Bi-directional anti-Hebbian plasticity rule for KC→MBON connections. In our standard training bout, a 30-s CS + presentation was paired with an unconditioned stimulus (shock) starting 3 s after CS + onset, followed by a 30-s CS − presentation. With either attractive or repulsive odours, KC1 is activated by the CS + and KC2 by the CS − , but DAN responses depend on the odour valence. During training with attractive odours, DANs integrate the inhibitory effects of the CS + and excitatory effects of the shock, yielding weak depression of the KC1→MBON synapse. The unpaired CS − presentation suppresses DAN firing, facilitating the KC2→MBON connection. With repulsive odours, joint CS + –US presentations strongly activate the DAN, greatly depressing the KC1→MBON synapse. A repulsive CS − mildly depresses the KC2→MBON weight.
