The trial spacing effect in delayed matching-to-sample by pigeons is dependent upon the illumination condition during the intertrial interval.

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Abstract
Two experiments examined the effects of chamber illumination during the intertrial interval (ITI) of delayed matching-to-sample by pigeons. Experiment 1 demonstrated that accurate choice of the matching stimulus was disrupted by illumination at long ITI, but was unaffected by illumination at short ITI. This effect was obtained regardless of the ITI illumination condition during training. The disruption produced by long illuminated ITI was constant across the delay intervals tested. A strong direct linear relationship between matching accuracy and the log ITI/delay ratio was obtained when the ITI was dark, but not when it was illuminated. In experiment 2, ITI illumination was more disruptive when it occurred at the end rather than at the beginning of the ITI. The view that trial spacing effects are not due to competing memories from previous trials was supported. A change in ITI illumination may disrupt encoding of the sample stimulus and offset the beneficial effects of trial spacing.