Effects of visual fixation cues on the detectability of simulated breast lesions

A discrepancy exists between two studies that investigated psychophysical detection of simulated lesions (e.g. gaussians or designer nodules) embedded in filtered noise images (Johnson et al, 2002; Burgess et al, 2003). Johnson et al, 2002 identified a significant difference in the slope of the contrast detail plots (CD plots) as the presentation methodology in a 2AFC task was changed from the unlike background (unpaired) to identical backgrounds (paired). In comparable experiments, Burgess et al, 2003 challenged the results by finding no difference between the slopes (both positive) of the CD plots when using paired backgrounds or unpaired backgrounds. We found that a significant difference between the two studies, namely the presence of a circular fixation cue was responsible for the discrepancy. The detection noise due to positional uncertainty was sufficient to reduce subject's threshold for small target diameters. This effect was amplified in the paired background, switching the CD plot from a negative slope (without fixation) to a positive slope (with fixation). The effect was less dramatic with the unpaired backgrounds, however intra-observer variability seemed to be reduced with fixation cues. These results significantly reduce the discrepancies in C-D characteristics between the two studies.