Observer detection performance loss: target-size uncertainty

The effect of target size and target-size uncertainty on human observers' ability to detect Gaussian and disk targets in spatially uncorrelated and correlated noise was measured. Disk and Gaussian targets were centered in circular areas (diameter, 128 pixels) of uncorrelated noise and uncorrelated noise filtered to resemble CT noise. Size uncertainty was introduced in the target stimuli by presenting targets with effective areas that ranging from 15 to 10,000 pixels. A constant non-prewhitening, matched- filter signal-to-noise ratio (NPW-SNR) was maintained for all target sizes within each trial by adjusting target contrast. Stimulus sets were rendered on the gray-scale monitor of the computer workstation used to collect observer responses. Observers rated for each stimulus the likelihood that a target was present. The observer ratings were analyzed using a multiple-distributing extension of the bi- normal ROC curve fitting procedure. A control experiment evaluated the influence of the circular noise area on performance with size-specified targets. Observer detection performance loss, the ratio of d' to NPW-SNR, decreased for small and large targets. Variations of target shape and noise correlation had no significant effect on performance loss. Observer performance when target was uncertain was the same as observer performance when the target size was specified. In the control trials the investigator specifies the target size to the observer, yet the observer cannot exploit that information. The observer apparently uses same perceptual resources in both the experimental and control trials to render a rating and consequently performs similarly in size-uncertain and size-specified trials. These results suggest a substantial role of higher order mechanisms in the detection of compact targets in noisy backgrounds.