Párraga C A, Tolhurst D J
Department of Physiology, University of Cambridge, Downing Street, Cambridge CB2 3EG, UK.
Perception. 2000;29(9):1101-16. doi: 10.1068/p2904.
It has been suggested (Tadmor and Tolhurst, 1994 Vision Research 34 541-554) that the psychophysical task of discriminating changes in the slope of the amplitude spectrum of a complex image may be similar to detecting differences in the degree of blur. It has also been suggested that human observers may perform this discrimination by detecting changes in the effective contrast within single narrow spatial-frequency bands, rather than by detecting changes in the slope per se which would involve the use of contrast information across many different frequency bands. To distinguish between these two possibilities, we have developed an experiment where observers were asked to discriminate changes in the spectral slope while different amounts of random contrast variation were introduced, with the purpose of disrupting their performance. This disruptive effect was designed to be particularly manifest if the observer really was performing a single-frequency-band contrast discrimination but to be unnoticeable if the observer was discriminating the change of slope per se. Our results imply that the observers do not usually detect changes in contrast in just one narrow spatial-frequency band when they discriminate changes in the slope of the amplitude spectrum. Rather, they must compare contrast between bands or, at least, they use contrast information from more than one band. However, for edge-enhanced (whitened) pictures, there is some evidence to suggest that observers rely on contrast changes in only a limited low-spatial-frequency band.
有人提出（塔德莫尔和托尔赫斯特，1994年《视觉研究》34卷541 - 554页），辨别复杂图像幅度谱斜率变化的心理物理学任务可能类似于检测模糊程度的差异。也有人提出，人类观察者可能通过检测单个窄空间频率带内有效对比度的变化来进行这种辨别，而不是通过检测斜率本身的变化，因为检测斜率本身的变化需要使用跨越许多不同频带的对比度信息。为了区分这两种可能性，我们设计了一个实验，要求观察者在引入不同量的随机对比度变化时辨别频谱斜率的变化，目的是干扰他们的表现。如果观察者真的是在进行单频带对比度辨别，这种干扰效应会特别明显；而如果观察者是在辨别斜率本身的变化，这种干扰效应则不会被注意到。我们的结果表明，观察者在辨别幅度谱斜率变化时，通常不会只在一个窄空间频率带内检测对比度的变化。相反，他们必须比较不同频带之间的对比度，或者至少，他们使用来自多个频带的对比度信息。然而，对于边缘增强（白化）的图片，有一些证据表明观察者仅依赖于有限的低空间频率带内的对比度变化。
