Positive local contrast and s- duration: Theories of local contrast and a resolution

Pigeons were exposed to a simple sequence in which an S+, presented for a fixed duration, was alternated with three durations of an S-. Both positive and negative local contrast appeared as distinctive patterns of responding and S+ responding depended upon the duration of the preceding S-. Response rates in S+ were higher after the two longer S- durations than after the briefest S-. The effect of S- duration was partially independent of the presence or absence of a clear pattern of positive local contrast in S+. Additionally, all birds showed Skinnerian induction in S+ (depressed responding during the first half of the presentation) when the preceding S- was the briefest of the three durations. Additivity theory and response competition theory are both unable to account for these and other local contrast phenomena. However, a liberal interpretation of additivity theory, better termed a "Pavlovian-process" theory, can do so in a very straightforward way.