Levi D M, Klein S A, Wang H
College of Optometry, University of Houston, TX 77204-6052.
Vision Res. 1994 Dec;34(24):3293-313. doi: 10.1016/0042-6989(94)90066-3.
Many computational models of normal vernier acuity make predictions based on the just-noticeable contrast difference. Recently, Hu, Klein and Carney [(1993) Vision Research, 33, 1241-1258] compared vernier acuity and contrast discrimination (jnd) in normal foveal viewing using cosine gratings. In the jnd stimulus the test grating was added in-phase to the (sinusoidal) pedestal, whereas in the vernier stimulus the same test grating was added with an approx. 90 deg phase shift to the pedestal. In the present experiments, we measured thresholds for discriminating changes in relative position and changes in relative contrast for abutting, horizontal cosine gratings in a group of amblyopes using the Hu et al., test-pedestal approach. The approach here is to ask whether the reduced vernier acuity of amblyopes can be understood on the basis of reduced contrast sensitivity or contrast discrimination. Our results show that (i) abutting cosine vernier acuity is strongly dependent on stimulus contrast. (ii) In both anisometropic and strabismic amblyopes, abutting cosine vernier discrimination thresholds are elevated at all contrast levels, even after accounting for reduced target visibility, or contrast discrimination. (iii) For both strabismic and anisometropic amblyopes, the vernier Weber fraction is markedly degraded, while the contrast Weber fraction is normal or nearly so. (iv) In anisometropic amblyopes the elevated vernier thresholds are consistent with the observers' reduced cutoff spatial frequency, i.e. the loss can be accounted for on the basis of a shift in spatial scale. (v) In strabismic amblyopes and in the normal periphery, there appears to be an extra loss, which can not be accounted for by either reduced contrast sensitivity and contrast discrimination or by a shift in spatial scale. (vi) This extra loss cannot be quantitatively mimicked by "undersampling" the stimulus. (vii) Surprisingly, in some strabismics, and in the periphery, at relatively high spatial frequencies, vernier thresholds appear to lose their contrast dependence, suggesting the possibility that there may be qualitative differences between the normal fovea and these degraded visual systems. (viii) This contrast saturation can be mimicked by "undersampling" the target, or by introducing strips of mean luminance between the two vernier gratings, thus mimicking a "scotoma". Taken together with the preceding paper, our results suggest that the extra loss in position acuity of strabismic amblyopes and the normal periphery may be a consequence of noise at a second stage of processing, which selectively degrades position but not contrast discrimination.
许多正常游标视力的计算模型都是基于刚好可察觉的对比度差异来进行预测的。最近，胡、克莱因和卡尼[（1993年）《视觉研究》，33卷，1241 - 1258页]使用余弦光栅比较了正常中央凹注视时的游标视力和对比度辨别（jnd）。在jnd刺激中，测试光栅与（正弦）基座同相添加，而在游标刺激中，相同的测试光栅以大约90度的相移添加到基座上。在本实验中，我们使用胡等人的测试 - 基座方法，测量了一组弱视患者中相邻水平余弦光栅的相对位置变化和相对对比度变化的辨别阈值。这里的方法是询问弱视患者降低的游标视力是否可以基于对比度敏感度或对比度辨别力的降低来理解。我们的结果表明：（i）相邻余弦游标视力强烈依赖于刺激对比度。（ii）在屈光参差性和斜视性弱视患者中，即使在考虑了目标可见度降低或对比度辨别力降低之后，相邻余弦游标辨别阈值在所有对比度水平下都升高。（iii）对于斜视性和屈光参差性弱视患者，游标韦伯分数明显降低，而对比度韦伯分数正常或接近正常。（iv）在屈光参差性弱视患者中，升高的游标阈值与观察者降低的截止空间频率一致，即这种损失可以基于空间尺度的变化来解释。（v）在斜视性弱视患者和正常周边区域，似乎存在额外的损失，这不能通过对比度敏感度和对比度辨别力的降低或空间尺度的变化来解释。（vi）这种额外的损失不能通过对刺激进行“欠采样”来定量模拟。（vii）令人惊讶的是，在一些斜视患者以及周边区域，在相对较高的空间频率下，游标阈值似乎失去了对对比度的依赖性，这表明正常中央凹与这些退化视觉系统之间可能存在质的差异。（viii）这种对比度饱和可以通过对目标进行“欠采样”，或者在两个游标光栅之间引入平均亮度条带来模拟，从而模拟“暗点”。与前文一起，我们的结果表明，斜视性弱视患者和正常周边区域位置视力的额外损失可能是处理第二阶段噪声的结果，该噪声选择性地降低了位置辨别但不降低对比度辨别。
