Christian Apologetics Related to Science
Inverted Human Eye
a Poor Design?
22-600 State Rt. 34
Archbold, OH 53402-9542
on Science and Christian Faith 52 (March 2000): 18-30.
© 2000 by the
American Scientific Affiliation
It is often claimed
that the human retina is poorly designed because it appears to be placed in the eye
backwards. Its design, therefore, requires that light travel through the nerves and blood
vessels to reach the photoreceptor cells located behind the eyes wiring. We now know
that specific functional reasons exist for this so-called backward placement of the
photoreceptors. A major reason for the retina reversal is that it allows the rods and
cones to interact with the retinal pigment epithelial cells that provide nutrients to the
retina, recycle photopigments, provide an opaque layer to absorb excessive light, and
perform other functions. This design is superior to other systems, because it allows close
association with the pigmented epithelium required to maintain the photoreceptors. It is
also critical in both the development and normal function of the retina.
A major argument for the existence
of a Creator is called the Argument from Design. Proponents claim that the design existing
in creation proves the existence of an Intelligent Designer. Darwinists try to disprove
this observation by providing examples of what they claim is poor design in order to
demonstrate that the natural world is in fact not designed, but is the result of blind,
natural, impersonal forces. This view is called the blind watchmaker thesis by
Dawkins.1 One of the most common examples
of poor design used by Darwinists is the human retina. A common claim made in both the
popular and scientific literature to support the blind watchmaker thesis is that the
vertebrate eye is functionally sub- optimal because the retina photoreceptors are oriented
away from incoming light.2 Dawkins
explains why he considers this an example of poor design:
Any engineer would
naturally assume that the photocells would point towards the light, with their wires
leading backwards towards the brain. He would laugh at any suggestion that the photocells
might point away from the light, with their wires departing on the side nearest the
light. Yet this is exactly what happens in all vertebrate retinas. Each photocell is, in
effect, wired in backwards, with its wire sticking out on the side nearest the light. The
wire has to travel over the surface of the retina, to a point where it dives through a
hole in the retina (the so-called blind spot) to join the optic nerve. This
means that the light, instead of being granted an unrestricted passage to the photocells,
has to pass through a forest of connecting wires, presumably suffering at least some
attenuation and distortion (actually probably not much but, still, it is the principle of
the thing that would offend any tidy-minded engineer!). I dont know the exact
explanation for this strange state of affairs. The relevant period of evolution is so long
Williams claimed the
retina is not just an example, but one of the best examples of poor
design in vertebrates, proving that the blind watchmaker, not an intelligent
creator, created life. He notes:
organism shows features that are functionally arbitrary or even maladaptive
chosen classic is the vertebrate eye. It was used by Paley as a particularly forceful
part of his theological argument from design. As he claimed, the eye is surely a superbly
fashioned optical instrument. It is also something else, a superb example of maladaptive
historical legacy. The retina consists of a series of special layers in the functionally
appropriate sequence. A layer of light-sensitive cells (rods and cones) stimulate nerve
endings from one or more layers of ganglion cells that carry out initial stages of
information processing. From these ganglia, nerve fibers converge to form the main trunk
of the optic nerve, which conveys the information to the brain. All layers are served by
blood capillaries that provide their metabolic requirements. Unfortunately for
Paleys argument, the retina is upside down. The rods and cones are the bottom layer,
and light reaches them only after passing through the nerves and blood vessels (emphasis
Williams admits that
the vertebrate eye still functions very well despite the backward retina design, but
argues that this fact does not negate his basic argument. He says:
the fact of
maladaptive design, however minimal in effect, spoils Paleys argument that the eye
shows intelligent prior planning, and the visual effect is real and routinely
This topic is of great
interest to creationists. As Diamond notes:
[of all of our
features] none is more often cited by creationists in their attempts to refute natural
