Nonadditivity of masking by narrow-band noises

Characterization of the visual system as a linear system has many consequences. One property implied by such a characterization is that signal-to-noise ratio at threshold is constant, so that the contrast energy of a sinusoidal grating at threshold depends on the effective noise passed by the filter used to detect the target. When the contrast energy in an external noise is sufficiently high, the contribution of internal noise may be conveniently ignored. In such circumstances, the effectiveness of a given noise is measured by its ability to mask the target. One consequence of the linearity assumption is that if the energy of the effective noise is increased by a factor k, then threshold energy of the signal is increased by the same amount. The contrast energy at threshold in the presence of a masker created by adding two maskers should be the sum of the threshold energies in the individual maskers. We have tested this hypothesis for spectrally nonoverlapping maskers. We find that the contrast energy required to detect the target in the presence of the combined maskers is much greater than the sum of the threshold energies for the two maskers. This "excess masking" violates the linearity assumption. On the other hand, when maskers do have spectral overlap, no excess masking is found.