Are the principles of Pavlovian conditioning in insects conserved with those in mammals?

Pavlovian conditioning, in which a relatively insignificant stimulus (the conditioned stimulus, CS) is paired with a biologically significant stimulus (the unconditioned stimulus, US), is a ubiquitous form of associative learning found in many animal phyla. Modern theories in mammals suggest that the prediction error, i.e., the discrepancy between the predicted unconditioned stimulus (US) and the actual US, drives conditioning, and that the conditioned response (CR) to the conditioned stimulus (CS) is flexibly guided by the expectation of the US. These theories were proposed to overcome limitations of conventional theories, which assume that contingency or correlation between the CS and US is sufficient to account for the achievement of conditioning and that the strength of the CS-US association is enough to determine the magnitude of the CR. It remained unclear, however, whether the modern theories account for Pavlovian conditioning in invertebrates. Here, I address this issue by reviewing recent studies in insects. It has been demonstrated that the error correction learning rule achieves the conditioning in crickets, and the production of the CR is guided by the current value of the US in fruit flies, crickets, and honey bees. From these findings, I conclude that the principles of Pavlovian conditioning in insects are, in essence, conserved with those in mammals. A crucial question to be addressed is how sophisticated forms of Pavlovian conditioning, comparable to those achieved in large-scale neural networks in mammalian brains, are accomplished by small-scale neural networks in insects.