VISUAL SIGNAL-DETECTION IN STRUCTURED BACKGROUNDS .2. EFFECTS OF CONTRAST GAIN-CONTROL, BACKGROUND VARIATIONS, AND WHITE-NOISE

Studies of visual detection of a signal superimposed on one of two ide ntical backgrounds show performance degradation when the background ha s high contrast and is similar in spatial frequency and/or orientation to the signal. To account for this finding, models include a contrast gain control mechanism that pools activity across spatial frequency, orientation and space to inhibit (divisively) the response of the rece ptor sensitive to the signal. In tasks in which the observer has to de tect a known signal added to one of M different backgrounds due to add ed visual noise, the main sources of degradation are the stochastic no ise in the image and the suboptimal visual processing. We investigate how these two sources of degradation (contrast gain control and variat ions in the background) interact in a task in which the signal is embe dded in one of M locations in a complex spatially varying background ( structured background). We use backgrounds extracted from patient digi tal medical images. To isolate effects of the fixed deterministic back ground (the contrast gain control) from the effects of the background variations, we conduct detection experiments with three different back ground conditions: (1) uniform background, (2) a repeated sample of st ructured background, and (3) different samples of structured backgroun d. Results show that human visual detection degrades from the uniform background condition to the repeated background condition and degrades even further in the different backgrounds condition. These results su ggest that both the contrast gain control mechanism and the background random variations degrade human performance in detection of a signal in a complex, spatially varying background. A filter model and added w hite noise are used to generate estimates of sampling efficiencies, an equivalent internal noise, an equivalent contrast-gain-control-induce d noise, and an equivalent noise due to the variations in the structur ed background. (C) 1997 Optical Society of America.