PROSPECTIVE FACTORS CONTRIBUTE LITTLE TO WITHIN-SESSION CHANGES IN RESPONDING

Five rats pressed levers for food delivered by a multiple variable int erval 1-min variable interval 1-min schedule. In the unpredictable con ditions, sessions were 20, 40, 60, 80, or 100 min long, determined ran domly at the beginning of each session. In the predictable conditions, each of these session durations was presented for 15 consecutive sess ions. Rate of responding changed systematically within the session eve n when the end of the session was unpredictable. This implies that pro spective factors related to anticipation of the end of the session are not necessary for producing within-session changes in responding. Wit hin-session patterns of responding were also similar for the predictab le and unpredictable conditions. This suggests that prospective factor s contributed little to the form of the within-session patterns under the present conditions.