Ohzawa I, Sclar G, Freeman R D
J Neurophysiol. 1985 Sep;54(3):651-67. doi: 10.1152/jn.1985.54.3.651.
We have examined the idea that the adaptation of cortical neurons to local contrast levels in a visual stimulus is functionally advantageous. Specifically, cortical cells may have large differential contrast sensitivity as a result of adjustments that center a limited response range around a mean level of contrast. To evaluate this notion, we measured contrast-response functions of cells in striate cortex while systematically adapting them to different contrast levels of stimulus gratings. For the majority of cortical neurons tested, the results of this basic experiment show that contrast-response functions shift laterally along a log-contrast axis so that response functions match mean contrast levels in the stimulus. This implies a contrast-dependent change in the gain of the cell's contrast-response relationship. We define this process as contrast gain control. The degree to which this contrast adjustment occurs varies considerably from cell to cell. There are no obvious differences regarding cell type (simple vs. complex) or laminar distribution. Contrast gain control is almost certainly a cortical function, since lateral geniculate cells and fibers exhibit only minimal effects. Tests presented in the accompanying paper (37) provide additional evidence on the cortical origin of the process. In another series of experiments, the effect of contrast adaptation on physiological estimates of contrast sensitivity was evaluated. Sustained adaptation to contrast levels as low as 3% was capable of nearly doubling the thresholds of most of the cells tested. Adaptation may therefore be an important factor in determinations of the contrast sensitivity of cortical neurons. We tested the spatial extent of the mechanisms responsible for these gain-control effects by attempting to adapt cells using both a large grating and a grating patch limited to that portion of a cell's receptive field from which excitatory discharges could be elicited directly (the central discharge region). Adaptation was found to be an exclusive property of the central region. This held even in the case of hypercomplex cells, which received strong influences from surrounding regions of the visual field. Finally, we measured the time course of contrast adaptation. We found the process to be rather slow, with a mean time constant of approximately 6 s. Once again, there was considerable variability in this value from cell to cell.
我们研究了这样一种观点，即皮层神经元对视觉刺激中局部对比度水平的适应在功能上具有优势。具体而言，由于围绕对比度平均水平对有限响应范围进行中心化的调整，皮层细胞可能具有较大的差异对比度敏感性。为了评估这一概念，我们测量了纹状皮层中细胞的对比度响应函数，同时系统地使它们适应不同对比度水平的刺激光栅。对于大多数测试的皮层神经元，这个基础实验的结果表明，对比度响应函数沿着对数对比度轴横向移动，从而使响应函数与刺激中的平均对比度水平相匹配。这意味着细胞对比度响应关系的增益存在对比度依赖性变化。我们将这个过程定义为对比度增益控制。这种对比度调整发生的程度在细胞之间有很大差异。在细胞类型（简单细胞与复杂细胞）或层状分布方面没有明显差异。对比度增益控制几乎肯定是一种皮层功能，因为外侧膝状体细胞和纤维仅表现出最小的影响。随附论文（37）中给出的测试为该过程的皮层起源提供了额外证据。在另一系列实验中，评估了对比度适应对对比度敏感性生理估计值的影响。持续适应低至3%的对比度水平能够使大多数测试细胞的阈值几乎翻倍。因此，适应可能是决定皮层神经元对比度敏感性的一个重要因素。我们通过尝试使用大光栅和仅限于细胞感受野中能直接引发兴奋性放电的部分（中央放电区域）的光栅小块来使细胞适应，测试了负责这些增益控制效应的机制的空间范围。发现适应是中央区域的专有特性。即使在超复杂细胞的情况下也是如此，超复杂细胞受到视野周围区域的强烈影响。最后，我们测量了对比度适应的时间进程。我们发现这个过程相当缓慢，平均时间常数约为6秒。同样，这个值在细胞之间也有很大差异。
