Perceptual feedback (by circling) of chest image areas receiving prolonged gaze duration significantly increases pulmonary nodule detection performance. Other methods of perceptual cueing do not lead to such dramatic increases in performance. The mechanisms by which circling influences detection performance were examined. The results of a number of experiments indicate that circling improves nodule detection because (1) the circle isolates the nodule-containing region from the rest of the image, making the disembedding and perceptual integration of nodule features more likely, (2) the circle insulates the region-of-interest from distracters in the chest anatomy outside of the circle boundary which tend to interfere with attention and detection processes, and (3) the circle increases the precision with which the eye fixates relevant nodule features within the region-of-interest, and decreases the dispersion of fixations within this area. The facilitative effects of circling thus seem to influence some basic visual processes. The results should be generalizable to other types of radiological search and detection tasks.