Mareschal I, Baker C L
McGill Vision Research Unit, Department of Ophthalmology, Montreal, Quebec, Canada.
Nat Neurosci. 1998 Jun;1(2):150-4. doi: 10.1038/401.
Object boundaries in the natural environment are often defined by changes in luminance; in other cases, however, there may be no difference in average luminance across the boundary, which is instead defined by more subtle 'second-order' cues, such as changes in the contrast of a fine-grained texture. The detection of luminance boundaries may be readily explained in terms of visual cortical neurons, which compute the linear sum of the excitatory and inhibitory inputs to different parts of their receptive field. The detection of second-order stimuli is less well understood, but is thought to involve a separate nonlinear processing stream, in which boundary detectors would receive inputs from many smaller subunits. To address this, we have examined the properties of cortical neurons which respond to both first- and second-order stimuli. We show that the inputs to these neurons are also oriented, but with no fixed orientational relationship to the neurons they subserve. Our results suggest a flexible mechanism by which the visual cortex can detect object boundaries regardless of whether they are defined by luminance or texture.
自然环境中的物体边界通常由亮度变化来界定；然而，在其他情况下，边界两侧的平均亮度可能并无差异，此时边界是由更为细微的“二阶”线索来界定的，比如细粒度纹理对比度的变化。亮度边界的检测可以很容易地用视觉皮层神经元来解释，这些神经元会计算其感受野不同部分的兴奋性和抑制性输入的线性总和。二阶刺激的检测则不太为人所理解，但据认为涉及一个单独的非线性处理流，其中边界检测器会从许多较小的亚单位接收输入。为了解决这个问题，我们研究了对一阶和二阶刺激都有反应的皮层神经元的特性。我们发现，这些神经元的输入也是有方向的，但与它们所服务的神经元没有固定的方向关系。我们的结果表明存在一种灵活的机制，通过这种机制，视觉皮层能够检测物体边界，而不管它们是由亮度还是纹理界定的。
