Sakai H M, Wang J L, Naka K
Department of Ophthalmology, New York University Medical Center, New York 10016, USA.
J Gen Physiol. 1995 Jun;105(6):815-35. doi: 10.1085/jgp.105.6.815.
Control of contrast sensitivity was studied in two kinds of retina, that of the channel catfish and that of the kissing gourami. The former preparation is dominantly monochromatic and the latter is bichromatic. Various stimuli were used, namely a large field of light, a spot-annulus configuration and two overlapping stimuli of red and green. Recordings were made from horizontal, amacrine, and ganglion cells and the results were analyzed by means of Wiener's theory, in which the kernels are the contrast (incremental) sensitivity. Modulation responses from horizontal cells are linear, in that the waveform and amplitude of the first-order kernels are independent of the depth of modulation. In the N (sustained) amacrine and ganglion cells, contrast sensitivity was low for a large modulation input and was high for a small modulation input, providing an example of contrast gain control. In most of the cells, the contrast gain control did not affect the dynamics of the response because the waveform of the first-order kernels remained unchanged when the contrast sensitivity increased more than fivefold. The signature of the second-order kernels also remained unchanged over a wide range of modulation. The increase in the contrast sensitivity for the second-order component, as defined by the amplitude of the kernels, was much larger than for the first-order component. This observation suggests that the contrast gain control proceeded the generation of the second-order nonlinearity. An analysis of a cascade of the Wiener type shows that the control of contrast sensitivity in the proximal retinal cells could be modeled by assuming the presence of a simple (static) saturation nonlinearity. Such a nonlinearity must exist somewhere between the horizontal cells and the amacrine cells. The functional implications of the contrast gain control are as follows: (a) neurons in the proximal retina exhibit greater sensitivity to input of lower contrast; (b) saturation of a neuronal response can be prevented because of the lower sensitivity for an input with large contrast, and (c) over a large range of modulation depths, the amplitude of the response remains approximately constant.
在两种视网膜中研究了对比敏感度的控制，即斑点叉尾鮰和吻鲈的视网膜。前一种标本主要是单色的，后一种是双色的。使用了各种刺激，即大面积光场、点环配置以及红色和绿色的两个重叠刺激。从水平细胞、无长突细胞和神经节细胞进行记录，并根据维纳理论分析结果，其中核是对比（增量）敏感度。水平细胞的调制响应是线性的，因为一阶核的波形和幅度与调制深度无关。在N（持续）无长突细胞和神经节细胞中，对于大的调制输入，对比敏感度低，而对于小的调制输入，对比敏感度高，这提供了一个对比增益控制的例子。在大多数细胞中，对比增益控制不影响响应的动力学，因为当对比敏感度增加超过五倍时，一阶核的波形保持不变。二阶核的特征在很宽的调制范围内也保持不变。由核的幅度定义的二阶分量的对比敏感度增加比一阶分量大得多。这一观察结果表明，对比增益控制先于二阶非线性的产生。对维纳型级联的分析表明，近端视网膜细胞中对比敏感度的控制可以通过假设存在简单（静态）饱和非线性来建模。这种非线性必须存在于水平细胞和无长突细胞之间的某个地方。对比增益控制的功能意义如下：(a)近端视网膜中的神经元对较低对比度的输入表现出更高的敏感度；(b)由于对高对比度输入的较低敏感度，可以防止神经元反应的饱和，并且(c)在很大范围的调制深度内，响应的幅度保持近似恒定。
