We continue to evaluate fundamental factors that affect the ability of human observers in digital stereomammography. A 2-alternative forced choice (2-AFC) observer study for discrimination of simulated objects in the presence of x-ray quantum noise was performed. In our previous contrast-detail and 2-AFC studies investigating the detection of simulated lesions, we observed that at the same total dose observers perform similarly for stereoscopic and monoscopic imaging. The current experiments were designed to investigate discrimination tasks. Three or four observers attended a series of sessions, each consisting of 300-400 image pairs. We sequentially evaluated discrimination of images based on object shape and contrast. In each trial, two images were presented, each containing a small disk of known size and position, but which differ in terms of blurring or contrast to background. The observers indicated the image containing the disk with greater blurring or higher contrast. The experiments were repeated for 3 or 4 different values of signal-to-noise ratio (SNR), and for 3 different diameters. The fraction of correct responses was computed for each test condition. Detection performance was compared in terms of the linear fit of d’ as a function of SNR. Preliminary results again confirmed the advantage of stereoscopy. For the discrimination of blurred objects, the ratio of d’(SNR) averaged over all conditions took values in the range of 1.22-1.73 for the three observers (average 1.45), compared to the theoretically expected value of 1.41. No advantage was seen for discrimination of contrast (average 1.02). It appears that suppression of quantum noise in stereoscopically viewed simulated images by the human visual system enables advantages in discrimination of small lesions with different shape. It is possible, therefore, to match the dose of a stereo pair to the dose required for a single mammogram.