S H Cheung, G E Legge
University of Minnesota, Department of Psychology, Minneapolis, United States
Purpose. Most visual-field tests assume that the patient fixates accurately with the fovea. Since this assumption is often false for people with central-field loss (CFL), there is a need for a simple test to estimate the field location and accuracy of non-foveal fixation. We are developing a new field test, that finds and uses the physiological blind spot (PBS) as a reference, rather than the fovea. Preliminary results on normal subjects have been used to estimate accuracy and reliability.
Methods. The test presumes a well preserved symmetrical PBS surrounded by visually responsive retina. We use a letter-shadowing task simultaneous with the test probe presentation to optimize fixation stability. The probe is a dark circle (0.5° diameter) presented on a uniform gray field for 100 ms. A search algorithm "finds" the PBS and makes on-line adjustments to the estimates for the parameters of its shape and the fixation location.
Results. Preliminary results with three normal subjects showed that averaged errors in the estimation of the horizontal and vertical positions of fixation were -0.89° ± 1.20° and 0.35° ± 0.93° respectively. Averaged within-subject test-retest variations of the horizontal and vertical estimations were 0.12° ± 0.08° and 1.69° ± 1.22° respectively.
Conclusions. The small estimation errors and within-subject variations confirmed the accuracy and reliability of estimation of fixation. The results encourage us to extend this method to the estimation of the retinal location of fixation in those CFL patients with well-established preferred retinal loci (PRLs).
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