Using the distortion of visual probes to analyse human visual pathways

在人类感知或心理物理实验中，我们精确地定义了呈现给人类受试者的时间频率、空间频率、波长和亮度方面的视觉刺激。通过沿着某个维度操纵刺激，并测量受试者反应的变化，我们可以推断出潜在视觉系统的一些特性。不幸的是，这种对完整/完整视觉系统的测量不能让我们直接接触到在刺激和反应之间进行干预的过程。相比之下，生理学家的优势在于，他们可以通过将电极直接插入视觉通路，在(非人类的)视觉系统内进行记录。视觉非线性的使用在某种程度上纠正了这种不平衡。有了它们，心理物理学家也有了一个“电极”，用来记录和引入视觉通路中的信号，从而在功能上“解剖”人类的视觉系统。在这个项目中，我们提出了视觉非线性的新用途来从内部研究人类的两条主要视觉路径--色度路径和亮度路径。视觉非线性是视觉通路中的一个阶段，它扭曲通过它的信号，产生输入端不存在的新信号分量。我们将研究的两个非线性会扭曲正弦闪烁信号，以产生--除了闪烁--颜色或亮度的稳定变化，这些变化似乎叠加在闪烁上，并在闪烁关闭时消失。例如，红色的光盘(约670 nm)在闪烁时会显得更黄(颜色失真)，而绿黄色的圆盘(约565 nm)则会显得更亮(亮度增强)。下面的图1和图3说明了在幅度上缓慢变化的闪烁(幅度调制闪烁)的这些影响。通过仔细操作呈现给受试者的刺激，从而在非线性位置产生新的信号分量，将有可能分别测量每个非线性之前和之后的时间频率响应。我们将研究M锥和L锥通道中的两种非线性：一种是压缩的，导致闪光爆发的表观颜色和饱和度发生变化；另一种是扩张的，导致闪光的表观亮度发生变化。我们的期望是，我们将能够分别确定人类两条主要的接受后通路：假定的颜色和亮度通路的早期和晚期特性，从而为人类视觉系统的接受后组织和在其中进行的处理变化提供新的见解。我们将研究的颜色和亮度变化是不同的：第一，因为它们可以单独发生，第二，因为它们与不同类型的非线性(在颜色的情况下是压缩的或减速的；在亮度的情况下是膨胀的或加速的)和不同的感知是一致的。总体而言，这些观察结果表明，这两种非线性存在于人类视觉系统中的两条主要的感受器后通路--亮度和色度通路中，这是一种令人兴奋的可能性。我们相信，这个项目将为了解人类视觉系统的内部工作提供一个独特的窗口，并将确定关键信号和位置，这些信号和位置将有助于其他物种的生理和/或解剖学研究。

In human perceptual or 'psychophysical' experiments, we precisely define the visual stimuli that are presented to human subjects in terms of of temporal frequency, spatial frequency, wavelength and brightness. By manipulating the stimuli along some dimension, and measuring the change in the subject's response, we can infer something about the properties of the underlying visual system. Unfortunately, such measures of the intact/complete visual system do not grant us direct access to the processes that intervene between the stimulus and response. Physiologists, by contrast, have the advantage that they can make recordings within the (non-human) visual system by inserting electrodes directly into visual pathways. The use of visual nonlinearities goes some way to redress this imbalance. With them, the psychophysicist also has an 'electrode' with which to record and introduce signals within the visual pathways and so functionally 'dissect' the human visual system. In this project, we propose the novel use of visual nonlinearities to study the two primary human visual pathways, the chromatic and luminance pathways--from the inside. A visual nonlinearity is a stage in the visual pathway that distorts signals that pass through it to produce new signal components that were not present at its input. The two nonlinearities that we will study distort sinusoidal flicker signals to produce--in addition to flicker--a steady change either in colour or in brightness, which appear superimposed on the flicker and disappear when the flicker is turned off. For example, a red disc of light (c. 670 nm) can appear yellower when it is flickered (colour distortion) or a greenish-yellow disc (c. 565 nm) can appear brighter ('brightness enhancement'). Figures 1 and 3, below, illustrate these effects for flicker that slowly varies in amplitude ('amplitude-modulated flicker'). By carefully manipulating the stimulus presented to the subject, and thus the new signal components produced at the nonlinear site, it will be possible to measure separately the temporal frequency responses before and after each nonlinearity. Two nonlinearities will be investigated in the M- and L-cone pathways: one, which is compressive, causes flicker bursts to change in apparent colour and saturation, while the other, which is expansive, causes them to change in apparent brightness. Our expectation is that we will be able to determine separately the early and late properties of the two principal human postreceptoral pathways: the putative chromatic and luminance pathways, and thus provide new insights into the postreceptoral organization of the human visual system and the processing changes going on in them. The colour and brightness changes that we will investigate are distinct: first, because they can occur separately, and, second, because they are consistent with different types of nonlinearities (compressive or decelerating in the case of colour; expansive or accelerating in the case of brightness) and different percepts. Taken as a whole, these observations suggest the exciting possibility that the two nonlinearities are in the luminance and chromatic pathways, the two principal postreceptoral pathways in the human visual system. We believe that this project will provide a unique window into the internal workings of the human visual system, and will identify key signals and sites that will yield to physiological and/or anatomical study in other species.