Previous experiments using highpass nose have either suggested that humans cannot compensate for anti-correlated noise in images or were inconclusive. These results may have been misleading because of the use of a single noise component. For large exponents of fn, image noise within the bandwidth of the signal amplitude for detection. This situation does not correspond to CT or SPECT imaging cases where patient structure with a lowpass spectrum is also present and limits detection accuracy. In addition, humans have two forms of internal noise that limit detection and this may have been the source of poor human performance. So, in this work, experiments were done with two noise components - one broadband to ensure that task performance was always limited by external noise. The experiments were designed to be more precise test of compensation for anti- correlated noise and to provide a more sensitive test of existing observer models. In all cases, separate experiments were done to estimate observer internal noise. The new results show a marked asymmetry between lowpass and highpass noise effects and are consistent withthe view that internal noise is the cause of poor highpass noise performance.