Gray R, Regan D
Department of Psychology, York University, BSB Room 375, 4700 Keele Street, North York, ON M3J 1P3, Canada.
Vision Res. 1998 Sep;38(17):2601-17. doi: 10.1016/s0042-6989(97)00461-6.
We describe evidence consistent with the proposal that the visual system contains a parallel array of size-tuned mechanisms sensitive to orientation texture-defined (OTD) form, and propose that the relative activity of these mechanisms determines spatial frequency discrimination threshold for OTD gratings. Using a pattern of short lines we measured spatial frequency discrimination thresholds for OTD gratings and luminance-defined (LD) gratings. For OTD gratings, the orientation of texture lines varied sinusoidally across the bars of the gratings, but line luminance was constant. For LD gratings, line orientation was constant, but line luminance varied sinusoidally across the bars of the grating. When the number of texture lines (i.e. spatial samples) per grating cycle was below about six, spatial sampling strongly affected both the spatial frequency discrimination and grating detection thresholds for OTD and LD gratings. However, when the number of spatial samples per grating cycle exceeded about six, plots of both discrimination threshold and detection threshold were different for OTD and LD gratings. For an OTD grating of any given spatial frequency, spatial frequency discrimination threshold fell as the number of samples per grating cycle was increased while holding texture line length constant: the lower limit was reached at six to ten samples per cycle. When we progressively increased the viewing distance (keeping the cycles per degree (cpd) constant), spatial frequency discrimination threshold reached a lower limit and increased thereafter. We propose that this minimum threshold represents a balance between opposing effects of the number of samples per grating cycle and the length of texture lines, and approaches the absolute physiological lower limit for OTD gratings. Spatial frequency discrimination was possible up to at least 7 cpd. Grating acuity for an OTD grating was considerably lower than the physiological limit for LD gratings, presumably because detectors of OTD form include a spatial integration stage following the processing of individual lines. For an LD grating, discrimination threshold fell as the number of samples per grating cycle was increased and asymptoted at six to ten samples per cycle. Spatial frequency discrimination thresholds for OTD and LD gratings were similar at low spatial frequencies (up to 3-4 cpd), but increased more steeply for OTD gratings at high spatial frequencies. For both OTD and LD gratings, discrimination threshold fell steeply as the number of grating cycles was increased from 0.5 to ca. 2.5 cycles, and thereafter decreased more slowly or not at all suggesting that, for both OTD and LD gratings, spatial frequency discrimination can be regarded as a special case of line interval or bar width discrimination. As orientation contrast was progressively increased, discrimination threshold for an OTD grating fell steeply up to about four to five times grating detection threshold, then saturated. This parallels the effect of luminance contrast on discrimination threshold for an LD grating.
视觉系统包含一系列对由方向纹理定义（OTD）的形状敏感的大小调谐机制并行阵列，并提出这些机制的相对活动决定了OTD光栅的空间频率辨别阈值。我们使用短线图案测量了OTD光栅和亮度定义（LD）光栅的空间频率辨别阈值。对于OTD光栅，纹理线的方向在光栅条上呈正弦变化，但线亮度恒定。对于LD光栅，线方向恒定，但线亮度在光栅条上呈正弦变化。当每个光栅周期的纹理线数量（即空间样本）低于约六条时，空间采样强烈影响OTD和LD光栅的空间频率辨别和光栅检测阈值。然而，当每个光栅周期的空间样本数量超过约六条时，OTD和LD光栅的辨别阈值和检测阈值图均有所不同。对于任何给定空间频率的OTD光栅，在保持纹理线长度不变的情况下，随着每个光栅周期样本数量的增加，空间频率辨别阈值下降：每个周期六到十个样本时达到下限。当我们逐渐增加观察距离（保持每度周期数（cpd）不变）时，空间频率辨别阈值达到下限，此后增加。我们提出，这个最小阈值代表了每个光栅周期样本数量和纹理线长度的相反效应之间的平衡，并接近OTD光栅的绝对生理下限。空间频率辨别至少可达7 cpd。OTD光栅的光栅敏锐度远低于LD光栅的生理极限，大概是因为OTD形状的探测器在处理单个线条之后包括一个空间整合阶段。对于LD光栅，随着每个光栅周期样本数量的增加，辨别阈值下降，并在每个周期六到十个样本时渐近。OTD和LD光栅在低空间频率（高达3 - 4 cpd）时的空间频率辨别阈值相似，但在高空间频率时OTD光栅的增加更为陡峭。对于OTD和LD光栅，当光栅周期数从0.5增加到约2.5个周期时，辨别阈值急剧下降，此后下降更慢或根本不再下降，这表明对于OTD和LD光栅，空间频率辨别都可被视为线间隔或条宽度辨别的一种特殊情况。随着方向对比度逐渐增加，OTD光栅的辨别阈值急剧下降，直至约为光栅检测阈值的四到五倍，然后饱和。这与亮度对比度对LD光栅辨别阈值的影响相似。
