Recognition of forms against a complex background.

Although many visual tasks require the observer to search for and attempt to recognize one of a particular class of objects of interest, the problem has received little systematic investigation. Basic visibility data, based upon known shapes presented in known positions, certainly do not apply. In this experiment, observers were repeatedly presented with a background array of curvilinear forms, among which a “critical target” was present 50% of the time. The critical targets consisted of one or another of five examples of six geometrical forms, all with straight sides and sharp corners, any one of which was equally likely to appear in any part of the visual field. Probability of correct recognition was determined as a function of the number of background forms, viewing time, visual angle, and form density. Forms were selected on the basis of a preliminary experiment in which basic recognition data were gathered on 240 different forms.