Bottleneck effect from first-order to second-order contrast sensitivity function in amblyopia

The aim of the study was to determine whether amblyopes have reduced second-order spatial information and, if present, whether the loss is explained by loss in first-order spatial contrast. Forty-four amblyopic observers (16 strabismic; 18 anisometropic; 10 mixed types) with a mean age of 15.5 years (SD = 8.7) and 33 health controls (mean = 14.4 years; SD = 5.8) were evaluated. First-order contrast sensitivity was measured for 0.4, 1.6, 3.2, 6.4, and 12.8 cpd luminance sinusoidal grating. For second-order contrast sensitivity, we used 0.4, 1.6, 3.2, and 6.4 cpd luminance sinusoidal grating contrast modulated a pink-noise (1/f) carrier. In a 2-AFC presentation, a psychophysical staircase procedure was used to measure the contrast sensitivity for each spatial frequency. Information transmission (bits) was calculated between the first- and second-order contrast sensitivity in each spatial frequency for the fellow and amblyope eyes. Comparisons between groups were performed with ANOVA and significant differences were specified with the Fisher LSD test. Dominant/Fellow Eye showed that the strabismic group presented higher contrast sensitivity than controls and other amblyopic groups for low frequencies 0.4 cpd (F = 3.37; p = 0.021). Controls have greater contrast sensitivity than the strabismic and mixed groups for medium frequencies, 1.6 cpd (F = 2.16, p = 0.042) and 3.2 cpd (F = 2.74, p = 0.047) and for the high 6.4 cpd (F = 16.30, p < 0.001) and 12.8 cpd (F = 57.81, p < 0.001), controls differ from the other groups. For the Non-Dominant/Amblyopic Eye, controls presented higher contrast sensitivity than those of the strabismic and anisometropic amblyopia groups for low frequencies 0.4 cpd (F = 3.45; p = 0.004), medium 1.6 cpd (F = 3.37, p = 0.022) and 3.2 cpd (F = 3.84, p = 0.012) control greater contrast sensitivity than all other groups; for high frequencies 6.4 cpd (F = 3.65, p = 0.015) and 12.8 cpd (F = 12.93, p < 0.001), controls differing from all other groups (Effect size η 2 = 0.642). Significant reduction of transmission contrast information occurred for fellow eyes and amblyopic eyes. Reductions in first- and second-order contrast sensitivity occurred for both eyes when controls were compared with amblyopies. Different types of amblyopia show different profiles of reductions in contrast sensitivity. Also, reductions in information transmission from sensorial to low perceptual level are more significant in amblyopes than in controls, showing a bottleneck between early and posterior visual areas.