(A) Model of positive modulation. The Target stimulus acquires a direct excitatory association (arrow) and a gated inhibitory association with the US; the gated inhibitory association is composed of an excitatory association between the target and an intermediate hidden unit, and an inhibitory association between this hidden unit and the US (blocked line). The gated inhibitory association can be inhibited by input from the Feature stimulus to the hidden unit. (B) Model of extinction. During acquisition, the target CS acquires a direct excitatory association with the US. During extinction a gated inhibitory association is established between the CS and the US; the intermediate hidden unit requires input from the CS and from the extinction Context for activation of the inhibitory association between the control element and the US.
