rod-cells

New function for rods in daylight

Vision – so crucial to human health and well-being – depends on job-sharing by just a few cell types, the rod cells and cone cells, in our retina. Botond Roska and his group have identified a novel function for rod photoreceptor cells in the retina in daylight. Driven by cones and mediated by horizontal cells, rods help to increase contrast information at times when they are not directly sensing light. The retina thus repurposes its cells in different light conditions to increase the amount of visual information about the environment.

(Caption: Horizontal cells in the retina)

Task sharing in the retina seemed clear: Two different kinds of photoreceptor cells take on two different visual tasks. Rods allow us to see at night, cones operate during the day and enable color vision. However, the question as to why there are about 20 times more rods than cones in a human retina, when daytime vision is much more relevant for us, has usually led to a shrug of shoulders. It seemed a waste of resources.

Botond Roska and his group at the Friedrich Miescher Institute for Biomedical Research, could now show in a study published recently in Nature Neuroscience that the rods in mouse take on an important function during daytime vision as well.

The scientists showed that in bright light, the rods mediate a so called surround inhibition. Surround inhibition is an important feature in the retina because it allows not only to transmit information about whether a photoreceptor is exposed to light, but also about contrast. While the cone cells hyperpolarize in bright light and thus send a visual signal to the inner retina, the rods depolarize, inversely matching the activity pattern of the cone cells. The response in the rods is driven by cone cells and mediated through horizontal cells. These horizontal cells connect rods and cones through their dendrites and long axons, and at the same time form a mesh of connections among each other. The hyperpolarization of one cone thus leads to the depolarization of many surrounding rods.

During bright light conditions, the cells of the inner retina receive therefore information through two pathways: First through the well-established cone pathway, and second through this newly identified rod pathway. “We think that the surround information relayed to the inner retina through the rod pathway has different functional properties than the information obtained through the cone pathway,” comments Roska. “In any case it is fascinating to see how the retina repurposes the rod cells during bright light conditions to increase contrast information, at times when they are not directly sensing light.”

After all, these large numbers of rods don’t seem to be present in the retina in vain.

The red eye shine seen in alligators arises when light enters the eye and hits a layer of cells called the tapetum lucidum. This membrane is located beneath the photoreceptor cells (rods and cones) in the retina and reflects light back into these cells to increase the amount of light detected, which improves an alligator’s vision in low light conditions.

Several species exhibit this phenomenon, with different colour ‘shines’ observed. Most species with eyeshine are night hunters who must make use of limited light.

-Jean

Photos by Larry Lynch (http://www.lynchphotos.com/) and David Moynahan (http://www.davidmoynahan.com/)

Figuring out the rules of bacterial cell division

Bacteria “know” where to divide into daughter cells using a concentration gradient formed by the protein MinD, which oscillates back and forth in a rod-shaped cell with maxima at the ends and minima in the middle, where the cell divides. Here’s a figure of it in action with green-fluorescent protein-tagged MinD in an E.coli cell over time: 

Researchers from the Netherlands have pushed the limits of how MinD is able to define the cell division boundary by custom-growing E.coli cells into different shapes and sizes. This required chemically suppressing the E.coli’s ability to maintain its rod shape and form a new cell wall between the dividing cells, and then injecting a single cell into bacterium-scale (micrometer, or um) moulds of the desired shape, which they would then expand to fill:

Again, using GFP-tagged MinD its oscillations were tracked over time in the artificially shaped cells. Definite patterns could be seen, with the MinD preferring to travel along symmetry axes:

The cell dimensions of rectangular cells were systematically altered and different patterns were observed, with one common characteristic -for cell lengths of ~3-6um, about the length of normal dividing E.coli cells,  MinD forms 2 poles. Multiple poles appeared at 7um or greater, and lack of poles occurred at <2.5um.

