Stereo-anomaly is found more frequently in tasks that require discrimination between depths

Within the population of humans with otherwise normal vision, there exists some proportion whose ability to perceive depth from binocular disparity is poor or absent. The prevalence of this "stereo-anomaly" has been investigated in previous studies, some finding the proportion to be as small as 2%, others finding it to be as great as 30%. In this study, we set out to investigate the possible reason for the wide range of results found in these studies. We used a digital stereoacuity measurement tool that could measure performance in tasks requiring either the detection of disparity or the discrimination of the sign of disparity. The stimulus design was otherwise similar between the two tasks. In a cohort of 228 participants, we found that 98% were able to consistently perform the detection task. In contrast, only 69% consistently performed the discrimination task. The 31% of participants who had difficulty with the discrimination task could further be divided into 17% who were consistently unable to perform the task (seeming to behave at chance), and 14% who showed some ability to perform the task. We propose that the greater prevalence of stereo-anomaly is revealed when tasks require the judgement of the direction of disparity. ### Competing Interest Statement Alex S. Baldwin (ASB) and Robert F. Hess (RFH) are both inventors on patent(s) and other intellectual property (including Hess and Baldwin, 2020) concerning stereovision and the measurement and treatment of disorders of binocular vision such as amblyopia. Some of these technologies have been commercially licensed by McGill University to Novartis International AG. The inventors have (separate from the work described here) worked to develop these technologies for clinical use through a research agreement involving Novartis and the Research Institute of the McGill University Health Centre.