Levi D M, Waugh S J
University of Houston, College of Optometry, TX 77204-6052, USA.
Vision Res. 1996 Feb;36(4):573-88. doi: 10.1016/0042-6989(95)00128-x.
Vernier acuity for opposite-contrast polarity stimuli clearly poses problems for local contrast models of relative position processing. In Expt 1 we show that vernier thresholds for abutting, or closely separated features of opposite-contrast polarity, are degraded across a wide range of stimulus strengths and configurations; but for widely separated stimuli they are more or less independent of contrast polarity (confirming and extending previous work). In Expts 2 and 3 we use a one-dimensional spatial noise masking paradigm to investigate to what extent the same mechanisms masked by this noise contribute to the relative position processing of same and opposite polarity stimuli. The orientation tuning functions determined using this paradigm are quite different for same and opposite polarity targets, for both line vernier acuity, and closely spaced two-dot alignment. However, for widely separated targets (24 min arc or more), they are similar. Over a range of separations from 3 to 30 min arc, for same and opposite polarity dots, masking is strongest at a spatial frequency of about 10 c/deg. Our results are consistent with the notion that signals from early (and relatively high spatial frequency) linear filters are collected in a second-stage nonlinear mechanism, which collates information along an orientation trajectory. We suggest that different properties of the mechanisms at each level of processing, can constrain positional acuity at small and large separations.
对于相反对比度极性刺激的游标敏锐度显然给相对位置处理的局部对比度模型带来了问题。在实验1中，我们表明，对于相邻或对比度极性相反的紧密间隔特征，游标阈值在广泛的刺激强度和配置范围内都会降低；但对于间隔较大的刺激，它们或多或少与对比度极性无关（证实并扩展了先前的研究）。在实验2和3中，我们使用一维空间噪声掩蔽范式来研究被这种噪声掩蔽的相同机制在多大程度上有助于相同和相反极性刺激的相对位置处理。对于相同和相反极性目标，使用该范式确定的方向调谐函数对于线游标敏锐度和紧密间隔的两点对齐都有很大不同。然而，对于间隔较大的目标（24分视角或更大），它们是相似的。在从3到30分视角的一系列间隔范围内，对于相同和相反极性的点，掩蔽在大约10周/度的空间频率处最强。我们的结果与以下观点一致：来自早期（以及相对较高空间频率）线性滤波器的信号在第二阶段非线性机制中被收集，该机制沿着方向轨迹整理信息。我们认为，处理各层次机制的不同属性可以在小间隔和大间隔时限制位置敏锐度。
