Benchmarking spatial discrimination thresholds of two-frame motion defined forms compared to luminance and stereoscopic defined forms

Two-frame motion defined form is a type of discontinuous motion contrast rendering objects visible through short distance displacement of a region within two otherwise identical sequentially presented random-dot patterns. The resulting illusory object is transiently perceived before fading. The purpose of this study was to benchmark visual spatial acuity for two-frame motion defined form compared to luminance and disparity defined forms. Random dot patterns of equal dot density were used for all contrast types to ensure equivalence of spatial sampling. Stimulus durations in the luminance and stereoscopic conditions were set at the subjective duration (∼270 ms) of the two-frame motion defined form measured in a control experiment. In the main experiment, 25 participants made forced choice judgements on the aspect ratios of the target defined by each of the contrasts using the method of constant stimuli. Participants indicated whether the target square was 'wider' or 'taller' as a function of the aspect ratio of the target while controlling for trial-to-trial variations in the distance between the vertical/horizontal edges of the target and the boundaries of the surrounding dots. Group mean just-noticeable differences (JNDs) for luminance and stereoscopic conditions translated to changes in aspect ratio of 4.8% and 4%, respectively. They were significantly smaller than the two-frame motion defined condition, 5.8%. Spatial acuity for two-frame motion defined forms is only slightly inferior to other contrast types when equated for spatial sampling and duration. This study is the first benchmark of visual acuity for two-frame motion defined form against luminance and disparity defined form.