Qian N, Andersen R A
Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge 02139.
J Neurosci. 1994 Dec;14(12):7367-80. doi: 10.1523/JNEUROSCI.14-12-07367.1994.
We investigated how the primate visual system solves the difficult problem of representing multiple motion vectors in the same part of the visual space--the problem of motion transparency. In the preceding companion article we reported that displays with locally well-balanced motion signals in opposite directions are perceptually nontransparent (i.e., one does not see two coherent moving surfaces) and that transparent displays always contain locally unbalanced motion signals. This is exemplified by our paired and unpaired dot patterns. Although both types of stimuli contain two sets of dots moving in opposite directions, the former is locally well balanced and appears like flicker while the latter gives a perception of two transparent surfaces. In this article we report our physiological recordings from areas V1 and MT of behaving monkeys, comparing single-cell responses to the paired and the unpaired dot patterns. Although a small proportion of directionally selective V1 cells responded differently to the two types of patterns, the average V1 responses could not reliably distinguish between the paired and the unpaired stimuli. A large fraction of MT cells, on the other hand, responded significantly better to the unpaired dot patterns than to the paired ones. Furthermore, the average response of all MT cells to the unpaired dot patterns was significantly higher than that to the paired dot patterns. These results demonstrate a neural correlate of the perceptual transparency at the level of MT. On the other hand, V1 cells do not generally discriminate between the transparent and nontransparent stimuli, indicating that V1 activity is not well correlated with the perception of motion transparency. Our results are consistent with a two-stage model for motion processing: the first stage measures local motion and the second stage introduces suppression if different directions of motion are present at a local region of the visual field. The first stage is located primarily in V1 and the second stage primarily in MT. Finally, we found a strong and negative correlation between the degree of the opponent-direction suppression of MT cells and their responses to flicker noise stimuli. This result suggests that one of the fundamental roles of the opponent-direction suppression in MT is noise reduction.
我们研究了灵长类动物视觉系统如何解决在视觉空间同一部分表示多个运动矢量这一难题——运动透明度问题。在前一篇相关文章中，我们报告称，具有局部方向相反且平衡良好的运动信号的显示在感知上是不透明的（即看不到两个连贯的运动表面），而透明显示总是包含局部不平衡的运动信号。我们的配对和非配对点图案就是例证。尽管这两种类型的刺激都包含两组向相反方向移动的点，但前者在局部是平衡良好的，看起来像闪烁，而后者则会让人感知到两个透明表面。在本文中，我们报告了对行为猴子的V1区和MT区进行的生理记录，比较了单细胞对配对和非配对点图案的反应。尽管一小部分方向选择性V1细胞对这两种图案的反应有所不同，但V1的平均反应无法可靠地区分配对和非配对刺激。另一方面，很大一部分MT细胞对非配对点图案的反应明显比对配对点图案的反应更好。此外，所有MT细胞对非配对点图案的平均反应明显高于对配对点图案的反应。这些结果证明了MT水平上感知透明度的神经关联。另一方面，V1细胞通常不会区分透明和不透明刺激，这表明V1的活动与运动透明度的感知没有很好的相关性。我们的结果与运动处理的两阶段模型一致：第一阶段测量局部运动，第二阶段如果在视野的局部区域存在不同的运动方向则引入抑制。第一阶段主要位于V1，第二阶段主要位于MT。最后，我们发现MT细胞的方向拮抗抑制程度与其对闪烁噪声刺激的反应之间存在强烈的负相关。这一结果表明，MT中方向拮抗抑制的一个基本作用是降噪。
