Delayed matching to sample in pigeons: Effects of delay of reinforcement and illuminated delays

Abstract

Pigeons show relatively poor memory when trained on 0-s delayed matching and tested with longer delays. We hypothesized that one reason for the effect of delay may be a loss of association between sample responding and reinforcement. To test this hypothesis, we compared the effect of standard delays inserted between the offset of the sample and the onset of the comparison stimuli with similar delays inserted between the comparison response and reinforcement. We also manipulated whether the delay was dark or lit because there is some evidence that lit delays are disruptive but filled intervals may also help bridge delays. In Experiment 1, pigeons were trained with a 0-s delay and were tested with longer delays. When both delays and delay illumination were novel, we found no effect of delay of reinforcement and only a small effect of delay illumination. In Experiment 2, to eliminate the effect of the novelty of the delay and delay illumination, pigeons were trained with delays from the start. Results indicated once again that delay of reinforcement had little effect on matching accuracy. However, in Experiment 2, delay illumination produced a general decline in matching accuracy and when it preceded comparison choice its effect increased with increasing delay. The results indicate that the rate of forgetting seen in the typical delayed matching experiment is not due to delay of reinforcement for attention to the sample. The results also confirm that lighting the delay after the sample is detrimental to delayed matching. Surprisingly perhaps, lighting the delay after the comparison response is also detrimental to delayed matching. Finally, it was concluded that a more accurate assessment of the rate of forgetting in delayed matching can be obtained by training with variable delays from the outset.