Elliott Sarah L, Werner John S, Webster Michael A
Institute for Mind & Biology, University of Chicago, Chicago, IL, USA.
J Vis. 2012 Aug 17;12(8):11. doi: 10.1167/12.8.11.
Precortical color channels are tuned primarily to the LvsM (stimulation of L and M cones varied, but S cone stimulation held constant) or SvsLM (stimulation of S cones varied, but L and M cone stimulation held constant) cone-opponent (cardinal) axes, but appear elaborated in the cortex to form higher-order mechanisms tuned to both cardinal and intermediate directions. One source of evidence for these higher-order mechanisms has been the selectivity of color contrast adaptation for noncardinal directions, yet the degree of this selectivity has varied widely across the small sample of observers tested in previous studies. This study explored the possible bases for this variation, and in particular tested whether it reflected age-related changes in the distribution or tuning of color mechanisms. Observers included 15 younger (18-22 years of age) and 15 older individuals (66-82), who adapted to temporal modulations along one of four chromatic axes (two cardinal and two intermediate axes) and then matched the hue and contrast of test stimuli lying along eight different directions in the equiluminant plane. All observers exhibited aftereffects that were selective for both the cardinal and intermediate directions, although selectivity was weaker for the intermediate axes. The degree of selectivity increased with the magnitude of adaptation for all axes, and thus adaptation strength alone may account for much of the variance in selectivity among observers. Older observers showed a stronger magnitude of adaptation thus, surprisingly, more conspicuous evidence for higher-order mechanisms. For both age groups the aftereffects were well predicted by response changes in chromatic channels with linear spectral sensitivities, and there was no evidence for weakened channel tuning with aging. The results suggest that higher-order mechanisms may become more exposed in observers or conditions in which the strength of adaptation is greater, and that both chromatic contrast adaptation and the cortical color coding it reflects remain largely intact in the aging visual system.
皮层下颜色通道主要调谐至L与M（L和M视锥细胞的刺激变化，但S视锥细胞刺激保持恒定）或S与LM（S视锥细胞的刺激变化，但L和M视锥细胞刺激保持恒定）视锥细胞对立（主要）轴，但在皮层中似乎得到细化，以形成调谐至主要和中间方向的高阶机制。这些高阶机制的一个证据来源是颜色对比适应对非主要方向的选择性，但在先前研究中测试的少量观察者样本中，这种选择性的程度差异很大。本研究探讨了这种差异的可能原因，特别是测试它是否反映了颜色机制分布或调谐中与年龄相关的变化。观察者包括15名年轻人（18 - 22岁）和15名年长者（66 - 82岁），他们适应沿四个色轴之一（两个主要轴和两个中间轴）的时间调制，然后匹配等亮度平面中沿八个不同方向的测试刺激的色调和对比度。所有观察者都表现出对主要和中间方向均有选择性的后效应，尽管对中间轴的选择性较弱。对于所有轴，选择性程度随适应幅度增加，因此仅适应强度可能就可以解释观察者之间选择性差异的大部分原因。年长观察者表现出更强的适应幅度，因此，令人惊讶的是，有更明显的高阶机制证据。对于两个年龄组，后效应都可以通过具有线性光谱敏感性的色通道中的响应变化得到很好的预测，并且没有证据表明随着年龄增长通道调谐会减弱。结果表明，在适应强度较大的观察者或条件下，高阶机制可能会更明显地表现出来，并且色对比适应及其所反映的皮层颜色编码在衰老的视觉系统中基本保持完整。
