Retroactive interference and rate of forgetting in delayed matching-to-sample performance

Previous evidence suggests that a disruptive stimulus presented during the delay interval of a delayed matching-to-sample trial increases the rate of forgetting by pigeons. However, disruptive events have generally been presented for a period of time proportional to the delay interval. Thus, the observed increase in forgetting may be the result of greater exposure to these events at longer delays than at shorter ones. This possibility was examined by comparing the effects of houselight illumination for the entire delay, half the delay, or a constant 1.5 sec of each delay on pigeons' delayed matching-to-sample accuracy. Presenting the houselight for a period of time proportional to each delay (i.e., the entire delay or half the delay) impaired accuracy more at longer delays than at shorter delays. By contrast, when the houselight was illuminated for 1.5 sec, irrespective of delay length, there was a greater impairment in accuracy at shorter delays than at longer delays. Thus, the increased rate of forgetting previously reported in the literature may be the result of unequal application of a disrupting stimulus across delays.