Research
中视视的视杆和视锥机制
Our visual system has an incredible capacity to detect small changes in light intensity (contrast sensitivity) over a remarkably broad range of light conditions. 这是通过杆状细胞和视锥细胞分别在昏暗和明亮的光线条件下工作而实现的, and by a process known as adaptation that controls the sensitivity and speed of the visual responses in rods, cones, and downstream neurons. In dim lights, 当视杆细胞驱动视觉反应时(暗视), 对比敏感度很差，仅限于缓慢变化的光线水平(见图)。. 在明亮的光线下，视锥细胞介导视觉反应(感光视觉), 对比灵敏度增加并扩展到光照水平的快速变化. 在中视光水平，视杆细胞和视锥细胞都对视觉有贡献. Rod photoreceptors are highly specialized for discrete detection of single photons when dark adapted and have slow responses under these conditions. 视锥细胞对光不太敏感，具有内在的更快的反应，特别是在白天. Thus, 几十年来，人们一直认为视杆细胞在昏暗的环境中起辅助作用, detecting only slow contrast changes, 而视锥细胞在对比度变化迅速的中观和明亮场景中起辅助作用.
Figure 1. 视觉敏感度取决于平均光照水平, 光的速度变化和光感受器的类型. Rods drive vision to slow variations in scotopic (dim) lights (purple sensitivity function) while cones drive responses in photopic (bright) lights (green sensitivity function). x轴表示光的时间变化频率.
However, 美国人86%的时间都呆在室内, exposed to artificial lighting, including that of our many video screens. At indoor luminance levels (100-200 cd/m2) rods are producing ~1000 to 10000 R*/s. 而早期的研究表明，哺乳动物的视杆细胞在这种光照水平下是饱和的，没有反应, we and others have shown that, in fact, rods respond to light variations. Hence, 中观范围远远超出了暮光条件, 进入我们大部分时间都在使用的室内照明范围. The goal of our lab is to understand how rod and cone- driven vision operates at the intermediate (mesopic) lights, 人在室内环境下的主要视觉模式(图1). We 测试视觉背后的细胞机制 使用包括视网膜电生理技术在内的综合方法在介视光下, transgenic mouse models, animal visual behavior, and mathematical modeling.Neural mechanisms underlying mesopic visual behavior in mouse:
为了更广泛地有效地探索视网膜机制的复杂性，从而揭示中观视觉行为, 我们开发了一种操作性条件反射方法来测量小鼠的时间对比敏感性. 小鼠的视运动反射被用作视网膜颞加工的代理. However, 皮层下反射通路驱动视运动反应，而不是皮层和决策区. In addition, the use of the optomotor response assay is sub-optimal for the study of temporal contrast sensitivity (TCS) because of a)在速度处理过程中相互作用的潜力 vs 漂移光栅刺激的时间频率特性 b)动态反应范围大大小于视网膜反应. To solve this problem, 我们已经开发了一种操作性条件反射试验，可以确定行为小鼠的TCS功能. Indeed, 视运动响应的时间分辨率限制在~12 Hz, 而用我们的新操作方法测量的视觉反应扩展到~50 Hz.
行为小鼠的时间对比敏感性与人类的心理物理学具有相同的基本特性. Umino et al., 2018, eNeuro.
(A)我们的操作行为实验通过强制选择视觉任务来测量行为小鼠的TCS. 我们应用信号检测理论来估计可判别因子(d*)，是一种独立于反应偏差和动机的绩效衡量标准. (B) With this approach, we established in the mouse a model of human vision that shares fundamental properties of human temporal psychophysics such as Weber adaptation in response to low temporal frequency flicker and illumination dependent increases in critical flicker frequency as predicted by the Ferry–Porter law. (C)人体TCS功能改编自Kelly (1961).
自由行为小鼠瞳孔光对稳定光的反应. Bushnell et al., 2016, J. Neurosci. Methods .
因为轻微的约束或轻微的麻醉会影响瞳孔的反应, the ability to determine retinal illumination levels in freely behaving mice is critical to our goal of relating retinal neuronal activity to behavior. We implemented a system to measure the pupillary light response to steady lights of freely behaving mice using a custom-built, 可以自动获取眼睛特写图像的便携设备.
In our system, 一只自由活动的老鼠在一个定制的房间里探索房间侧壁的一个开口. Outside the chamber and suspended from the lid of the chamber is a video camera that captures infrared images of the lateral view of the mouse every time its nose crosses the aperture. LEDs provide IR illumination. 平均瞳孔面积离线测量，并绘制为亮度的函数.
