A Laboratory For the Study of Objective Measurement of Visual Acuity

[00:82 . 20]视觉灵敏度（VA）是衡量视觉系统分辨距离近的物体的能力。Snellen视力表是测量成人VA最常用的方法，但它在测量婴儿和其他不能说话的患者VA时是无用的。优先观察、光动力学眼球震颤和光筛检查已被用来评估婴儿的VA，然而，所有这些方法都被发现有其缺点。弱视等疾病，在2-3%的人口中发生，如果在6岁之前没有诊断出来，可能导致永久性视力丧失，这可能导致学习和行为困难。标准的学前视力筛查计划可能无法诊断出多达2%的人口的眼部疾病。视觉诱发电位（VEP）是大脑视觉皮层对视觉刺激产生的一种电位，已被用于测量婴儿的VA。这是通过确定刺激中引发可检测的VEP的最大空间频率来完成的。然而，这种方法不能产生非常可重复的测量结果，并且比VA的心理物理测量具有更高的可变性。PI的团队开发了VEP检测器，能够在比其他检测器高得多的刺激空间频率下检测非常低的VEP；然而，仍然需要较长的测量时间。本提案的目标是获得研究基于视觉系统新模型的视觉灵敏度客观测量所需的实验室设备。如果该模型得到验证，则可以使用VEP通过简单地计算其时间频谱来测量VA（这比现有的扫描空间频率方法快得多）。眼睛中光感受器的间距并不能很好地预测空间分辨率的限制。视网膜中央凹上视锥细胞的平均间距实际上比通常引用的60圈/度的视觉分辨率限制要高得多。所提出的模型是基于这样一个前提，即眼睛是不断运动的，因此落在视网膜上任何给定的感光器上的亮度是随着时间不断变化的。主要的假设是，中央凹视觉的分辨率完全由视觉系统的时间动态决定。该模型由一个光感受器、一组时间带通滤波器和滤波器组输出端的检测器组成。时间带通滤波器与空间频率滤波器具有相同的效果，它们被用来模拟视觉系统。第二个假设将基于时间滤波器的VA模型与VEP联系起来。提出了一些心理物理和电生理实验，旨在验证这两个假设。其中一个实验是基于经典的Blakemore和Campbell的心理物理适应实验，但我们提出适应恒定的空间频率，而不是恒定的时间频率。这将通过稳定刺激光栅相对于眼球运动的漂移率来实现。通过测量两种不同漂移速率和固定空间频率下的对比灵敏度函数（CSF），时间滤波器模型可以预测空间频率通道模型完全无法解释的结果。还将进行心理物理和电生理时空脑脊液的对比实验，以确保心理物理结果延续到电生理领域。本研究所需的设备包括高精度双Perkinje图像眼动仪、光学图像稳定器、高扫描速率、快速荧光粉视频监视器和心理物理图形生成系统。PI所在的机构同意分担项目总成本的30%。这项研究将对理解人类视觉的基本过程产生重要的理论影响。此外，它还将影响医疗保健的质量，使电生理测量婴儿和非语言患者VA的准确方法成为可能。

0100820DavilaVisual acuity (VA) is a measure of how well the visual system can resolve closely spaced objects. The Snellen eye chart is the most common method of measuring VA in adults however it is useless when measuring VA in infants and other non-verbal patients. Preferential looking, optokinetic nystagmus, and photoscreening have been used to assess VA in infants, however all of these methods have been found to have their shortcomings. Diseases such as amblyopia, which occurs in 2-3% of the population, if not diagnosed by age 6 can lead to permanent visual loss, and this can lead to learning and behavioral difficulties. Standard pre-school visual screening programs can fail to diagnose ocular disease in as any as 2% of the population. The visual evoked potential (VEP), an electrical potential generated in the visual cortex of the brain in response to a visual stimulus, has been used to measure VA in infants. This is done by determining the maximum spatial frequency in the stimulus which elicits a detectable VEP. However, this method does not yield very repeatable measures and has a higher variability than psychophysical measures of VA. The PI's group has developed VEP detectors which are capable of detecting very low-level VEP's at much higher stimulus spatial frequencies than other detectors; nevertheless, long measurement times are still required. The goal of this proposal is to obtain the laboratory equipment necessary to study objective measurement of visual acuity based on a new model of the visual system. This model, if validated, will enable VA to be measured using the VEP by simply computing its temporal frequency spectrum (which is much faster than existing swept spatial frequency methods). The spacing of photoreceptors in the eye is not a good predictor of limits on spatial resolution. The average spacing of cones on the foveal of the retina would actually predict a much higher visual resolution limit than the often quoted value of 60 cycles/deg. The proposed model is based on the premise that the eyes are constantly in motion and hence the luminance falling on any given photoreceptor in the retina is constantly changing with time. The main hypothesizes is that resolution in foveal vision is determined entirely by the temporal dynamics of the visual system. The model for VA consists of a photoreceptor, a ban of temporal bandpass filters, and a detector at the output of the filter bank. The temporal bandpass filters have the same effect as the spatial frequency filters which have been used to model the visual system. A second hypothesis associates the temporal filter-based model for VA with the VEP. A number of psychophysical and electrophysiologic experiments are proposed which are designed to validate the two hypotheses. One experiment is based on the classical psychophysical adaptation experiments of Blakemore and Campbell but rather than adapting to a constant spatial frequency, we propose to adapt to a constant temporal frequency. This will be done by stabilizing the drift rate of the stimulus grating with respect to eye motion. By measuring the contrast sensitivity function (CSF) at two different drift rates and a fixed spatial frequency, the temporal filter model would predict a result which the spatial frequency channel model would completely fail to explain. Experiments co paring psychophysical and electrophysiologic spatiotemporal CSF's will also be performed which are designed to insure that the psychophysical results carry over to the electrophysiologic domain. The equipment needed for this research consists of a high accuracy dual Perkinje image eye tracker, an optical image stabilizer, a high scan rate, fast phosphor video monitor, and a psychophysical graphics generation system. The PI's institution has agreed to cost-share 30% of total project costs. This research will have important theoretical ramifications in understanding the basic processes in human vision. In addition, it will also impact the quality of health care by enabling an accurate method of electrophysiologically measuring VA in infants and non-verbal patients.