Behavioural contrast and preference in a double chained schedule

Pigeons peck faster during a signal for reward (S+) when that signal alternates with one for absence of reward (S-). This “contrast effect” has been shown to involve diminished preference for S + compared with a stimulus not involved in a discrimination. The present experiment demonstrates that the signal produced by pecks to S+ in a chained schedule is responded to in proportion to the contrast effect during S +. The result suggests that a prior interpretation of contrast, in terms of Amsel's frustration theory, is not the correct one.