Hyperacuity thresholds and gap functions using an adaptive staircase method

Abstract

The human visual system has the ability to perceive differences in the relative positions of spatially localized objects that are smaller than the size or spacing of foveal cones under ideal conditions. It is referred to as “hyperacuity”. Vernier acuity, a form of hyperacuity, is sensitive to spatial alignment of lines or dots and observers can judge misalignments of about 2 arc seconds. However, thresholds depend upon the psychophysical experimentation method as well as stimuli parameters such as the size, shape, color and contrast etc. We are working to standardize the test for clinical use by developing an adaptive staircase psychophysical procedure which involves a response-oriented positioning of the stimulus in a 3-Down and 1-Up design. Responses were recorded using a 3-alternative forced choice technique for 7 different gap sizes (vertical separation) of the stimulus features ranging from 128 to 2 arc minutes with a test time of about 15-20 minutes. The standard deviations of the reported aligned responses were defined as a threshold in arc seconds. We performed this pilot study on five normal, healthy subjects for method validation. Thresholds were measured for different stimuli parametric conditions. The gap size versus thresholds functions were plotted. The mean difference between the high and reverse contrast was statistically significant only for a gap size of 32 arc minutes. This pilot study aimed at developing and validating an adaptive staircase technique using a user-friendly software for clinical use in screening diseases such as glaucoma in developing nations.