A single perceptual comparison is sufficient, but not necessary, to induce lasting memory distortion.

Humans constantly compare their visual memories with similar visual inputs to modify their behavior adaptively. However, basic and applied studies suggest that these comparisons can produce systematic distortions in observers' memory reports that persist across time and contaminate subsequent reports. This has led to the hypothesis that perceptual comparisons, while adaptive, are both necessary and sufficient to produce persistent memory distortions. In three experiments, we tested this hypothesis by independently manipulating demands for perceptual comparisons and memory reports across a pair of memory tests separated by a 24-h delay. At each test, observers were cued to recall colored object silhouettes that they had learned previously. At the first test, following cued recall, the silhouette was presented again in a similar color to serve as a perceptual probe to be compared with memory or to be used in a control task where comparisons were omitted and attention to the probe varied. Afterwards, observers either reported their memory of the recalled silhouette or not. Memory reports at the first test exhibited attractive biases towards the probe after comparisons and each of the control tasks. While biases persisted into the second test following comparisons, as previously demonstrated, they also persisted when the control task required processing of the probe that was commensurate with comparisons. In both cases, persistence was not contingent upon making an initial memory report. Therefore, a single perceptual comparison is indeed sufficient for memory distortions to persist, but persistence may be primarily determined by attentional engagement with similar visual inputs.