Conventional model and our model of classical conditioning in insects. (A) A model proposed to account for the roles of intrinsic and extrinsic neurons of the mushroom body in olfactory conditioning in fruit flies . OA neurons and DA neurons (“OA/DA” neurons) convey signals for appetitive and aversive US, respectively. “CS” neurons, which convey signals for CS, make synaptic connections with “CR” neurons that induce a conditioned response (CR), the efficacy of the connection being strengthened by conditioning (open triangles, marked as modifiable). “OA/DA” neurons make synaptic connections with axon terminals of “CS” neurons. (B) Our model proposed to account for classical conditioning in crickets. The model assumes that efficacy of synaptic transmission from “CS” neurons to “OA/DA” neurons is strengthened by conditioning and that coincident activation of “OA/DA” neurons (open triangles, marked as modifiable) and simultaneous activation of “CS” neurons and “OA/DA” neurons is needed to activate “CR” neurons to lead to a CR (AND gate). No synaptic plasticity is assumed for synapses with filled triangles. Modified from Mizunami et al. .