Individual objective versus subjective fixation disparity as a function of forced vergence

Area of Science:

• Ophthalmology
• Neuroscience
• Visual Science

Background:

• Fixation disparity, an inaccuracy in vergence eye position, can persist even with a fusion stimulus.
• Objective fixation disparity is measured using eye trackers, while subjective fixation disparity is assessed via psycho-physical procedures.
• The relationship between objective and subjective fixation disparity measurements remains a subject of debate.

Purpose of the Study:

• To investigate the relationship between objective and subjective fixation disparity measurements.
• To determine if subjective fixation disparity can accurately predict objective fixation disparity.
• To analyze individual differences in the response to forced vergence.

Main Methods:

• Simultaneous measurements of objective and subjective fixation disparity were conducted in 12 observers.
• Prisms were used to induce forced vergence, varying absolute disparity from 1 degree divergent to 3.4 degrees convergent.
• Frequent, repeated measurements were performed to allow for individual analyses.

Main Results:

• Fixation disparity values and prism effects were consistently smaller in subjective than in objective measures.
• Individual regressions revealed an average subjective-to-objective slope of 8%, with some observers showing up to 18%.
• Systematic differences in prism-induced curve characteristics were observed between subjective and objective measures in some individuals.

Conclusions:

• Sensory fusion appears to shift visual direction by the full amount of objective fixation disparity, enabling single vision.
• The dichoptic nonius targets, however, indicate only a partial percentage of objective fixation disparity due to incomplete shifts in central monocular lines.
• The subjective-to-objective ratio is an individual characteristic, suggesting subjective measures cannot precisely predict objective fixation disparity on a group level.