Flexibility in the choice of distinctive features in visual search with blocked and random designs.

An experiment is reported which demonstrates that the features used to detect a character in arrays of similar forms are a function of the stimulus context in which the target is embedded. With the use of blocked and randomised design it was shown that prior knowledge of the background aided search when the feature which was relevant for one backgroind was irrelevant for another, but no significant effect of prior knowledge was obtained when the same feature was useful for the backgrounds.