Monocular Ghost Image Offset Thresholds: Dependent on Target Size and Ghost Image Relative Brightness.

SIGNIFICANCE: This is the first report of monocular ghost image offset thresholds measured using O optotypes. Monocular diplopia is a complaint of patients and is the result of a variety of etiologies. Furthermore, monocular image doubling also can be a confounding variable of vectographic stereoacuity tests, warranting an investigation of ghost image offset thresholds.

PURPOSE: To measure ghost image offset thresholds of normal observers and how they are affected by offset orientation, target size, and ghost image relative luminance.

METHODS: Participants were five individuals without ocular abnormalities aged 21 to 32 years. Stimuli were viewed monocularly and consisted of Sloan "O" optotypes generated on a computer monitor with varying levels of image doubling. Ghost image offset thresholds were determined using a spatial 2-alternative forced-choice paradigm and probit analysis of the frequency of seeing data.

RESULTS: Under close-to-optimal conditions, monocular ghost image offset thresholds ranged between 14 and 22 arc seconds, a level that might be considered a hyperacuity. Ghost image offset detection thresholds demonstrated a U-shaped relationship with optotype size, with optimum thresholds occurring for optotypes sizes of approximately -0.15 logarithmic minimum angle of resolution. There was no measurable effect of offset orientation on ghost image offset detection thresholds. Monocular ghost image offset detection thresholds decreased as ghost image relative luminance increased.

CONCLUSIONS: Ghost image offset detection thresholds can be quite low even when viewing under monocular conditions and relatively low ghost image relative luminance. This should be considered when designing and interpreting the results of vectographic stereoacuity tests.