Psychophysical measurement of the size and shape of the human foveal avascular zone.

We have examined two psychophysical procedures for assessing the size and shape of the human foveal avascular zone (FAZ). Both procedures used a Maxwellian view system with a rotating beam to create a high contrast entoptic view of the retinal vasculature. Most subjects readily report a clear avascular zone surrounding their fixation point. The size of this FAZ was measured by (a) reducing the circular Maxwellian view field stop until it appeared to coincide with the edge of the FAZ, and (b) by tracing the boundary of the FAZ with a point source. Consistent with earlier angiographic and anatomical studies, the first method (N = 34 eyes) showed a mean FAZ diameter of 0.736 mm (area 0.42 mm2). FAZ diameter ranged from 0.46 to 1.13 mm (area 0.166-1.00 mm2) and the FAZ from the right and left eyes of each individual subject were very similar (r2 = 0.863). The tracing technique (n = 24 eyes) showed that FAZ's were generally not spherical but well fit by ellipses with, on average, a major axis 17% longer than the minor axis. The maximum diameters of the FAZ tracings were very similar and significantly correlated with the circle diameters matched to the FAZ in experiment 1.