selection than the human eye. In their opinion, so complex and perfect an organ could only
have been created by design. Yet while its true that our eyes serve us well, we
would see even better if they werent flawed by some bad design. Like other cells in
our bodies, the retinas photoreceptor cells are linked to a network of blood vessels
and nerves. However, the vessels and nerves arent located behind the photo-
receptors, where any sensible engineer would have placed them, but out in front of them,
where they screen some of the incoming light. A camera designer who committed such a
blunder would be fired immediately. By contrast, the eyes of the lowly squid, with the
nerves artfully hidden behind the photoreceptors, are an example of design perfection. If
the Creator had indeed lavished his best design on the creature he shaped in his own
image, creationists would surely have to conclude that God is really a squid.6
Thwaites argues that
the inverted retina problem hits at the core of the design argument, and that
historically the design argument was a major basis of theism. He says:
straight out of creationist tracts involves the vertebrate eye that humans must share with
the other vertebrates
the vertebrate eye shows poor design when compared to the eye
evolved by the cephalopods. The vertebrate eye has a blind spot where the retinal nerves
and the blood vessels exit the eye. There is no comparable blind spot in the cephalopod
eye. The structures of the retinas spell the difference. Everything a vertebrate sees is
seen through the nerves and blood vessels of the retina since the photosensitive elements
of the retina are on the far side of the retina away from the light source. Clearly the
cephalopod solution to retinal structure is more logical, for they have the photosensitive
elements of the retina facing the light. Certainly the creationists need to explain why we
got the inferior design. I had thought that people were supposed to be the Creators
Williams adds that
our eyes, and those of all other vertebrates, have the functionally stupid
upside-down orientation of the retina and that the functionally sensible
arrangement is in fact what is found in the eye of a squid and other mollusks.8 An evaluation of this
argument reveals it is not only naive but grossly erroneous.
inversion of the retina is considered a suboptimal design primarily due to its simplistic
comparison with a camera. In Diamonds words, placing the rods and cones at the
bottom layer and requiring light to pass through the nerves and blood vessels is the
opposite of how a sensible engineer would have designed the eye. He adds a
camera designer who committed such a blunder would be fired immediately.9
And Edinger concluded: The vertebrate eye is like a camera with the film loaded
if an engineer at Nikon designed a camera like that, he would be
conclusion is based not only on the assumption that placing nerves and blood vessels in front
of the retina reduces the retinas overall effectiveness but that another design as a
whole would be superior.
that face the eye lens are called verted, and photoreceptors that face the back of
the eye are called inverted.11 Verted eyes are wired so the photoreceptors face toward the light and the
nerves are placed behind the photoreceptor layer.12 Most invertebrates possess a verted-type eye; most
vertebrates (including mammals, birds, amphibians and fish) possess an inverted-type eye.
Most verted eye types are very simple, although a few types, such as the cephalopod eye
(squids and octopus), are almost as complex as the vertebrate eye. Verted eyes tend to be
functionally inferior, a conclusion usually determined by measuring performance in
response to visual stimuli. Even the better verted eyes are still overall quite
inferior to the vertebrate eye.13
In contrast to
the claims of Dawkins et al., no evidence exists to support the claim that even the most
advanced verted eye is superior to the inverted eye.
The most advanced
invertebrate eye is that of certain cephalopods.14 The major anatomical difference between the human eye and
the advanced cephalopod eye, such as the octopus, is the retina, which is not only verted
but also lacks a fovea centralis. As an underwater animal which usually lives on the ocean
bottom, the eye of the octopus is designed to detect motion, not detail as is true of
human eyes, and must maximize its utilization of light since the ocean usually has little
or no light at its lower depths. Barnes notes:
The cephalopod eye
undoubtedly forms an image, but the animals visual perception is certainly quite
different from that of man, which is greatly dependent upon interpretation by the brain.
