Eyes....Soul's mirror.

                                                                         

Compare and contrast the properties of rod and cone vision

As we open our eyes and look around, we see a panorama of light images with the help of the retina - a light-sensitive tissue at the back of each eye which converts light energy into neural signals and then transfers these signals to the brain. This is done through the photoreceptor cells, which are in control of detecting attributes like colour and  light intensity. These photoreceptor cells are of two types: rods and cones. We are now going to look deeper into each of their individual functions and differences.

Rods and cones differ in their construction, size and shape. The basic difference, as the name suggests, is that the outer structure of rods is straight and narrower whereas that of cones is of a conical shape and wider. Not only that but they differ in their arrangement, as well. Rods contain disks and cytoplasmic space whereas cones have invaginations of cell membranes. The pigments of rods are located in these disks and the pigments of cones situated in the infoldings of the cell membrane.

Both rods and cones are sites of the transduction of light energy into neuronal signals but they, in essence, work in opposite ways. Rods respond to low levels of light at all wavelengths to generate neuronal signals but cones require high levels of light for the very same signals. This happens because cones have lesser pigments than rods and need more light to detect images.

                                                                           Rod

                                                                            Cone

The most important distinguishing factor between rods and cones is that rods are not responsible for discriminating colours and work in conditions of low light (such as dusk and nighttime) but cones are certainly in charge of colour vision and work best during the daytime. In other words, rods are generate nocturnal vision and cones create high resolution diurnal vision.

You can also say, cones have high visual sharpness, whereas rods do not. But it is due to high sensitivity of rods to dimmest illumination, that enables vision .possible

Cones are less sensitive than rods. This is because rods are receptive enough to respond to a single photon of light whereas cones require tens to hundreds of photons to get triggered. However, rods are 6 to 15 times less sensitive than cones when it comes to light increment.

To make it clearer, rods are sensitive to scattered light and saturate only in daylight, while cones are sensitive to direct axial rays and saturate only in intense light.

Rods are comparatively abundant than cones and there are about 110 to 130 million present in the human eye. Cones, on the other hand, are known to be approximately 5 to 7 million.

Cones, being narrower, are concerned with the direction of light reaching them, whereas rods are not so bothered and respond well to dispersed light.

Rods also have only one kind of photosensitive pigment, which is completely responsible for night vision and seeing black, white and shades of gray. They allow us to see when it’s very dark. In contrast, cones sense mixtures of light waves and have to do with colour vision. They have three types of pigments: red (64%), green (32%) and blue (2%). Together, all three type of these cones enable us to see a spectrum of colours.

Every cone is served by one neuron while in contrast, sets of rods are served by a single common neuron. Rods are also known to have highly-convergent retinal pathways while cones have less convergent retinal pathways.

The density of the rods is a lot higher than that of cones all over the retina but in the fovea, cone density reaches 200-fold and beats rods by miles. Moreover, cones are concentrated in the centre of the retina in the fovea while rods are located everywhere in the retina except in and around the fovea. The foveola, in fact, is entirely free of rods. Rods are also more easily procured than cones and their parts are comparatively easier to separate than the parts of cones.

Due to the fact that the sensitivity of the rod system is lesser than that of cones for higher illumination, rods do not recover quickly from bleaching lights - unlike cones. This has to do with two chemicals: iodopsin (in the cones) and rhodopsin (in the rods). Being in the light bleaches these chemicals. When you go into the dark, the bleaching stops and the chemicals restore their original levels. This is also known as dark adaptation.

Similarly, when we move from a dark area to a well-lit area, the glare gets too much to handle and it takes a bit of time for our eyes to adjust. This is because the visual inception in the cones increases due to the bleaching of iodopsin. It takes about five minutes for the eyes to acclimate. This is known as light adaptation.

The central field of vision is solely performed by cones, under high illumination, producing visual acuity of sharpest image with perfect color sensitivity. It is as much as ten times better than peripheral vision. Whereas peripheral field of vision performed by rods is highly sensitive to fading illumination, has insignificant color identification and lacks shape formation feature.

Since we have concluded that rods are responsible for night vision, the loss of rods causes night blindness, while, similarly, the loss of cones causes legal blindness. Both of these photoreceptors together work towards normal eye vision and make for the perfect visual system.

Why We Need Two Systems

The human eye is basically a specialized transducer, with the capability of converting signals of light with varying wavelengths into visual images. In order to act on electromagnetic radiations emitted from the surrounding spectrum, eye’s visual system converts such signals into visible images. For carrying out such highly specialized functions, the structure of the eye is studded with immensely delicate units of rods and cones. Both these specialized optical units perform the delicate function of converting light impulses from surrounding objects and illuminant surfaces into chemically mediated nervous signals, ultimately sending those signals to the visual cortex of brain.

