Two experiments employed an autoshaping procedure with pigeons to examine the effects of changes in the inter-stimulus interval (ISI) upon the blocking phenomenon (Kamin, 1968). The introduction of a novel stimulus into a reinforced compound with the pretrained stimulus was accompanied by a decrease (Experiment I) or by increases (Experiment II) in the ISI. A decrease and a relatively small increase in the ISI attenuated the blocking of the added stimulus. The results were related to the major theories of stimulus selection and were interpreted to have added variations in the ISI to the list of operations during compound conditioning that cause “surprise” and, hence, attenuate blocking.
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