Figure 2.

A. Effects of noise and amblyopia. Threshold contrast for letter identification as a function of noise level (power spectral density). The open gray circle on the horizontal axis indicates the equivalent input noise level of a normal observer. The solid gray line represents normal threshold. Each curve represents linear dependence of squared threshold contrast on noise level, c² ∝ N + N_eq, which produces a bent line in the log-log coordinates of Figure 2. When the noise level is less than (to the left of) the equivalent noise, it has little effect on threshold; when the noise exceeds the equivalent noise, threshold contrast increases in proportion to the square root of noise level. The dashed line has reduced efficiency, and the dotted line has increased equivalent noise (solid circle on the horizontal axis). B and C. Letter threshold contrast as a function of noise level for a strabismic (B) and an anisometropic (C) amblyope. Open symbols are the nonamblyopic eyes; solid symbols are the amblyopic eyes. Both amblyopes show a marked loss of efficiency (vertical shift), accompanied by little (C) or no (B) increase in equivalent noise (diagonal shift). Letter size 0.5 deg.