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Masking: The visibility of a spatial target is usually reduced by adding background patterns. Such an effect is called masking.  For example, the line segment is easy to see in a uniform field, but difficult to tell under a noise pattern as shown in figure 1.

Fig. 1

Frequency Masking:  Published research results in the literature have shown that the masking effect is spatial frequency specific. When the background patterns, or called maskers, have similar frequency components to the target, the masking effect is strong.  Thus, researchers in this field frequently use the term spatial frequency masking, and the effect is explained with spatial frequency channels in the biological visual system. 
      In this paragraph we elaborate our way of thinking about frequency masking. Assuming that the human visual system can perform  image analysis in a frequency domain somehow, a sine-wave is represented by a dot in this domain. Due to spatial inhomogeneities in the visual system, this dot will spread to its surrounds, producing frequency components that were not in the original sine-wave. Figure 2 demonstrates the effect of frequency masking in the frequency domain as an analogy to the perception in the space domain. The center dot represents a frequency masker. When there is no frequency spread, the visibility for each of the two target dots on its right side is very high (left panel). After adding some spread, the closer target  is barely discriminable, while the far dot is still easy to tell (right panel).

 Fig. 2

Implicit Masking:  It is well-known from psychophysical experiments that the visibility of a spatial target is largely determined by the luminance contrast of the target (i.e., the ratio between the target signal and its background signal strength), but not the absolute luminance of the target. Along this line, traditional vision models assume that the early stage of visual processing is to extract luminance contrast of a visual stimulus as the signal to later visual processing. The potential limitation with such models is that they would not show the effect of absolute luminance on visual detection, if there are any.
         Actually, experimentally obtained contrast thresholds do vary with absolute luminance of the stimuli.  For example, Van Nes and Bouman (1967) studied the change of the contrast threshold of  a sinusoidal grating with its background luminance and spatial frequency. The results showed that the contrast thresholds are barely affected by the absolute luminance only with coarse patterns (i.e. low spatial frequencies) at high luminance levels.
       What visual functions can be used to simulate the luminance effects? Yang and Makous (1994) proposed that the mean luminance level of a stimulus can be treated in the frequency domain along with all other frequency components of the stimulus, as a component at 0 cpd. As illustrated in figure 2, such a component can act as a frequency masker to other frequency components, especially to those low frequency components which are close to the 0 cpd component in the frequency domain.  As the component at 0 cpd is involved in the very nature of any visual stimulus and is not deliberately intended as a stimulus,  its desensitizing effects is called as implicit masking (Makous, 1997). Based on this thought, the luminance effects are also explained by frequency masking. With this thought it is easy to describe the change of contrast sensitivity functions with the luminance level (Yang, Qi, and Makous, 1995).  Implicit masking provides a coherent framework for developing visual performance simulators to model and compute luminance-dependent detection and perception.

Makous, W. L. (1997).  Fourier models and the loci of adaptation.  J. Opt. Soc. Am. A, 14,  2323-2345.
Van Nes, F. L. and Bouman,  M.A. (1967).  Spatial modulation transfer in the human eye.  J. Opt. Soc. A, 57,  401-406.
Yang, J. & Makous, W. (1994). Spatiotemporal separability in contrast sensitivity. Vision Research, 34, 2569-2576.
Yang, J.,  Qi,  X. & Makous, W. (1995). Zero frequency masking and a model of contrast sensitivity.  Vis. Res., 35, 1965-1978.