Burkhardt Dwight A, Fahey Patrick K, Sikora Michael A
Departments of Psychology and Graduate Program of Neuroscience, University of Minnesota, Minneapolis, Minnesota 55455, USA.
Vis Neurosci. 2006 Jan-Feb;23(1):35-47. doi: 10.1017/S0952523806231043.
Intracellular recordings were obtained from 57 cone-driven bipolar cells in the light-adapted retina of the land-phase (adult) tiger salamander (Ambystoma tigrinum). Responses to flashes of negative and positive contrast for centered spots of optimum spatial dimensions were analyzed as a function of contrast magnitude. On average, the contrast/response curves of depolarizing and hyperpolarizing bipolar cells in the land-phase animals were remarkably similar to those of aquatic-phase animals. Thus, the primary retinal mechanisms mediating contrast coding in the outer retina are conserved as the salamander evolves from the aquatic to the land phase. To evaluate contrast encoding in the context of natural environments, the distribution of contrasts in natural images was measured for 65 scenes. The results, in general agreement with other reports, show that the vast majority of contrasts in nature are very small. The efficient coding hypothesis of Laughlin was examined by comparing the average contrast/response curves of bipolar cells with the cumulative probability distribution of contrasts in natural images. Efficient coding was found at 20 cd/m2 but at lower levels of light adaptation, the contrast/response curves were much too shallow. Further experiments show that two fundamental physiological factors-light adaptation and the nonlinear transfer across the cone-bipolar synapse are essential for the emergence of efficient contrast coding. For both land- and aquatic-based animals, the extent and symmetry of the dynamic range of the contrast/response curves of both classes of bipolar cells varied greatly from cell to cell. This apparent substrate for distributed encoding is established at the bipolar cell level, since it is not found in cones. As a result, the dynamic range of the bipolar cell population brackets the distribution of contrasts found in natural images.
在陆栖阶段（成年）虎螈（Ambystoma tigrinum）适应光照的视网膜中，从57个视锥驱动的双极细胞获得了细胞内记录。分析了对具有最佳空间尺寸的中心光斑的负对比度和正对比度闪光的反应，作为对比度大小的函数。平均而言，陆栖阶段动物中去极化和超极化双极细胞的对比度/反应曲线与水生阶段动物的曲线非常相似。因此，随着蝾螈从水生阶段进化到陆栖阶段，介导外视网膜对比度编码的主要视网膜机制得以保留。为了评估自然环境背景下的对比度编码，测量了65个场景中自然图像的对比度分布。结果与其他报告总体一致，表明自然界中绝大多数对比度非常小。通过将双极细胞的平均对比度/反应曲线与自然图像中对比度的累积概率分布进行比较，检验了Laughlin的有效编码假说。在20 cd/m2时发现了有效编码，但在较低的光适应水平下，对比度/反应曲线过于平缓。进一步的实验表明，两个基本的生理因素——光适应和视锥-双极突触的非线性传递，对于有效对比度编码的出现至关重要。对于陆栖和水栖动物来说，两类双极细胞的对比度/反应曲线动态范围的程度和对称性在细胞之间差异很大。这种明显的分布式编码底物在双极细胞水平上就已确立，因为在视锥细胞中未发现。因此，双极细胞群体的动态范围涵盖了自然图像中发现的对比度分布。
