A Laboratory For the Study of Objective Measurement of Visual Acuity

戴维拉视觉敏锐度(VA)是用来衡量视觉系统分辨近距离物体的能力。斯内伦眼表是测量成年人视力的最常见方法，但在测量婴儿和其他非语言患者的视力时，它毫无用处。优先观察法、视动性眼震和光筛选法已被用于评估婴儿的VA，然而所有这些方法都被发现有其缺点。弱视等疾病发生在2%-3%的人口中，如果在6岁前没有诊断出来，可能会导致永久性视力丧失，这可能会导致学习和行为困难。标准的学龄前视力筛查计划可能无法诊断多达2%的人口的眼部疾病。视觉诱发电位(VEP)是大脑皮层对视觉刺激产生的电势，已被用于测量婴儿的VA。这是通过确定刺激中引起可检测的视觉诱发电位的最大空间频率来完成的。然而，这种方法产生的测量不是非常可重复的，并且比VA的心理物理测量具有更高的可变性。PI的团队已经开发了VEP检测器，它能够在比其他检测器更高的刺激空间频率下检测到非常低水平的VEP；然而，仍然需要很长的测量时间。这项提议的目标是获得必要的实验室设备，以研究基于一种新的视觉系统模型的客观视力测量。如果得到验证，这个模型将能够通过简单地计算其时间频谱(这比现有的扫描空间频率方法快得多)，使用VEP来测量VA。眼睛中光感受器的间距并不能很好地预测空间分辨率的极限。视网膜中央凹上视锥细胞的平均间距实际上预示着比经常引用的60个周期/度的值高得多的视觉分辨率极限。提出的模型是基于这样的假设，即眼睛不断地运动，因此落在视网膜中任何给定光感受器上的亮度都会随着时间不断变化。主要假设是中心凹视觉的分辨率完全由视觉系统的时间动力学决定。VA的模型由一个光感受器、一个时间带通滤波器组和一个位于滤波器组输出端的探测器组成。时间带通滤光器与空间频率滤光器具有相同的效果，后者已被用于对视觉系统建模。第二个假设将基于时间滤波的VA模型与VEP相关联。为了验证这两个假说，我们提出了一些心理物理和电生理实验。其中一个实验是基于Blakemore和Campbell的经典心理物理适应实验，但我们建议适应恒定的时间频率，而不是适应恒定的空间频率。这将通过稳定刺激光栅相对于眼睛运动的漂移率来实现。通过测量两种不同漂移率和固定空间频率下的对比敏感度函数(CSF)，时间滤波模型将预测空间频率信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比较实验，以确保心理物理结果延续到电生理领域。本研究所需要的设备包括高精度的双珀金野图像眼球跟踪器、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具有重要的理论意义。此外，它还将通过实现一种准确的电生理方法来测量婴儿和非语言患者的VA，从而影响卫生保健的质量。

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.