Friedman Howard S, Zhou Hong, von der Heydt Rüdiger
Krieger Mind/Brain Institute, Department of Biomedical Engineering, Johns Hopkins University, Baltimore, MD 21218, USA.
J Physiol. 2003 Apr 15;548(Pt 2):593-613. doi: 10.1113/jphysiol.2002.033555. Epub 2003 Feb 28.
Psychophysical studies indicate that perception of the colour and brightness of a surface depends on neural signals evoked by the borders of the surface rather than its interior. The visual cortex emphasizes contrast borders, but it is unclear whether colour surface signals also exist, whether colour border signals are orientation selective or mainly non-oriented, and whether cortical processing tends to separate colour and form information. To address these questions we examined the representation of uniform colour figures by recording single neuron activity from areas V1 and V2 in alert macaque monkeys during behaviourally induced fixation. Three aspects of coding were quantified: colour, orientation and edge selectivity. The occurrence of colour selectivity was not correlated with orientation or edge selectivity. The fraction of colour-selective cells was the same (64 % in layers 2 and 3 of V1, 45 % in V2) for oriented and non-oriented cells, and for edge-selective and surface-responsive cells. Oriented cells were often highly selective in colour space, and about 40 % of them were selective for edge polarity or border ownership. Thus, contrary to the idea of feature maps, colour, orientation and edge polarity are multiplexed in cortical signals. The results from V2 were similar to those from upper-layer V1, indicating that cortical processing does not strive to separate form and colour information. Oriented cells were five times more frequent than non-oriented cells. Thus, the vast majority of colour-coded cells are orientation tuned. Based on response profiles across a 4 deg square figure, and the relative frequency of oriented and non-oriented cells, we estimate that the cortical colour signal is 5-6 times stronger for the edges than for the surface of the figure. The frequency of oriented colour cells and their ability to code edge polarity indicate that these cells play a major role in the representation of surface colour.
心理物理学研究表明，对一个表面颜色和亮度的感知取决于该表面边界所诱发的神经信号，而非其内部。视觉皮层强调对比度边界，但尚不清楚是否也存在颜色表面信号、颜色边界信号是否具有方向选择性或主要是非定向的，以及皮层处理是否倾向于分离颜色和形状信息。为了解决这些问题，我们通过在行为诱导的注视过程中记录警觉猕猴V1和V2区域的单个神经元活动，来研究均匀颜色图形的表征。对编码的三个方面进行了量化：颜色、方向和边缘选择性。颜色选择性的出现与方向或边缘选择性无关。对于定向和非定向细胞，以及边缘选择性和表面反应性细胞，颜色选择性细胞的比例相同（V1区第2和3层为64%，V2区为45%）。定向细胞在颜色空间中通常具有高度选择性，其中约40%对边缘极性或边界归属具有选择性。因此，与特征图的观点相反，颜色、方向和边缘极性在皮层信号中是多重编码的。V2区的结果与上层V1区的结果相似，表明皮层处理并不试图分离形状和颜色信息。定向细胞的数量是非定向细胞的五倍。因此，绝大多数颜色编码细胞是方向调谐的。基于4度方形图形上的反应曲线，以及定向和非定向细胞的相对频率，我们估计皮层颜色信号在图形边缘处比在图形表面强5 - 6倍。定向颜色细胞的频率及其编码边缘极性的能力表明，这些细胞在表面颜色的表征中起主要作用。
