Fig. 5. Non-associative controls and the effects of inter-stimulus interval. (A) Stimulus schedules (see Fig. 1C for key to shading) for three nonassociated control groups (CS alone, US alone and CS/US unpaired groups) and for five differential conditioning groups with different relationships between CS and US (backward, coincident and forward 5, 10 and 20 s groups). (B) Performance indices of eight groups at 2 h after training. Values are means + S.E.M. (N=26, 21, 27, 25, 25, 23 and 20 from left to right). CS, conditioned stimulus; US, unconditioned stimulus.