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A fundamental everyday visual task is to detect specific target objects within a background scene. Under natural conditions, both the properties of the background and the amplitude of the target (if present) are generally different on every occasion. To gain some understanding of detection under such natural conditions we determined the amplitude thresholds in natural images of a matched-template detector, as a function of the three local background properties: luminance, contrast, and phase-invariant similarity to the target. We found that threshold (which is equal to the standard deviation of the template response) is a linear separable function (the product) of all three dimensions—" multidimensional Weber's law." This fact poses a serious problem for detecting targets under natural conditions, where both the properties of the background and the target amplitude are uncertain. Specifically, good performance …