DeVries Steven H, Qi Xiaofeng, Smith Robert, Makous Walter, Sterling Peter
Department of Ophthalmology, Northwestern University, 303 East Chicago Avenue, Chicago, IL 60611, USA.
Curr Biol. 2002 Nov 19;12(22):1900-7. doi: 10.1016/s0960-9822(02)01261-7.
Cone photoreceptors are noisy because of random fluctuations of photon absorption, signaling molecules, and ion channels. However, each cone's noise is independent of the others, whereas their signals are partially shared. Therefore, electrically coupling the synaptic terminals prior to forward transmission and subsequent nonlinear processing can appreciably reduce noise relative to the signal. This signal-processing strategy has been demonstrated in lower vertebrates with rather coarse vision, but its occurrence in mammals with fine acuity has been doubted (even though gap junctions are present) because coupling would blur the neural image.
In ground squirrel retina, whose triangular cone lattice resembles the human fovea, paired electrical recordings from adjacent cones demonstrated electrical coupling with an average conductance of approximately 320 pS. Blur caused by this degree of coupling had a space constant of approximately 0.5 cone diameters. Psychophysical measurements employing laser interferometry to bypass the eye's optics suggest that human foveal cones experience a similar degree of neural blur and that it is invariant with light intensity. This neural blur is narrower than the eye's optical blur, and we calculate that it should improve the signal-to-noise ratio at the cone terminal by about 77%.
We conclude that the gap junctions observed between mammalian cones, including those in the human fovea, represent genuine electrical coupling. Because the space constant of the resulting neural blur is less than that of the optical blur, the signal-to-noise ratio can be markedly improved before the nonlinear stages with little compromise to visual acuity.
由于光子吸收、信号分子和离子通道的随机波动，视锥光感受器存在噪声。然而，每个视锥的噪声相互独立，而它们的信号部分共享。因此，在正向传输和后续非线性处理之前，将突触终端进行电耦合，可以显著降低相对于信号的噪声。这种信号处理策略已在视觉较为粗糙的低等脊椎动物中得到证实，但在视力敏锐的哺乳动物中（尽管存在缝隙连接），其是否存在一直受到质疑，因为耦合会使神经图像模糊。
在三角视锥晶格类似于人类中央凹的地松鼠视网膜中，对相邻视锥进行成对电记录，结果显示存在电耦合，平均电导约为320皮西门子。这种程度的耦合所导致的模糊具有约0.5个视锥直径的空间常数。采用激光干涉测量法绕过眼睛光学系统的心理物理学测量表明，人类中央凹视锥经历的神经模糊程度相似，且与光强度无关。这种神经模糊比眼睛的光学模糊更窄，我们计算得出它应能使视锥终端的信噪比提高约77%。
我们得出结论，在包括人类中央凹视锥在内的哺乳动物视锥之间观察到的缝隙连接代表真正的电耦合。由于由此产生的神经模糊的空间常数小于光学模糊的空间常数，因此在非线性阶段之前可以显著提高信噪比，而对视敏度的影响很小。
