Allard Rémy, Faubert Jocelyn
University of Montreal, Montreal, Quebec, Canada.
Behav Res Methods. 2008 Aug;40(3):735-43. doi: 10.3758/brm.40.3.735.
Visual psychophysics often manipulates the contrast of the image on a digital display screen. Therefore, the limitation of the number of different luminance intensities displayable for most computers (typically, 256) is frequently an issue. To avoid this problem, experimenters generally need to purchase special hardware (graphic cards) and/or develop specific computer programs. Here, we describe an easy-to-implement method, consisting of adding noise to the displayed stimulus, that we call the noisy-bit method. This random dithering method, generalized to 256 luminance intensities, is equivalent to displaying continuous luminance intensities plus a certain amount of noise. Psychophysical testing using a standard spatiotemporal resolution (60 Hz and 1,024 x 768 pixels) demonstrated that the noise introduced by the noisy-bit method has no significant impact on contrast threshold and is not visible. We conclude that the noisy-bit method, combined with the standard 256 luminance levels, is perceptually equivalent to an analog display with a continuous luminance intensity resolution when the spatiotemporal resolution is high enough that the noise becomes negligible (which is easily attainable with the typical spatiotemporal resolutions of present-day computers).
视觉心理物理学常常会操控数字显示屏上图像的对比度。因此，大多数计算机可显示的不同亮度强度数量有限（通常为256种）这一限制常常成为一个问题。为避免这个问题，实验者通常需要购买特殊硬件（图形卡）和/或开发特定的计算机程序。在此，我们描述一种易于实施的方法，即给显示的刺激添加噪声，我们称之为噪声位方法。这种随机抖动方法，推广到256种亮度强度，等同于显示连续的亮度强度加上一定量的噪声。使用标准时空分辨率（60赫兹和1024×768像素）进行的心理物理学测试表明，噪声位方法引入的噪声对对比度阈值没有显著影响，并且不可见。我们得出结论，当时空分辨率足够高以至于噪声可忽略不计时（当今计算机的典型时空分辨率很容易达到这一点），噪声位方法与标准的256种亮度水平相结合，在感知上等同于具有连续亮度强度分辨率的模拟显示屏。