杆状和锥状驱动的视觉反应在中视光中互换角色. Pasquale et al., 2020, JNeurosci.
我们用瞳孔计和操作行为法测量了WT小鼠的TCS, GNAT1 KO mice (dysfunctional rods) and GNA2 KO (or GNAT2cpfl3) mice (with dysfunctional cones) to investigate the contributions of rods and cones to mesopic vision. (A), 在昏暗的灯光下，WT和G2小鼠的杆状体传递相对缓慢的时间变化. (B), However, in daylight conditions, 杆状通路对快速的时间变化表现出高度的敏感性，而对缓慢的时间变化则不敏感, 而锥体驱动的响应弥补了杆状驱动的灵敏度对缓慢时间变化的损失. (C), Our findings highlight the dynamic interplay of rod- and cone-driven vision as light levels rise from night to daytime levels. 功能图说明了杆和锥驱动的响应作为光照和频率的函数. For example, cones drive the responses to low frequencies (<6 Hz) at 10000 ph/s/um2 while rods responses to high frequencies (> 6 Hz) (indicated by the grey arrow).
Furthermore, the fast, 道路驱动信号不需要过去提出的道路与锥体之间的Cx36间隙连接, but rather, 是否可以通过替代的不依赖于cx36的杆状通路(此处未显示, but see Pasquale et al 2020 for details).
In summary, using transgenic mice to selectively assess the functional contributions of rods and cones to visual behavior, 我们发现，在室内(中观)光水平, rods drive the visual responses to fast — not slow — temporal variations. Remarkably, 锥细胞对快光变化的敏感性较差 at mesopic intensities. 当光照水平从夜间上升到中等水平时, rod vision shifts from low frequency detection to high frequency and cones take over as the low frequency detectors.
目前在实验室的项目建立在这些有趣的发现和关注以下问题:
1)中观杆快速反应的神经机制?
2) How do rod-driven visual responses to high frequencies in mesopic lights determine behavioral sensitivity in natural environments?
Keep posted for future results!
Another major line of research in the lab investigates the retinal mechanisms that determine temporal resolution and limit contrast sensitivity. Umino et al., 2019, J Neurosci.
We determined that R9AP95 mice (with fast photoresponse recovery kinetics caused by overexpression of the transducin GAP complex) exhibit increased behavioral TCS to (A) low (6 Hz) but not (B) high (21 Hz) temporal frequencies at retinal irradiance levels ranging from 100 to 4000 ph/s/mm2. 这些光线水平对应于老鼠视觉的中视范围. This is also the range where mouse TCS exhibits rod-driven Weber adaptation in response to 6 Hz flickering lights (TCS remains constant as irradiance increases). TCS to 21 Hz flicker did not adapt to background light levels and TCS to 21 Hz flicker is independent of the level of R9AP expression. Our results established that rod photoresponse kinetics limit temporal contrast sensitivity to low temporal frequencies in mesopic vision.
The photoresponse recovery time-constant controls the magnitude (and phase) of the pharmacologically isolated ERG flicker responses. Umino et al., 2019, J Neurosci.
(A), The pharmacologically isolated flicker ERG responses of WT and transgenic mice with fast photoresponse recovery kinetics (R9AP95 line) grew differentially at irradiance levels > 80 ph/s/mm2. (B), Flicker responses of R9AP95 mice had markedly higher amplitudes and faster responses (as inferred from the relative phase advance of the waveforms) than WT mice. Recordings with mice with the GNAT2cpfl3 背景资料证实这些反应来自杆状细胞. (C), A simple quantitative model satisfactorily explains the increase in the magnitude of the flicker responses in R9AP95 mice in terms of their faster photoresponse kinetics.
Mesopic temporal contrast sensitivity increases 尽管视网膜色素变性小鼠模型具有快速棒恢复动力学的光感受器变性. Pasquale et al., 2021, eNeuro.
Uncharacteristically fast rod recovery kinetics are facets of both human patients and transgenic animal models with a prevalent cause of retinitis pigmentosa: a P23H rhodopsin mutation (RhoP23H/+). We found that these mice exhibit a 1.2 to 2-fold increase in retinal (A, B)和视运动(C) TCS，尽管有明显的光感受器变性. A simple linear-non-linear model suggests that the increase in sensitivity can be explained by the change in rod kinetics. Measurement of TCS could be used as a non-invasive early diagnostic tool indicative of rod dysfunction in some forms of retinal degenerative disease.
A current goal of the lab is to identify the phototransduction and membrane mechanisms that increase the magnitude of rod contrast responses when the kinetics speed up, in health and disease conditions.