Zanker J M
Max-Planck-Institut für Biologische Kybernetik, Tübingen, Germany.
Perception. 1995;24(4):363-72. doi: 10.1068/p240363.
The subjective strength of a percept often depends on the stimulus intensity in a nonlinear way. Such coding is often reflected by the observation that the just-noticeable difference between two stimulus intensities (JND) is proportional to the absolute stimulus intensity. This behaviour, which is usually referred to as Weber's Law, can be interpreted as a compressive nonlinearity extending the operating range of a sensory system. When the noise superimposed on a motion stimulus is increased along a logarithmic scale (in order to provide linear steps in subjective difference) in motion-coherency measurements, observers often report that the subjective differences between the various noise levels increase together with the absolute level. This observation could indicate a deviation from Weber's Law for variation of motion strength as obtained by changing the signal-to-noise ratio in random-dot kinematograms. Thus JNDs were measured for the superposition of uncorrelated random-dot patterns on static random-dot patterns and three types of motion stimuli realised as random-dot kinematograms, namely large-field and object 'Fourier' motion (all or a group of dots move coherently), 'drift-balanced' motion (a travelling region of static dots), and paradoxical 'theta' motion (the dots on the surface of an object move in opposite direction to the object itself). For all classes of stimuli, the JNDs when expressed as differences in signal-to-noise ratio turned out to increase with the signal-to-noise ratio, whereas the JNDs given as percentage of superimposed noise appear to be similar for all tested noise levels. Thus motion perception is in accordance with Weber's Law when the signal-to-noise ratio is regarded as stimulus intensity, which in turn appears to be coded in a nonlinear fashion. In general the Weber fractions are very large, indicating a poor differential sensitivity in signal-to-noise measurements.
感知的主观强度通常以非线性方式依赖于刺激强度。这种编码方式常常体现在这样的观察结果中：两个刺激强度之间的刚可察觉差异（JND）与绝对刺激强度成正比。这种行为通常被称为韦伯定律，它可以被解释为一种压缩性非线性，扩展了感觉系统的工作范围。在运动连贯性测量中，当叠加在运动刺激上的噪声沿着对数尺度增加时（以便在主观差异上提供线性步长），观察者通常会报告说，各种噪声水平之间的主观差异会随着绝对水平的增加而增加。这一观察结果可能表明，在通过改变随机点运动图中的信噪比来获得运动强度变化时，与韦伯定律存在偏差。因此，针对不相关随机点图案叠加在静态随机点图案上以及三种作为随机点运动图实现的运动刺激（即大场和物体“傅里叶”运动，所有或一组点连贯移动；“漂移平衡”运动，一个静态点的移动区域；以及悖论性“θ”运动，物体表面的点与物体本身的移动方向相反），测量了JND。对于所有类型的刺激，当以信噪比差异表示时，JND结果随信噪比增加，而以叠加噪声百分比给出的JND在所有测试噪声水平下似乎相似。因此，当将信噪比视为刺激强度时，运动感知符合韦伯定律，而刺激强度反过来似乎是以非线性方式编码的。一般来说，韦伯分数非常大，表明在信噪比测量中差异敏感性较差。
