Pavlov theory
In this review, I briefly summarize our attempts to characterize associative processes that account for the CR in crickets and propose that associations that are formed by conditioning and govern the CR in crickets are fundamentally similar to those in mammals. However, the question about the nature of associative processes governing the CR has received little attention until very recently in insects. Adaptive significance of Pavlovian conditioning, as well as its cost (such as decreased longevity associated with increased capability of long-term memory formation in the fruit-fly, Lagasse et al., 2012), has been examined in some insects including the grasshopper Schistocerca americana ( Dukas and Bernays, 2000) and the fruit-fly ( Mery and Kawecki, 2005 Lagasse et al., 2012).
For example, in the fruit-fly Drosophila melanogaster, the use of advanced transgenic technologies allowed detailed analysis of neural and molecular mechanisms of Pavlovian conditioning, and it has been demonstrated that neural circuits of the mushroom bodies, highly organized multisensory associative centers of the insect brain, play critical roles for achieving conditioning ( Hige, 2017 Eschbach et al., 2020 Modi et al., 2020). In this regard, insects have provided useful experimental animals to investigate basic neural mechanisms of Pavlovian conditioning and its adaptive significance ( Menzel, 2012). Elucidation of questions such as what are the underlying neural mechanisms, what is its adaptive significance, what is learned during learning or what kind of associations underlie learned behavior is a fundamental issue of behavioral neuroscience. Pavlovian conditioning is a basic form of associative learning ubiquitous among many vertebrates and invertebrates. Usually, the CR is similar to the response elicited by the US. Pavlovian (or classical) conditioning, first reported by Pavlov in 1902 ( Pavlov, 1927), refers to a learning process in which pairing of a biologically significant stimulus (unconditioned stimulus, US) with a relatively neutral stimulus (conditioned stimulus, CS) results in the CS eliciting a response (conditioned response, CR). Adaptive significance and evolutionary implications for our findings are discussed. We also found that the nature of the habitual response after extended Pavlovian training in crickets is not the same as that after extended instrumental training in mammals concerning the context specificity. The control, however, diminishes with extension of training and hence the CR becomes insensitive to the US value. Our pharmacological analysis suggested that octopamine neurons mediate US (water) value signals and control execution of the CR after standard training. An increase of behavioral automaticity by extended training has not been reported in Pavlovian conditioning in any other animals, but it has been documented in instrumental conditioning in mammals.
We showed that after a standard amount of Pavlovian training, crickets exhibited no response to odor CS when water US was devalued by providing it until satiation, whereas after extended training, they exhibited a CR after US devaluation. In this article, results of our studies in the cricket Gryllus bimaculatus are reviewed. Whether these theories account for Pavlovian conditioning in invertebrates has remained unclear. The other theory, called S-R theory, assumes an association or connection between the CS center and the CR center, allowing the CS to elicit the CR. One theory, called S-S theory, assumes an association between a conditioned stimulus (CS) and internal representation of an unconditioned stimulus (US), allowing the animal to adjust the CR depending on the current value of the US. In Pavlovian conditioning in mammals, two theories have been proposed for associations underlying conditioned responses (CRs). Faculty of Science, Hokkaido University, Sapporo, Japan.