M.Sc Thesis

M.Sc StudentLeibovitz Elad
SubjectSpatial Resolution within the Fovea Using a Host of Visual
Acuity Tests
DepartmentDepartment of Biomedical Engineering
Supervisor ASSOCIATE PROF. Moshe Gur
Full Thesis textFull thesis text - English Version


Visual acuity is a measure of the spatial resolution of the visual system which determines the smallest visible feature one can discern. The retina,in the back of the eye, is a mosaic of two basic types of photoreceptors: rods, and cones. The 6 to 7 million cones which provide the eye's color sensitivity, are much more concentrated in the "fovea centralis ", a 0.3 mm diameter rod-free area in the center of the retina.

 Not much is known about visual acuity within the fovea. The aim of this research is to measure the visual acuity at different eccentricities within the fovea up to 1° of eccentricity using two tests: spatial acuity (Landolt's C) and face recognition. In each test we used a single measure:  in Landolt's C - the gap size is the proportion of the missing part of the circle, in face recognition- the size of the face shown.

In each visual acuity test the subject was presented with a target having different gaps for 130ms at different eccentricities (0°, 0.25°,0.5°,0.75°, 1° and 1.5°, 2° as references). The subject had to choose between two options. Using an eye tracker, eye movements were measured and the epochs in which the subject's eye was fixating correctly were selected.

Using probit analysis we found in each eccentricity the performance threshold: the Landolt C gap or the face size in which the subject did well on 75% of the trials (the average between guessing (50%) and certainty (100%).

The results in both tests show that it is possible to see changes in visual acuity even within the fovea. Performance deteriotes as a function of distance form the center of the fovea. This trend is clearer in the face recognition test. Those results match the trend of the cones distribution but do not replicate it. We assume this is due to the fundemental limit imposed by the human eye's inability to stay absolutely focused on a small target.