Figure 3.

A model to account for the finding that the CR is sensitive to reward devaluation after standard training but not after extended training. The model was modified from our previous model proposed to account for the finding that activation of OA neurons is needed for the execution of the CR after standard training [22,23]. Our original model contained two types of OA neurons, ‘OA1’ neurons that participate in the learning process and ‘OA2’ neurons that participate in execution of the CR, and in this model, we focus on ‘OA2’ neurons, which we assume to mediate current value of the US. (a) The basic assumptions of the model are that: (i) it represents neural circuits in the lobe of the mushroom body (MB): ‘CS’ neurons that encode CS and ‘CR’ neurons for which activation leads to the CR represent Kenyon cells and output neurons of the MB lobe, respectively; (ii) two kinds of memory traces are formed by standard training, one being synaptic connection from ‘CS’ to ‘OA2’ neurons, which represents the CS–US (S–S) connection, and the other being synaptic connection from ‘CS’ neurons to ‘CR’ neurons, which represents the CS–CR (S–R) connection; (iii) simultaneous activation of ‘CS’ neurons and ‘OA2’ neurons is needed for activating ‘CR’ neurons (AND gate) and for producing the CR; and (iv) activation of ‘OA2’ neurons is inhibited when the animal has been satiated with water. (b,c) The CR becomes insensitive to reward devaluation by extended training owing to strengthening of the CS–CR connection and resulting elimination of the requirement of activation of ‘OA2’ neurons for the CR (b), or by development of a novel neural circuitry that produces the CR without activation of ‘OA2’ neurons independently of the neural circuitry where learning initially occurs (c).