At and beyond the neural limits: visual psychophysics using an adaptive-optics visual stimulator

我们所有人的眼睛的光学都在某种程度上是不完美的，即使是那些具有“20/20视力”的人。这些缺陷会在我们看到的图像中产生微小的、通常难以察觉的像差，并限制图像质量和视觉灵敏度。最初为天文学开发的新技术，称为“自适应光学”或简称AO，可以纠正这些像差，并在眼睛中产生近乎完美的无像差图像。大多数AO依赖于使用可以以受控方式变形的柔性反射镜。测量眼睛中存在的光学像差，并将这些畸变的倒数应用于镜子，从而校正图像质量并克服通常限制眼睛中图像质量的光学像差。AO仪器在视觉科学中有两种用途：要么看着眼睛，要么看着眼睛。看着眼睛，在眼睛后部对视网膜成像的仪器可以通过AO改进到可以分辨单个细胞的程度。从眼睛向外看，投射到视网膜上的视觉图像的分辨率可以通过AO增加到超过正常视力，产生所谓的“超视觉”。我们将设计和制造的仪器将是一个用户友好的AO视觉刺激器，我们可以做这两件事。该设备将包括一个数字光投影仪通常用于在电影院投影图像将超高分辨率图像直接投射到视网膜上。该仪器的主要目的是研究眼睛和大脑的基本特性，超出眼睛光学所施加的正常限制。实验将通过要求观察者对他们所能看到的做出判断来测试他们的视力。一旦设备建成，我们将开始对人类视觉性能进行一系列标准测量。与其试图测量对复杂视觉场景的反应，我们将测量观察者对由光和暗的周期性图案组成的简单空间图案的敏感度。这些简单的“正弦”模式是我们可以预测对任何复杂刺激的反应的基石。为了充分展示可能的视觉场景的广阔空间，简单的正弦图案将系统地从粗到细变化。对于每个图案，我们将确定观察者需要多少对比度才能看到图案。从这些测量中，我们得到了空间对比敏感度函数（或空间CSF），它定义了人类观察者的视觉性能。AO系统的重要性在于，我们可以使用它来测量空间CSF，而不受眼睛光学系统的限制。此外，我们可以对彩色刺激，非彩色刺激和由单一类别的光敏探测器检测到的刺激进行这些测量。通过使用不同的刺激，我们可以研究视觉系统中不同的神经通路。AO刺激器还将允许我们研究人类视觉系统的通常神经限制。由于图案对之间的相互作用或由于图案打开和关闭时表观强度的变化，可以间接看到由AO刺激器产生的视觉图案，这些视觉图案太精细而无法直接看到（因此是“不可见”的）。使用这些模式，我们将能够研究视觉系统中神经元的基本特性。同样，我们可以使用彩色刺激，非彩色刺激和由单一类别的光敏检测器检测到的刺激进行测量。这些“看不见的”模式的间接检测使我们能够探测路径的内部工作，回答有关视网膜如何工作的具体问题。

The optics of all of our eyes are to some degree imperfect, even those of individuals with "20/20 vision". These imperfection produce small, often imperceptible, aberrations in the images that we see and limits both image quality and visual acuity.New technology first developed for astronomy, called "adaptive optics" or AO for short, can correct those aberrations and produce nearly perfect, aberration-free images in the eye. Most AO relies on the use of a flexible mirror that can be deformed in a controlled away. The optical aberrations present in the eye are measured and the inverse of those distortions are applied to the mirror, thus correcting the image quality and overcoming the optical aberrations that usually limit image quality in the eye.AO instruments can be used in two ways in visual science: either to look into the eye or to look out of it. Looking into the eye, instruments that image the retina at the back of the eye can be improved by AO to the extent that individual cells can be resolved. Looking out of the eye, the resolution of a visual image projected onto to retina can be increased by AO to beyond normal visual acuity, producing so called "super-vision". The instrument that we will design and build will be a user-friendly AO visual stimulator with which we can do both of these things. The device will include a digital light projector-normally used to project images in cinemas-that will project super high resolution images directly onto the retina. The primary purpose of the instrument is to investigate the underlying properties of the eye and brain beyond the normal limits imposed by the optics of the eye. Experiments will be conducted to test how well observer are able to see by asking them to make judgments about what they can see. Once the device is constructed, we will embark on a series of standard measurements of human visual performance. Rather than trying to measure the response to complex visual scenes, we will measure an observer's sensitivity to simple spatial patterns made up of periodic patterns of light and dark. These simple "sinusoidal" patterns are the building blocks from which we can predict the responses to any complex stimuli. To fully characterise the vast space of possible visual scenes, the simple sinusoidal patterns will be systematically varied from coarse to fine. For each pattern, we will determine how much contrast the observer needs to just see the pattern. From these measurements, we derive the spatial contrast sensitivity function (or spatial CSF) that defines the visual performance of the human observer. The importance of the AO system is that we can use it to measure spatial CSFs without the measurements being limited by the optics of the eye. Moreover, we can make those measurements for chromatic stimuli, achromatic stimuli and stimuli detected by single classes of light-sensitive detectors. By using different stimuli, we can investigate different neural pathways in the visual system.The AO stimulator will also allow us to investigate human vision beyond the usual neural limits of the system. Visual patterns produced by the AO stimulator that are too fine to be seen directly (and are therefore "invisible") can be seen indirectly due to interactions between pairs of patterns or due to changes in apparent intensity when a pattern is turned on and off. Using these patterns we will be able to investigate the underlying properties of the neurons in the visual system. Again, we can make measurements using chromatic stimuli, achromatic stimuli and stimuli detected by single classes of light-sensitive detectors. The indirect detection of these "invisible" patterns allows us to probe the inner working of the pathways answer specific questions about how the retina works.