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This paper investigates signal detectability in a fixed structured background as a function of signal contrast, additive white noise and feature motion. We use a 4 AFC (alternative forced choice) detection task where the signal appeared at the center of one of four identical, clearly visible, simulated cylindrical artery segments. All four segments moved identically relative to the background in 32-frame image sequences displayed at 15 frames per second. The background in one condition was uniform and in a second condition was structured noise consisting of a single frame randomly selected from a group of clinical x-ray coronary angiograms. We studied two display formats, the 'moving artery' display in which the background was stationary and the cylinders moved back and forth, simulating the motion of the coronary arteries, and the 'stabilized artery' in which each frame of the sequence was translated to keep the cylinders stationary, while the background moved back and forth. The signal to be detected was a disk superimposed at the center of one of the simulated cylinders. Signal energy and the variance of additive Gaussian spatiotemporal white noise were manipulated. For each level of additive white noise the threshold signal energy for detection (at the 82% correct performance level) was determined. There was no time limit to reach decision. For all conditions the threshold signal energy increased linearly with added white noise variance, with a positive y-intercept. The presence of the structured background increased both the y-intercept and the slope of this relationship between threshold energy and added white noise variance. Thus, the presence of the structured background had a multiplicative effect, as well as an additive effect, on the degradation of performance due to added white noise. The multiplicative effect might be modeled by an increase in induced internal noise (noise proportional to the external noise) with the presence of the structured background. Such an effect, if it occurs in the setting of clinical coronary angiography, would cause changes in radiation exposure (and thus quantum noise) to affect visual perception more than expected from experiments with white noise alone. One possible mechanism for this effect may be that the added random noise interferes with the observer's use of spatiotemporal correlations to 'subtract' or 'read around' the structured noise.
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