Department of Neurobiology, Harvard Medical School Boston, MA, USA.
Department of Neurobiology, Harvard Medical School Boston, MA, USA ; Department of Ophthalmology, Boston Children's Hospital Boston, MA, USA.
Front Syst Neurosci. 2014 May 30;8:105. doi: 10.3389/fnsys.2014.00105. eCollection 2014.
Normalization has been proposed as a canonical computation operating across different brain regions, sensory modalities, and species. It provides a good phenomenological description of non-linear response properties in primary visual cortex (V1), including the contrast response function and surround suppression. Despite its widespread application throughout the visual system, the underlying neural mechanisms remain largely unknown. We recently observed that corticocortical feedback contributes to surround suppression in V1, raising the possibility that feedback acts through normalization. To test this idea, we characterized area summation and contrast response properties in V1 with and without feedback from V2 and V3 in alert macaques and applied a standard normalization model to the data. Area summation properties were well explained by a form of divisive normalization, which computes the ratio between a neuron's driving input and the spatially integrated activity of a "normalization pool." Feedback inactivation reduced surround suppression by shrinking the spatial extent of the normalization pool. This effect was independent of the gain modulation thought to mediate the influence of contrast on area summation, which remained intact during feedback inactivation. Contrast sensitivity within the receptive field center was also unaffected by feedback inactivation, providing further evidence that feedback participates in normalization independent of the circuit mechanisms involved in modulating contrast gain and saturation. These results suggest that corticocortical feedback contributes to surround suppression by increasing the visuotopic extent of normalization and, via this mechanism, feedback can play a critical role in contextual information processing.
归一化被提议作为一种跨不同脑区、感觉模态和物种的标准计算操作。它对初级视觉皮层 (V1) 中的非线性响应特性提供了很好的现象描述，包括对比响应函数和周围抑制。尽管它在整个视觉系统中得到了广泛的应用，但潜在的神经机制在很大程度上仍然未知。我们最近观察到，皮层间反馈有助于 V1 中的周围抑制，这增加了反馈通过归一化起作用的可能性。为了验证这个想法，我们在清醒的猕猴中描述了 V1 中的区域总和和对比响应特性，同时有和没有来自 V2 和 V3 的反馈，并将标准归一化模型应用于数据。区域总和特性很好地用一种除法归一化形式来解释，这种归一化形式计算神经元的驱动输入与“归一化池”的空间积分活动之间的比值。反馈失活通过缩小归一化池的空间范围来减少周围抑制。这种效应与被认为介导对比对区域总和影响的增益调制无关，反馈失活期间增益调制保持完整。感受野中心内的对比敏感度也不受反馈失活的影响，这进一步证明了反馈参与归一化的过程独立于调节对比增益和饱和的电路机制。这些结果表明，皮层间反馈通过增加归一化的视空间范围来促进周围抑制，并且通过这种机制，反馈可以在上下文信息处理中发挥关键作用。
