Betts Lisa R, Sekuler Allison B, Bennett Patrick J
Department of Psychology, Neuroscience, and Behaviour, McMaster University, Hamilton, Ont., Canada.
Vision Res. 2007 Jun;47(13):1769-80. doi: 10.1016/j.visres.2007.02.016. Epub 2007 Apr 26.
The current experiments measured orientation discrimination thresholds in younger (mean age approximately 23 years) and older (mean age approximately 66 years) subjects. In Experiment 1, the contrast needed to discriminate Gabor patterns (0.75, 1.5, and 3c/deg) that differed in orientation by 12deg was measured for different levels of external noise. At all three spatial frequencies, discrimination thresholds were significantly higher in older than younger subjects when external noise was low, but not when external noise was high. In Experiment 2, discrimination thresholds were measured as a function of stimulus contrast by varying orientation while contrast was fixed. The resulting threshold-vs-contrast curves had very similar shapes in the two age groups, although the curve obtained from older subjects was shifted to slightly higher contrasts. At contrasts greater than 0.05, thresholds in both older and younger subjects were approximately constant at 0.5deg. The results from Experiments 1 and 2 suggest that age differences in orientation discrimination are due solely to differences in equivalent input noise. Using the same methods as Experiment 1, Experiment 3 measured thresholds in 6 younger observers as a function of external noise and retinal illuminance. Although reducing retinal illumination increased equivalent input noise, the effect was much smaller than the age difference found in Experiment 1. Therefore, it is unlikely that differences in orientation discrimination were due solely to differences in retinal illumination. Our findings are consistent with recent physiological experiments that have found elevated spontaneous activity and reduced orientation tuning on visual cortical neurons in senescent cats (Hua, T., Li, X., He, L., Zhou, Y., Wang, Y., Leventhal, A. G. (206). Functional degradation of visual cortical cells in old cats. Neurobiology Aging, 27(1), 155-162) and monkeys (Yu, S., Wang, Y., Li, X., Zhou, Y. & Leventhal, A. G. (2006). Functional degradation of visual cortex in senescent rhesus monkeys. Neuroscience, 140(3), 1023-1029; Leventhal, A. G., Wang, Y., Pu, M., Zhou, Y. & Ma. Y. (2003). GABA and its agonists improved visual cortical function in senescent monkeys. Science,300 (5620), 812-815).
当前的实验测量了年轻（平均年龄约23岁）和年长（平均年龄约66岁）受试者的方向辨别阈值。在实验1中，针对不同水平的外部噪声，测量了辨别方向相差12度的不同空间频率（0.75、1.5和3c/度）的Gabor图形所需的对比度。在所有三个空间频率下，当外部噪声较低时，年长受试者的辨别阈值显著高于年轻受试者，但在外部噪声较高时则不然。在实验2中，通过固定对比度并改变方向来测量辨别阈值作为刺激对比度的函数。尽管从年长受试者获得的曲线向稍高的对比度偏移，但两个年龄组中得到的阈值-对比度曲线形状非常相似。在对比度大于0.05时，年长和年轻受试者的阈值在0.5度左右均保持恒定。实验1和2的结果表明，方向辨别中的年龄差异完全是由于等效输入噪声的差异。实验3使用与实验1相同的方法，测量了6名年轻观察者的阈值作为外部噪声和视网膜照度的函数。尽管降低视网膜照度会增加等效输入噪声，但这种影响远小于实验1中发现的年龄差异。因此，方向辨别差异不太可能仅仅是由于视网膜照度的差异。我们的发现与最近的生理学实验一致，这些实验发现衰老的猫（Hua, T., Li, X., He, L., Zhou, Y., Wang, Y., Leventhal, A. G. (206). Functional degradation of visual cortical cells in old cats. Neurobiology Aging, 27(1), 155 - 162）和猴子（Yu, S., Wang, Y., Li, X., Zhou, Y. & Leventhal, A. G. (2006). Functional degradation of visual cortex in senescent rhesus monkeys. Neuroscience, 140(3), 1023 - 1029; Leventhal, A. G., Wang, Y., Pu, M., Zhou, Y. & Ma. Y. (2003). GABA and its agonists improved visual cortical function in senescent monkeys. Science,300 (5620), 812 - 815）视觉皮层神经元的自发活动增加且方向调谐降低。
