The present experiment examined overall and local effects of omission of reinforcers in a choice situation. Pigeons' key-pecking responses were reinforced under concurrent fixed-interval and random-interval schedules of food presentation. After some weeks of baseline sessions in which the probability of reinforcement was 1.00, approximately 25% of food presentations from the fixed-interval schedule were omitted and replaced by timeout periods. In such omission sessions, the overall relative rates of responding to the fixed-interval schedule became lower than those in the baseline sessions. On the other hand, when relative rates of responding to the fixed-interval schedule in the omission sessions were calculated separately for fixed-interval cycles preceded by timeout periods and those preceded by food presentations, the relative rates in the former type of fixed-interval cycles were higher than those in the latter type for three out of four pigeons. These results mean that relative rates of responding cannot always be regarded as reflecting a relative value of an alternative, and that the overall effect of the omission of fixed-interval reinforcers is not reducible to the local effect of omission.
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