Welch L
Smith-Kettlewell Eye Research Institute, San Francisco, California 94115.
Nature. 1989 Feb 23;337(6209):734-6. doi: 10.1038/337734a0.
When viewed through a small aperture, the perceived motion exhibited by a long moving line or grating is ambiguous. This situation prevails because even a perfect machine could only detect motion perpendicular to a moving contour, so motion parallel to a contour is undetectable. The human visual system views the world through an aperture array--the neural receptive fields. Therefore a moving object is viewed through many small apertures and the motion within many of those apertures is ambiguous. This ambiguity may be resolved by monitoring the motion of a distinctive feature, such as a line-end or corner, and attributing to the larger object the motion of the feature. Alternatively, Adelson and Movshon have suggested that moving images are processed in two stages, that is, they are first decomposed into one-dimensional components which are later recombined to generate perceived object motion. For a moving plaid, defined as the sum of two drifting gratings, these alternative models generate different predictions concerning the resolution of the plaid's motion ambiguity. A feature monitor would respond to the motion of the intersections between gratings, whereas the two-stage motion processor would first decompose the plaid into its constituent gratings and subsequently recombine them to generate the perception of a moving plaid. Using speed discrimination to distinguish between the two models, I find that discrimination thresholds reflect the speed of a plaid's component gratings, rather than the speed of the plaid itself. This result supports the two-stage model. Although speed discrimination is limited by component processing, observers cannot directly access component speed. The only perceptually accessible velocity signal is generated by the second-stage pattern processing.
当通过一个小孔径观察时，一条长的移动线条或光栅所呈现出的感知运动是模糊的。这种情况之所以存在，是因为即使是一台完美的机器也只能检测到垂直于移动轮廓的运动，所以平行于轮廓的运动是无法检测到的。人类视觉系统是通过一个孔径阵列——神经感受野来观察世界的。因此，一个移动的物体是通过许多小孔径来观察的，并且这些孔径中的许多运动都是模糊的。这种模糊性可以通过监测一个独特特征（如线条末端或拐角）的运动，并将该特征的运动归因于更大的物体来解决。或者，阿德尔森和莫夫尚提出，移动图像是分两个阶段进行处理的，也就是说，它们首先被分解为一维分量，然后再重新组合以产生感知到的物体运动。对于一个定义为两个漂移光栅之和的移动方格图案，这些不同的模型对该方格图案运动模糊性的解决产生了不同的预测。一个特征监测器会对光栅之间交叉点的运动做出反应，而两阶段运动处理器会首先将方格图案分解为其组成的光栅，随后再将它们重新组合以产生一个移动方格图案的感知。通过使用速度辨别来区分这两种模型，我发现辨别阈值反映的是方格图案组成光栅的速度，而不是方格图案本身的速度。这一结果支持了两阶段模型。尽管速度辨别受到分量处理的限制，但观察者无法直接获取分量速度。唯一可在感知上获取的速度信号是由第二阶段的图案处理产生的。
