Ledgeway T, Smith A T
Department of Psychology, University of London, Egham, Surrey, England.
Vision Res. 1994 Oct;34(20):2727-40. doi: 10.1016/0042-6989(94)90229-1.
Current theories of second-order motion perception postulate that such motion is detected by either a high-level mechanism which computes the temporal correspondences between "features" extracted from the image, or low-level motion mechanisms which operate on a nonlinear, neural transformation of the luminance profile of the image. Theories which favour the latter strategy either suggest that first- and second-order motion are detected by a common mechanism or else that distinct mechanisms exist for the two types of motion, both operating on similar principles. The aim of this study was to differentiate between these possibilities. Observers were required to judge the direction of multiframe motion sequences in which the frames alternated between sinusoidal variations in luminance (first order) and similar variations in contrast (second order). On each frame the modulation signal was displaced by some fraction of its spatial period. The motion sequences were designed such that integration of both types of frame (first and second order) would lead to unambiguous motion in a particular direction whilst separate analysis of first- or second-order frames alone would yield ambiguous motion. The results show clearly that observers were unable to integrate the first- and second-order frames of such motion sequences. However, when observers were presented with motion sequences in which the frames alternated between two, different types of second-order image (variations in the contrast or size of the elements constituting a random noise field) perceived direction was always consistent with integration of both image types. This is taken as support for models that suggest that first- and second-order motion are processed by distinct mechanisms in the visual system and that each mechanism is only sensitive to one type of motion. It is suggested that several varieties of second-order motion stimuli may be regarded as equivalent to contrast-modulated images when considered in terms of the effects of local spatiotemporal filtering operations carried out by the human visual system. In this respect, our results are consistent with the "texture grabber" concept of Werkhoven, Sperling and Chubb [(1993) Vision Research, 33, 463-485].
当前关于二阶运动感知的理论假定，此类运动是通过一种高级机制检测到的，该机制计算从图像中提取的“特征”之间的时间对应关系，或者是通过低级运动机制检测到的，这些机制对图像亮度轮廓进行非线性神经变换。支持后一种策略的理论要么认为一阶和二阶运动由共同机制检测，要么认为这两种运动存在不同机制，但两者运作原理相似。本研究的目的是区分这些可能性。要求观察者判断多帧运动序列的方向，其中各帧在亮度的正弦变化（一阶）和对比度的类似变化（二阶）之间交替。在每一帧上，调制信号按其空间周期的一定比例移位。设计这些运动序列的目的是，两种类型的帧（一阶和二阶）的整合将导致在特定方向上的明确运动，而单独分析一阶或二阶帧则会产生模糊运动。结果清楚地表明，观察者无法整合此类运动序列的一阶和二阶帧。然而，当向观察者呈现各帧在两种不同类型的二阶图像（构成随机噪声场的元素的对比度或大小变化）之间交替的运动序列时，感知到的方向始终与两种图像类型的整合一致。这被视为支持如下模型：该模型认为一阶和二阶运动在视觉系统中由不同机制处理，且每种机制仅对一种运动类型敏感。有人提出，当从人类视觉系统执行的局部时空滤波操作的效果来看时，几种二阶运动刺激可能被视为等同于对比度调制图像。在这方面，我们的结果与韦尔霍芬、斯珀林和查布[（1993年）《视觉研究》，33卷，463 - 485页]的“纹理捕捉器”概念一致。
