Neural models 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 . Octopaminergic or dopaminergic neurons ('OA/DA' neurons) convey signals for appetitive or aversive unconditioned stimulus, respectively. 'CS' neurons, which convey signals for CS, make synaptic connections with 'CR' neurons that induce conditioned response (CR), the efficacy of the connection being strengthened by conditioning. 'OA/DA' neurons make synaptic connections with presynaptic terminals of 'CS' neurons. (B) A new model of classical conditioning, termed Mizunami-Unoki or M-U model. 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 and 'CS' neurons is needed to activate 'CR' neurons to lead to a CR (AND gate). (C) M-U model to account for second-order conditioning.