranged as context. Context generalization of this discrimination was previously shown to be dependent on the choice of colors for C/D and on the mushroom bodies, a prominent neuropil in the insect brain.
If C and D are chosen such that memory for the A/B discrimination IS NOT generalized between them, both mushroom bodyless as well as wildtype flies fail to retrieve the A/B memory, when the training context is re-established after sufficient exposure to the non-generalizing context. If C and D are chosen such that memory for the A/B discrimination IS generalized, the flies gradually cease to generalize with increasing number of context changes.
The latter result indicates that contexts can acquire discriminating power depending on the number of context presentations. Occasion setting is a form of higher order learning, in which a discriminator (e.g. C/D) indicates whether the A/B contingencies are valid or not. Extending the concept of context change towards occasion setting, it emerges that operant control of the discriminator facilitates occasion setting. Our results are among the first to demonstrate higher-order learning in invertebrates.
Varying the perceptual quality of the stimuli A/B or C/D in color space suggests that conditionability of both simple discrimination and occasion setting in Drosophila depends crucially on the overlap of the wavelength spectra of the stimuli used.
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