Directed versus free search for nodules in chest radiographs

Nodule detection in chest radiographs was compared under free-search and directed-search conditions. In directed search, the observer was told which quadrant of the chest to examine. Each of five observers read a test series of 96 radiographs seven times. Results were analyzed in terms of signal detection theory. NOdule detection was found to be significantly more accurate in directed search than in free search. The results were compared with those of previous experiments which found no such effect or an opposite effect. The present results do not support a previously hypothesized preattentive visual mechanism with better detection capabilities than focused visual attention.