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，即使是那些视力为20/20的人也是如此。这些缺陷会在我们看到的图像中产生微小的、往往难以察觉的像差，并限制图像质量和视觉灵敏度。最先为天文学开发的新技术，被称为“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、无像差的图像。大多数AO依赖于可以在受控范围内变形的灵活镜子的使用。测量眼睛中存在的光学像差，并将这些失真的逆像差应用到镜子上，从而校正图像质量并克服通常限制眼睛图像质量的光学像差。在视觉科学中，光学像差仪器有两种使用方式：看眼睛或看眼睛外。通过观察眼睛，在眼睛后面成像视网膜的仪器可以被AO改进到可以分解单个细胞的程度。从眼睛看去，投射到视网膜上的视觉图像的分辨率可以通过AO提高到超出正常视力的水平，从而产生所谓的超级视觉。我们将设计和制造的仪器将是一个用户友好的声光视觉刺激器，我们可以用它来做这两件事。该设备将包括一个数字投影仪-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调查眼睛和大脑的潜在属性，超出眼睛光学施加的正常限制。实验将通过要求观察者对他们所看到的做出判断来测试他们能够看到的程度。一旦该装置建成，我们将着手进行一系列人类视觉表现的标准测量。我们不会尝试测量对复杂视觉场景的反应，而是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。为了充分描述可能的视觉场景的广阔空间，简单的正弦图案将从粗略到精细系统地变化。对于每种图案，我们将确定观察者仅看到该图案所需的对比度。从这些测量中，我们得出了定义人类观察者视觉表现的空间对比敏感度函数(或空间CSF)。声光系统的重要之处在于，我们可以使用它来测量空间CSF，而不受人眼光学系统的限制。此外，我们还可以对彩色刺激、非彩色刺激和单类光敏探测器检测的刺激进行测量。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。声光刺激器也将允许我们研究超出系统通常神经限制的人类视觉。由于图案对之间的相互作用或由于打开和关闭图案时表观强度的变化，由声光刺激器产生的视觉图案太细而不能直接看到(因此是“不可见”的)，可以间接地看到。使用这些模式，我们将能够研究视觉系统中神经元的潜在属性。同样，我们可以使用彩色刺激、非彩色刺激和由单类光敏探测器检测到的刺激进行测量。对这些“看不见的”模式的间接检测使我们能够探索这些通路的内部工作原理，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.