Measurement of metamorphopsia in the presence of ocular media opacities

Variation in vernier acuity with age

Many visual functions deteriorate with increasing age, vernier acuity apparently being an exception. We used an improved experimental protocol to investigate vernier thresholds in subjects in four age groups. Vernier threshold was found to increase with increasing age, and the difference in this, compared with previous findings is probably due to the smaller inter-pixel angle used. Vernier acuities in young subjects in other studies seem to have been underestimated. As vernier acuity did not vary with retinal illuminance and the sensitivity loss does not seem to be a learning effect, neural rather than optical changes are likely to account for the deterioration. Measurement of vernier acuity may have clinical application in the determination of visual function behind cataract, and it will be necessary to consider the effect of age on vernier acuity.

Fixation disparity and nonius bias

Fixation disparity, i.e. the vergence error within Panum's area, can be measured psychophysically with two nonius (vernier) lines that are presented dichoptically, i.e. one to each eye. The observer adjusts these nonius lines to subjective alignment; the resulting physical nonius offset indicates the amount of fixation disparity. The present experiments investigate the relation between fixation disparity and the nonius bias, which is the physical offset of the nonius lines that is adjusted by the observer in order to perceive them as aligned when both nonius lines are presented to both eyes (binocular nonius bias) or both to the left or both to the right eye (monocular nonius bias). It was found that (1) the fixation disparity is correlated with the binocular nonius bias in the horizontal and vertical meridian and (2) the binocular nonius bias can be predicted from the average of the right eye and left eye monocular nonius bias. To remove the influence of the nonius bias on measured fixation disparity it is possible to calculate the fixation disparity relative to the individual binocular nonius bias, rather than to the physical coincidence of the nonius lines. This procedure tends to increase the correlation between fixation disparity and the tonic resting position of vergence. We discuss the clinical relevance of the dichoptic nonius method for measuring fixation disparity and its limitations as compared to physical recordings of eye position.