The additivity of wavelet subband quantization distortions was investigated in an unmasked detection task and in masked detection and discrimination tasks. Contrast thresholds were measured for both simple targets (artifacts induced by uniform quantization of individual discrete wavelet transform subbands) and compound targets (artifacts induced by uniform quantization of pairs of discrete wavelet transform subbands) in the presence of no mask and eight different natural image maskers. The results were used to assess summation between wavelet subband quantization distortions on orientation and spatial-frequency dimensions. In the unmasked detection experiment, subthreshold quantization distortions pooled in a non-linear fashion and the amount of summation agreed with those of previous summation-at-threshold experiments ((beta) equals 2.43; relative sensitivity equals 1.33). In the masked detection and discrimination experiments, suprathreshold quantization distortions pooled in a linear fashion. Summation increased as the distortions became increasingly suprathreshold but quickly settled to near-linear values. Summation on the spatial-frequency dimension was greater than summation on the orientation dimension for all suprathreshold contrasts. A high degree of uncertainty imposed by the natural image maskers precludes quantifying an absolute measure of summation.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.