Rods and cones are the basic and most vital units of the visual system. It is only these rods and cones which have the capability to catch electromagnetic impulses (photons) reaching our retina, and converting them into visual signals. The location of rods and cones is on outer retinal layer. The processing and perception of signals is done by the middle neuronal layer of retina. That only takes place when the light signals have been generated by rods and cones, after trapping photons from illuminant surfaces.

The normal visual capability of a human eye goes through a two-way processes. At first it needs to perceive strength of illuminant surface, and secondly it requires to assess strength of information of image signals. This dual role is effectively carried out by the two different systems within rods and cones. Of the two, rods have the specific function for dark vision, because of their lower electromagnetic perception threshold. Whereas cones fulfill a wide range of electromagnetic frequencies’ analysis and thus performing varied or non-specific visual activities particularly daytime visual perceptions.

The more concentrated central location of cones system at fovea, enables retina to create a more sharp and focused image with the best visual acuity. Where there is a decline in the number of cone receptors more peripherally, it makes it impossible to receive lower wavelength electromagnetic signals. It is at this region away from fovea, rods’ receptors are more abundant. So when eyes move sideways, the lower threshold for wavelength of light stimulus enables dimmer objects to be visualized easily. This is clear indication of such a delicate advantage of the dual optical system of retina.

It is these two systems of rods and cones, with stimuli from surrounding illuminant objects, that helps the retina to create sharpest of images at given time of day. The rods, with pigment rhodopsin inside enables the eye to see shades of gray, white and black. They also assist in making out the shape of electromagnetic photons, hence form surrounding objects and surfaces.

The cones system in retina is needed to perform the function of identifying colors around the focus of eyes and plays a major role for visual acuity. They work best with higher wavelength electromagnetic stimulus, hence they are meant for bright light vision. With three different types of blue, red and green color receptor cones, it becomes easier to recognize different color image stimuli.

It is quite clear that rods enable the retina of the eye to adapt to nighttime or dim illumination vision (scotopic). The limitation of rods to discriminate only in shades of white to black becomes obvious at twilight, when it is not possible to make out different colors.  The cones function in good illumination, performing color vision and resolution of finer details (photopic vision) of surrounding objects. They are so tuned up for each specific role, so no intermingling or wrong signals are transmitted for brain cortex during creation of an image.

The range between photopic and scotopic vision of the two systems of the eye is so wide that it allows remarkable working space for the human eye to changes in electromagnetic wavelength (brightness) to almost 1000,000,000 times. Having said that, unprotected exposure to laser or sun may cause severe damage to retina.

It will be relevant here to point out, that there are certain conditions, either congenital or acquired which may cause poor night time or dim light vision. Retinitis pigmentosa is one such congenital (by birth), where rod receptors are abnormal or damaged. In some cases cones are damaged. Consequently there is poor or no dark adaptation, loss of peripheral vision and in severe situation loss of central vision.

Similarly there are conditions, not uncommon, in which eyes are unable to distinguish different color spectrums. As we know that it is cones system which enables us to perceive a wide range of colors from illuminant surfaces. The three types of cones, blue, red and green, have different sensitivity to electromagnetic wavelengths. Light entering cones cells stimulates them simultaneously, sending signals to visual cortex. Here the brain cortex interprets signals into a wide range of colors. As rods have a very limited color sensitivity, the three-color cones system performs this remarkable function of recognizing the beautiful colorful world of ours.

Color blindness, as we call it, can either be due to abnormal cones or fault in the signal pathway from cones to visual cortex. This further emphasizes the existence vitality of the dual system rods and cones inside retina. Most color blind people are unable to perceive and distinguish shades of red and green.

Although rods and cones function between wide range of light wavelengths (498nm for rods and 555nm for cones) where both these receptors work independent. Still some misconception needs to cleared. There are intermediate illumination levels at which they perform the function of photon receptors simultaneously. This level of wavelength at which photopic (cones function) and scotopic (rods function) is at transition level of illumination of twilight (dusk) is known as mesopic vision. At the illumination level of dusk, both rods and cones system are not working at full potential, still they are active in visual perception. When these two systems are active together, it plays significant role during night aviation.

The range of intensities of perception from illuminant surfaces between scotopic (rods system) and photopic (cones system) becomes evident when light starts fading. At this level of photon (electromagnetic impulses) the color discrimination becomes impossible. Still, due to higher sensitivity of rods, even to dimmest of light sources, retina can perceive signals and send them to visual cortex in dim gray shades for image build up. This clearly shows the vitality of two systems vision in human retina.

Both these photoreceptors, so vital structures located inside retina, function to compliment and not weaken human vision. The structure, pigments and concentration of rods and cones in retina allows each to perform specific visual response. Rods furnishing dim light information and object movements, whereas cones giving sharp details and color patterns of surroundings

The manner in which both systems work is in perfect sequence and harmony. They never intermingle under specific daytime or night time conditions but adjunct each other at one particular time of day.