Equivalent intrinsic blur in amblyopia.

We used Gaussian blurred stimuli to explore the effect of blur on three tasks: (i) 2-line resolution; (ii) line detection; and (iii) spatial interval discrimination, in observers with amblyopia due to anisometropia, strabismus, or both. The results of our experiments can be summarized as follows. (i) 2-Line resolution: in normal foveal vision, thresholds for unblurred stimuli are approx. 0.5 min arc in the fovea. When the standard deviation (sigma) of the stimulus blur is less than 0.5 min, it has little effect upon 2-line resolution; however, thresholds are degraded when the stimulus blur, sigma, exceeds 0.5 min. We operationally define this transition point, as the equivalent intrinsic blur, or Bi. When the stimulus blur, sigma, is greater than Bi, then the resolution threshold is approximately equal to sigma. In all of the amblyopic eyes, 2-line resolution thresholds for unblurred stimuli were elevated, and the equivalent intrinsic blur was much larger. When the stimulus blur exceeds the equivalent intrinsic blur, resolution thresholds were similar in amblyopic and nonamblyopic eyes. (ii) Line detection: in both normal and amblyopic eyes, when the stimulus blur, sigma, is less than Bi, then the line detection threshold is approximately inversely proportional to sigma; i.e. (it obeys Ricco's law). When sigma is greater than Bi, the equivalent intrinsic blur, then the detection threshold is approximately a fixed contrast. All of the amblyopic eyes showed markedly elevated thresholds for detecting thin lines, but normal or near normal thresholds for detecting very blurred lines. Consequently, Ricco's diameter is larger in amblyopic than in normal eyes. (iii) Spatial interval discrimination: thresholds are proportional to the separation of the lines (i.e. Weber's law). At the optimal separation, spatial interval discrimination thresholds represent a "hyperacuity" (i.e. they are smaller than the resolution threshold). For unblurred lines, the optimal separation is approx. 2-3 times Bi. In the normal fovea, and in the amblyopic eyes of anisometropic amblyopes the optimal spatial interval discrimination threshold is about one-fifth of the resolution threshold (i.e. a hyperacuity); and over a wide range of separations, spatial interval discrimination thresholds begin to rise when the stimulus blur exceeds about one-third of the separation between the lines as long as the contrast is sufficiently high. In contrast, in strabismic amblyopes, like the normal periphery, the optimal spatial interval discrimination thresholds are worse (higher) than would be expected based upon the resolution limit of the strabismic amblyopic eye.(ABSTRACT TRUNCATED AT 400 WORDS)