The protein dynamics underlying this can be modeled by Turing’s reaction-diffusion equations

Although Turing is mostly known for his role in deciphering the Enigma coding machine and the Turing Test, the impact of his ‘reaction-diffusion theory’ on biology might be even more spectacular. He predicted how patterns in space and time emerge as the result of only two molecular interactions – explaining for instance how a zebra gets its stripes, or how an embryo hand develops five fingers. Such a Turing process also acts with proteins within a single cell, to regulate cell division.

Read more at: http://phys.org/news/2015-06-squares-triangles-bacteria-figure-alan.html#jCp

http://www.nature.com/nnano/journal/vaop/ncurrent/full/nnano.2015.126.html

25 June 2015

Switching Eyes On

Deep in the back layer of our eyeballs, light triggers cells called photoreceptors – the long, rod-shaped cells stained green in this microscope image. Once activated, the photoreceptors send messages via nerve cells into the brain that enable us to see. Sometimes these photoreceptors break down and stop working properly, causing sight loss. But because the nerves wiring them to the brain are still intact, researchers are testing whether new genetic engineering techniques – known as optogenetics – can switch light-sensitivity back on and restore sight. Using a modified virus, they’re adding a specially-designed light-activated protein molecule into cells at the back of the eye in blind mice. These molecular ‘light switches’ work amazingly well, turning on in response to light and bringing back the animals’ vision. Although it’s still early days, the exciting results bring hope that this technique could one day lead to new therapies for sight loss.

Written by Kat Arney

Image by Michiel van Wyk and colleagues
University of Bern, Switzerland
Originally published under a Creative Commons Licence (BY 4.0)
Research published in PLOS Biology, May 2015

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The red eye shine seen in alligators arises when light enters the eye and hits a layer of cells called the tapetum lucidum. This membrane is located beneath the photoreceptor cells (rods and cones) in the retina and reflects light back into these cells to increase the amount of light detected, which improves an alligator’s vision in low light conditions.

Several species exhibit this phenomenon, with different colour ‘shines’ observed. Most species with eyeshine are night hunters who must make use of limited light.

Photos by Larry Lynch and David Moynahan

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顯示器 LED 背光面板的藍光比較多?電腦王實際量測給你看
http://ift.tt/1L0UO7f 自從  LED  背光顯示面板問世之後,帶來輕薄以及省電之效,不過隨之而來的  PWM  調光造成眼睛不適的問題,以及白光  LED  的原罪--較高頻率的藍光強度較強。究竟真相為何,還是經過彩色濾光片就可調節,就來實際量測看看真相。

人眼中有 3 種主要負責感知色彩的錐狀細胞(Cone Cell),個別可感知不同波長的光,其中最敏感的波長大約分別是 565、535、420nm,加上主要感受亮度的桿狀細胞(Rod Cell),人眼就利用這 4 種細胞形成視覺。而目前顯示科技也根據此一原理,通常以紅、綠、藍 3 色光以一定比例組合模擬自然界的連續光,由於人眼運作原理的關係,我們會認為螢幕上的色彩與自然界的相似,可顯示許多種不同的色彩。

產生出 RGB 3 色的方式有很多種,以目前自體不發光,需要背光支援的液晶螢幕來說,可以使用螢光燈管和發光二極體 LED 這 2 種常見的光源,而近年來受惠於白光 LED 的研發和技術進步,LED 幾乎已完全取代螢光燈管成為液晶螢幕的主要背光來源。

白光 LED 的發光原理為產生藍光,再透過螢光粉將部份藍光轉為黃光,由於黃光部分可涵蓋到感受 565nm 和 535nm 的錐狀細胞,再加上原本的藍光,便可組合成白光。而螢光燈管的發光原理則為汞氣體放電在燈管內散發出紫外線,再交由塗布在管壁的螢光粉轉成可見光(色溫可由螢光粉比例調配)。這 2 種發光原理的特色,雖然人眼看起來都是白色的,但是實際上的光譜卻不同,螢光燈管以綠色光的強度最強,白光 LED 則是以藍光最強。


