Fixation disparity and nonius bias

Fixation disparity, i.e. the vergence error within Panum's area, can be mea sured psychophysically with two nonius (vernier) lines that are presented d ichoptically, i.e. one to each eye. The observer adjusts these nonius lines to subjective alignment; the resulting physical nonius offset indicates th e amount of fixation disparity. The present experiments investigate the rel ation 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 pe rceive them as aligned when both nonius lines are presented to both eyes (b inocular nonius bias) or both to the left or both to the right eye (monocul ar 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 rig ht eye and left eye monocular nonius bias. To remove the influence of the n onius bias on measured fixation disparity it is possible to calculate the f ixation 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 restin g position of vergence. We discuss the clinical relevance of the dichoptic nonius method for measuring fixation disparity and its limitations as compa red to physical recordings of eye position. (C) 1998 Elsevier Science Ltd. All rights reserved.