The effects of perceptual history on memory of visual objects.

We investigated how the recognition and perception of memory-stored visual objects are influenced by cumulative experience with similar stimuli. The memory of a face was established by training observers to identify a set of faces as either "friends" or "non-friends". Subsequently, for multiple daily sessions, observers continued to perform this identification task, in which presented faces included a sequence of morphed faces, gradually transforming from a friend face (source) to another initially distinguishable non-friend face (target), interleaved with other faces. Initially observers identified only the first part of the morph sequence as "friends". In experimental conditions for which the initial "friends" portion was at least 54% of the sequence, this portion increased along repeated daily practice, until eventually most of the sequence was identified as "friends". After this practice, perceived similarity between source and target faces was much higher than the average similarity between the other face images. These effects did not occur when the morph images were shown in random order using a similar protocol. In addition, corresponding recognition confusions between source and target faces were found. Our findings suggest that memories of objects can be changed as a result of exposure to similar stimuli and show the dependency of these changes on the order in which stimuli are presented and on their level of similarity.