Graham N, Robson J G
Department of Psychology, Columbia University, New York, NY 10027.
Vision Res. 1987;27(11):1997-2007. doi: 10.1016/0042-6989(87)90063-0.
In accounting for pattern thresholds it is necessary to consider probability summation (or equivalent nonlinear pooling) not only across detectors selective for different spatial frequencies but also across detectors in different spatial positions. Interestingly, calculation on this basis shows that the amount of summation between components of closely similar spatial frequency in a large grating is primarily determined by the variation in sensitivity of detectors at different spatial locations and is little affected by the spatial-frequency bandwidths of the detectors. To test this conclusion, we have measured the amount of summation between two components with spatial frequencies very close to 6 c/deg in two regions of the visual field: in the fovea (a region where sensitivity is very non-uniform) and in the periphery (where sensitivity is nearly uniform). As predicted, there was less summation between components of very closely similar frequencies in the nearly-uniform peripheral region than in the non-uniform foveal region. Measurements in the fovea of the summation of two components with spatial frequencies very near to either 1.5, 6 or 24 c/deg showed, as expected, that the amount of summation depends upon the ratio of the frequencies rather than their absolute difference, indicating that probability summation takes place over an area related to spatial frequency rather than over a fixed area.
在考虑模式阈值时，不仅有必要考虑对不同空间频率具有选择性的探测器之间的概率总和（或等效的非线性合并），还需考虑不同空间位置的探测器之间的概率总和。有趣的是，基于此进行的计算表明，大光栅中空间频率非常相似的成分之间的总和量主要由不同空间位置探测器灵敏度的变化决定，而受探测器空间频率带宽的影响很小。为了验证这一结论，我们测量了视野中两个区域内空间频率非常接近6周/度的两个成分之间的总和量：在中央凹（一个灵敏度非常不均匀的区域）和外周（灵敏度几乎均匀的区域）。正如所预测的，在几乎均匀的外周区域中，空间频率非常相似的成分之间的总和比在不均匀的中央凹区域中要少。在中央凹对空间频率非常接近1.5、6或24周/度的两个成分的总和进行测量，正如预期的那样，总和量取决于频率之比而非它们的绝对差值，这表明概率总和发生在与空间频率相关的区域而非固定区域。
