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.
