Bonin Vincent, Mante Valerio, Carandini Matteo
Smith-Kettlewell Eye Research Institute, San Francisco, California 94115, USA.
J Neurosci. 2005 Nov 23;25(47):10844-56. doi: 10.1523/JNEUROSCI.3562-05.2005.
The responses of neurons in lateral geniculate nucleus (LGN) exhibit powerful suppressive phenomena such as contrast saturation, size tuning, and masking. These phenomena cannot be explained by the classical center-surround receptive field and have been ascribed to a variety of mechanisms, including feedback from cortex. We asked whether these phenomena might all be explained by a single mechanism, contrast gain control, which is inherited from retina and possibly strengthened in thalamus. We formalized an intuitive model of retinal contrast gain control that explicitly predicts gain as a function of local contrast. In the model, the output of the receptive field is divided by the output of a suppressive field, which computes the local root-mean-square contrast. The model provides good fits to LGN responses to a variety of stimuli; with a single set of parameters, it captures saturation, size tuning, and masking. It also correctly predicts that responses to small stimuli grow proportionally with contrast: were it not for the suppressive field, LGN responses would be linear. We characterized the suppressive field and found that it is similar in size to the surround of the classical receptive field (which is eight times larger than commonly estimated), it is not selective for stimulus orientation, and it responds to a wide range of frequencies, including very low spatial frequencies and high temporal frequencies. The latter property is hardly consistent with feedback from cortex. These measurements thoroughly describe the visual properties of contrast gain control in LGN and provide a parsimonious explanation for disparate suppressive phenomena.
外侧膝状体核（LGN）中神经元的反应表现出强大的抑制现象，如对比度饱和、大小调谐和掩蔽。这些现象无法用经典的中心-外周感受野来解释，并且已归因于多种机制，包括来自皮层的反馈。我们询问这些现象是否都可以由一种单一机制——对比度增益控制来解释，这种机制是从视网膜继承而来的，并且可能在丘脑得到强化。我们构建了一个直观的视网膜对比度增益控制模型，该模型明确预测增益是局部对比度的函数。在该模型中，感受野的输出除以抑制场的输出，抑制场计算局部均方根对比度。该模型对LGN对各种刺激的反应提供了很好的拟合；使用一组参数，它就能捕捉到饱和度、大小调谐和掩蔽现象。它还正确地预测了对小刺激的反应随对比度成比例增加：如果没有抑制场，LGN的反应将是线性的。我们对抑制场进行了表征，发现它的大小与经典感受野的外周相似（比通常估计的大八倍），它对刺激方向没有选择性，并且它对广泛的频率做出反应，包括非常低的空间频率和高的时间频率。后一个特性很难与来自皮层的反馈相一致。这些测量全面描述了LGN中对比度增益控制的视觉特性，并为不同的抑制现象提供了一个简洁的解释。
