Eagle R A
Department of Experimental Psychology, University of Oxford, UK.
Vision Res. 1998 Jun;38(12):1775-87. doi: 10.1016/s0042-6989(97)00378-7.
How is the spatial-frequency content of a moving broadband pattern analysed by the visual system? Observers were asked to discriminate the direction of motion in random-noise patterns containing equal energy in each two-dimensional octave band. Uncorrelated noise could be introduced into either low- or high-frequency bands in order to force the visual system to rely on the outputs of putative mechanisms tuned to a narrow frequency range of the stimulus. In two experiments the dependent measure was the magnitude of dmax, the largest discrete displacement whose direction could be discriminated reliably. It was found that dmax was unaffected by the presence of high-frequency noise reaching down to 0.67 c/deg, but that the task became impossible thereafter. In the case of low-frequency noise, dmax fell as the noise was moved up towards about 2 c/deg, at which point the task became impossible at any displacement. This pattern of results would be expected if the system were using information from the lowest signal frequencies in all conditions. In experiment 2, dmax was measured for stimuli in which the spectral position and quantity of high-frequency noise were manipulated. It was found that only noise spectrally-adjacent to the signal band has a detrimental effect on dmax. Three different single-filter models of motion detection each failed to provide a satisfactory account of the spatial-frequency range of good direction discrimination performance. Rather, the modelling shows that the visual system can access the outputs of a low-frequency channel when the noise is high and a high-frequency channel when the noise is low.
视觉系统是如何分析移动宽带模式的空间频率内容的？研究人员要求观察者辨别随机噪声模式中的运动方向，这些模式在每个二维倍频程带中具有相等的能量。不相关噪声可以被引入低频或高频带，以迫使视觉系统依赖于调谐到刺激的窄频率范围的假定机制的输出。在两个实验中，因变量是dmax的大小，即其方向能够被可靠辨别的最大离散位移。研究发现，高达0.67 c/deg的高频噪声的存在不会影响dmax，但此后任务变得不可能完成。在低频噪声的情况下，随着噪声向约2 c/deg移动，dmax下降，此时在任何位移下任务都变得不可能完成。如果系统在所有条件下都使用最低信号频率的信息，那么这种结果模式是可以预期的。在实验2中，对其中高频噪声的频谱位置和数量被操纵的刺激测量了dmax。研究发现，只有频谱上与信号带相邻的噪声对dmax有不利影响。三种不同的运动检测单滤波器模型都未能对良好方向辨别性能的空间频率范围提供令人满意的解释。相反，建模表明，当噪声较高时，视觉系统可以访问低频通道的输出，而当噪声较低时，可以访问高频通道的输出。
