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The equations for nonwhite noise given in the previous chapter can also be used for an evaluation of the masking of one signal by the presence of another signal. This application will be used for the development of a model for contrast discrimination that will be given in this chapter. For contrast discrimination, a difference has to be observed between two nearly identical sinusoidal signals that differ only in modulation. This situation is different from the situation for contrast detection where only a difference has to be observed between the presence and the absence of a signal. In contrast discrimination experiments one signal has a fixed modulation and the modulation of the other signal is varied until a just-noticeable modulation difference between the two signals is observed. The threshold of the modulation difference is the decisive quantity for contrast discrimination. The signal with the fixed modulation is called the reference signal and the signal with the variable modulation is called the test signal. The threshold of the modulation difference appears to be a function of the modulation of the reference signal. Contrast detection can be considered as a form of contrast discrimination where the reference signal has zero modulation.
To obtain a model for contrast discrimination, first an evaluation will be made of the psychometric function occurring in contrast discrimination experiments. The psychometric function, which plays an important role in contrast detection, also plays an important role in contrast discrimination. From the psychometric function of contrast discrimination experiments much information can be obtained about the fundamental aspects of contrast discrimination. This information will be used to derive an equation for the functional structure of the contrast discrimination process. This equation will further be used in the evaluation of the contrast discrimination model.
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