The notion that repeated temporally paired stimulus presentation were a necessary and sufficient criterion for learning, was prevalent among learning theorists in the first half of this century. However, modern error-correcting learning rules are based on the principle of predictability (as described in the background section on the features of classical conditioning). Two phenomena are critical for falsifying this old idea and thus give support for the modern theories: Whereas a successful blocking experiment demonstrates that repeated temporal pairing of stimuli is not a sufficient criterion for acquiring associative strength, a successful sensory preconditioning experiment shows that it is not a necessary criterion, either (see the features of classical conditioning). Since we failed to find evidence for blocking in our operant learning setup, it is interesting to see whether sensory preconditioning can be observed.
As in the blocking experiments, we used patterns and colors as CS's. For sensory preconditioning, the flies were allowed to fly in closed loop for either 10 (a) or 16 minutes (b, pre-conditioning). The coloration of the arena was changed whenever the fly brought one of the two pattern orientations into its frontal visual field (CS1+CS2). In the subsequent training phase, either the color filter was removed (patterns alone, CS1+US), or the patterns were replaced by four identical vertical bars (colors alone, CS2+US). In the final phase, the flies were tested for the CS that had not been paired with the US previously.
It is apparent that the sensory preconditioning experiment was successful (b): Unreinforced pre-exposure of the colors (CS2) and patterns (CS1) compound (area shaded gray) yielded significant learning scores in one CS (test patterns or colors alone) when the other CS had been paired with the reinforcer in-between (training colors or patterns alone). Interestingly, the sensory preconditioning effect is dependent on the amount of preconditioning. There is no effect, of the preconditioning phase only lasts 10 instead of 16 minutes (a). Orange bars - training; yellow bars - test.
We thus have two opposing results so far, a blocking experiment, that did not show the expected results and a sensory preconditioning experiment that came out as expected. It will be interesting to see whether this is peculiar to operant conditioning in Drosophila or whether it is special for Drosophila in the flight simulator.
A detailed discussion of these results s available in our paper "Conditioning with compound stimuli in Drosophila at the flight simulator" (PDF).