Discipline of Biomedical Science, University of Sydney, Lidcombe, New South Wales, Australia.
PLoS One. 2012;7(4):e34466. doi: 10.1371/journal.pone.0034466. Epub 2012 Apr 9.
Many neurons in mammalian primary visual cortex have properties such as sharp tuning for contour orientation, strong selectivity for motion direction, and insensitivity to stimulus polarity, that are not shared with their sub-cortical counterparts. Successful models have been developed for a number of these properties but in one case, direction selectivity, there is no consensus about underlying mechanisms. We here define a model that accounts for many of the empirical observations concerning direction selectivity. The model describes a single column of cat primary visual cortex and comprises a series of processing stages. Each neuron in the first cortical stage receives input from a small number of on-centre and off-centre relay cells in the lateral geniculate nucleus. Consistent with recent physiological evidence, the off-centre inputs to cortex precede the on-centre inputs by a small (∼4 ms) interval, and it is this difference that confers direction selectivity on model neurons. We show that the resulting model successfully matches the following empirical data: the proportion of cells that are direction selective; tilted spatiotemporal receptive fields; phase advance in the response to a stationary contrast-reversing grating stepped across the receptive field. The model also accounts for several other fundamental properties. Receptive fields have elongated subregions, orientation selectivity is strong, and the distribution of orientation tuning bandwidth across neurons is similar to that seen in the laboratory. Finally, neurons in the first stage have properties corresponding to simple cells, and more complex-like cells emerge in later stages. The results therefore show that a simple feed-forward model can account for a number of the fundamental properties of primary visual cortex.
哺乳动物初级视觉皮层中的许多神经元具有一些特性，例如轮廓方向的尖锐调谐、对运动方向的强烈选择性以及对刺激极性不敏感，这些特性与它们的皮质下对应物不同。已经为许多这些特性开发了成功的模型，但在一个情况下，即方向选择性，关于潜在机制没有共识。我们在这里定义了一个模型，该模型解释了许多关于方向选择性的经验观察。该模型描述了猫初级视觉皮层的一个单一柱，包括一系列处理阶段。第一皮层阶段中的每个神经元都接收来自外侧膝状体核中的少量中心和偏心中继细胞的输入。与最近的生理证据一致，偏心输入到皮层的时间比中心输入早约 4 毫秒，正是这种差异赋予了模型神经元方向选择性。我们表明，所得到的模型成功匹配了以下经验数据：具有方向选择性的细胞比例；倾斜的时空感受野；对停留在感受野上的对比度反转光栅的响应中的相位提前。该模型还解释了其他几个基本性质。感受野具有拉长的子区域，方向选择性很强，并且神经元之间的方向调谐带宽分布与实验室中观察到的相似。最后，第一阶段的神经元具有与简单细胞相对应的特性，而更复杂的细胞则出现在后面的阶段。因此，结果表明，简单的前馈模型可以解释初级视觉皮层的许多基本特性。
