Burkhardt D A, Fahey P K
Department of Psychology, University of Minnesota, Minneapolis 55455, USA.
J Neurophysiol. 1998 Sep;80(3):1070-81. doi: 10.1152/jn.1998.80.3.1070.
Responses of bipolar cells, cone photoreceptors, and horizontal cells were recorded intracellularly in superfused eyecup preparations of the tiger salamander (Ambystoma tigrinum). Contrast flashes of positive and negative polarity were applied at the center of the receptive field while the entire retina was light adapted to a background field of 20 cd/m2. For small contrasts, many bipolar cells showed remarkably high contrast gain: up to 15-20% of the bipolar response was evoked by a contrast step of 1%. There was considerable variation from cell to cell but, on average, no striking differences in contrast gain were found between the depolarizing (Bd) and hyperpolarizing (Bh) bipolar cells. Quantitative comparisons of contrast/response measurements for cone photoreceptors and cone-driven bipolars suggest that the high contrast gain of bipolars is the consequence of a 5-10 x amplification of small signals across the cone-->bipolar synapse. Bipolar cells had a very restricted linear range of response and tended to saturate at stimulus levels that were within the linear range of the cone response. The contrast/response of horizontal cells was similar to that of cones and differed markedly from that of Bh cells. For steps of equal contrast, the latency of the Bh cells was approximately 20 ms shorter than that of the Bd cells regardless of the contrast magnitude. For both bipolar cells and cones, the effect of contrast polarity on latency seems largely due to the absolute value of the light step, delta L. In the large signal domain, properties of the contrast responses of bipolar cells varied appreciably, both within and between the Bd and Bh classes. Cells of either class could be positive- or negative-contrast dominant. These and additional results show that in the light-adapted retina, the bipolar population is functionally diverse and has the potential to provide a rich substrate for distributed encoding of visual images.
在虎螈（Ambystoma tigrinum）的灌注眼杯标本中，细胞内记录了双极细胞、视锥光感受器和水平细胞的反应。当整个视网膜适应20 cd/m²的背景场进行光适应时，在感受野中心施加正负极性的对比闪光。对于小对比度，许多双极细胞显示出非常高的对比度增益：1%的对比度步长可诱发高达15 - 20%的双极反应。细胞之间存在相当大的差异，但平均而言，去极化双极细胞（Bd）和超极化双极细胞（Bh）之间在对比度增益上没有显著差异。对视锥光感受器和视锥驱动的双极细胞的对比度/反应测量的定量比较表明，双极细胞的高对比度增益是小信号在视锥 - 双极突触处放大5 - 10倍的结果。双极细胞的反应线性范围非常有限，并且在视锥反应的线性范围内的刺激水平时往往会饱和。水平细胞的对比度/反应与视锥细胞相似，与Bh细胞明显不同。对于相等对比度的步长，无论对比度大小如何，Bh细胞的潜伏期比Bd细胞短约20毫秒。对于双极细胞和视锥细胞，对比度极性对潜伏期的影响似乎主要归因于光步长的绝对值ΔL。在大信号域中，双极细胞对比度反应的特性在Bd和Bh类别内部和之间都有明显变化。任何一类细胞都可能是正对比度或负对比度占主导。这些以及其他结果表明，在光适应的视网膜中，双极细胞群体在功能上是多样的，并且有可能为视觉图像的分布式编码提供丰富的基础。