▲人眼中 3 種錐狀細胞對於不同波長光的反應,注意單一錐狀細胞並不是僅對單一波長有所反應,而是有個範圍區間。(圖片取自維基百科)

使用光譜儀讀取各波長強度

螢光燈管衰退通常是變黃,而白光 LED 通常是偏藍,更不用說以成本為導向的螢幕,一出場就會有色偏現象,此狀況在過去只是看起來比較黃而已,現在除了畫面比較藍之外,竟然有了比較傷眼的疑慮。傷眼疑慮有二,其一為 PWM(Pulse width modulation)背光控制,現在已經有不錯的解決方法,在此不贅述,讀者可以看看我們過去的文章。其二為藍光屬於高頻率的光,通常高頻率代表的就是高能量,長期下來可能會影響水晶體或是黃斑部。在此筆者不討論健康議題,只是單純的使用儀器測量出結果比較。

ColorMunki Design 是款光譜儀,主要是用來量測螢幕、紙張上的顏色進行色彩校正。與另 1 款黑色 ColorMunki Photo 不同之處在於內建軟體功能以及它不是黑色的,與市面上更常見到的 Datacolor Spyder 系列來比較,Spyder 無法分辨顏色,僅能透過濾光片瞭解光的強度,因此濾光片的設計好壞與否就是關鍵,而 Design 和 Photo 內部則是有著類似三菱鏡的分光系統,能夠量測不同頻率的光的強度。附帶說明,Design 和 Photo 內部都有紫外線濾鏡,無法量測加入螢光劑(增白劑)的紙張。

搭配 ArgllCMS 量測和製圖

除了原廠搭配的程式之外,尚有第 3 方程式 ArgllCMS 可供校正顏色,不過本次並不使用校正功能,而是使用內附的軟體讀取螢幕上的白色後,直接輸出圖片觀察。使用的軟體指令參數為 spotread -e -H -S,量測之前會先使用 ColorMunki Design 把螢幕調整至接近 6500K 色溫。


▲BenQ E2220HD 的光譜圖。(橫軸為波長,單位 nm,縱軸為強度,單位 mW/(m2.sr.nm),以下的圖亦然。)


▲Eizo FlexScan S2001W 的光譜圖。

首先來看一下目前筆者找得到碩果僅存,採用 CCFL 背光的液晶顯示器,BenQ E2220HD 為目前筆者自己使用的螢幕,Eizo FlexScan S2001W 為編輯部數年前購買的產品。圖中可以看到,2 台的峰值都落在綠色的位置,接著是紅色,再者藍色的強度雖然不高,不過從 430~500nm 都有訊號,加起來也能夠誘發藍色錐狀細胞的反應。量測的結果合乎原先假設,綠色的強度最高,如果大家有興趣的話,家中日光燈或是所謂的省電燈泡,光譜的量測結果大致上也符合此模式。


▲Dell U2312HM 的光譜圖。


▲Lenovo x230 IPS 面板的光譜圖。

想想白光 LED 的發光原理,就不難理解 LED 背光的液晶面板為何有此種光譜特性,白光 LED 主要以藍、黃光為主,黃光透過彩色濾光片再過濾出紅、綠光,因此我們可以看到峰值為藍光,接著是綠光、紅光。

由以上比較可得知,新款採用 LED 背光的面板的藍光峰值確實是高了不少,大約都集中在 440~450nm 這個波長。網路上也有人猜測白光 LED 經過濾光片後藍光強度將變弱,與從前的螢幕無異,從此實驗結果也了解這種說法是不正確的。

有了光譜儀之後可以玩的事很多,譬如廣色域和一般色域的螢幕差在哪?濾光片的設計如何影響色域呈現以及和背光的配合?低藍光模式是否有效?亦或者比較 OLED 和液晶面板的異同,讀者不妨思索討論看看。

延伸閱讀:

LCD 大拆解面板、背光完全分析:LED、IGZO、In-cell 是什麼?

LCD 螢幕色域測試圖,我也看得懂!

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