FIGURE 4.

Evolutionary considerations for the S-R type and S-S type Pavlovian conditioning systems, which can be discriminated by US devaluation. Pavlovian conditioning of gill withdrawal response in the sea hare Aplysia can be judged as S-R type from its neural circuitry mechanisms (Kandel, 2001). Pavlovian conditioning systems in mammals and birds are in many cases an S-S type, though S-R type conditioning systems are also present (Holland, 2008; Mazur, 2017). Olfactory conditioning in crickets can be characterized as an S-S and S-R hybrid type. Pavlovian conditioning has been reported in planarians (Prados et al., 2013), which are evolutionary-basal bilaterian animals. Investigations of the effect of US devaluation on execution of a CR in planarians and other invertebrates are needed for obtaining more insights into the evolution of Pavlovian conditioning systems.