The cephalopod optic connections appear to be especially adapted for analyzing vertical
and horizontal projections of objects in the visual field.15
The visual system used
by cephalopods is poorly understood partly because understanding it is not a funding
priority and partly because it is so complex. Meglitsch notes: The cephalopods have
the most highly developed nervous systems to be found in invertebrates, and
correspondingly complex behavior patterns.16
In contrast to the
claims of Dawkins et al., no evidence exists to support the claim that even the most
advanced verted eye is superior to the inverted eye. As Ayoub asks:
of thousands of vertebrate speciesin a great variety of terrestrial, marine and
aerial environmentsreally see better with a visual system used by a handful of
exclusive marine vertebrates? In the absence of any rigorous comparative evidence all
claims that the cephalopod retina is functionally superior to the vertebrate retina
remains entirely conjectural.17
Judging by physiology,
the verted cephalopod retina is clearly inferior to the inverted retina. Wells notes:
Compared with the
vertebrate retina, the retina of Octopus is very simple. There are no equivalents of
amacrine, bipolar or ganglion cells in the cephalopod; peripheral processing of the visual
input must be much simpler.18
The octopus eye also
contains a complex nerve plexus posterior to the receptors.19 Wells adds that the optic lobes must assume
many of the functions of the inverted retina in vertebrates so that the apparent
relative simplicity of the cephalopod system is an illusion. It is a matter of stacking;
amacrines, bipolars and ganglion cells are all there, but stuck onto the outer layer of
the optic lobe rather than onto the back of the retina.20
that although cephalopods can perceive shape, light intensity, and texture, they lack many
of the advantages of an inverted retina, such as the ability to perceive small details.21 The visual system of the cephalopods is
designed very differently than the inverted eye in other ways to enable them to function
in their dark, water world. They can see only in black and white and have a narrow range
of vision compared to humans. Their photoreceptor cell population is composed of only
rods, and they contain a mere twenty million retina receptor cells compared to 126 million
in humans.22 The rods outer segments contain rhodopsin
pigment that has a maximum absorption in the blue-green part of the spectrum (475 nm),
which is the predominant color in their environment. Photons change the rhodopsin to
metarhodopsin and no further breakdown nor bleaching occurs.23
A second pigment in
the octopus, retinochrome, has an absorption maximum of 490 nm, which is more
sensitive to dim light. It evidently serves a supplementary role in the octopus vision
system.24 Humans have one rod
type and three cone types. One cone type has a light frequency of 430 nm (blue), another
530 nm (green), and the other 569 nm (red). Further, in bright light the cephalopods
pupils become thin and slit-shaped, and are held in a horizontal position by an organ
called a statocyst that uses gravity to determine the horizontal.25 Evidently they scan a thin but wide area for
information, indicating that their visual world is considerably different from that of
Grzimek notes that
their visual process is quite similar to that of the batrachians, reptiles and
insects. A photograph of the recorded image is not traced on the retina as in
man; instead cephalopods record and interpret as stimuli (pattern recognition ) only light
and color variations of a moving object.27 Importantly, the octopus will respond to certain motions as
if they were prey, but will not react to his normal food-objects when they are
motionless.28 This observation of the importance of motion in vision
function is in harmony with the observation that the octopus eye can be called a
compound eye with a single lens for the reason that the receptor cells are
surrounded by microvilli which form rhabdomeres.29 Each facet in a compound eye is either on or
off, and object movement produces a change in the on-and-off pattern, similar to how a
series of light bulbs produces the illusion of movement by changing on-and-off patterns.
How the eye
evolved from the primitive verted type common
to invertebrates into the inverted eye of vertebrates is
an unexplained mystery. No
evidence exists of any transitional forms, and all known animals have either verted or
Our ignorance about
the function of major parts of the cephalopod visual system, such as the optic lobe,
prevents researchers from completing a more detailed analysis of cephalopod vision. How
the eye evolved from the primitive verted type common to invertebrates into the inverted
eye of vertebrates is also so far an unexplained mystery. No evidence exists of any
transitional forms, and all known animals have either verted or inverted eyes. Prince
[one of the essential
and] most important differences between vertebrate and invertebrate eyes is that in the
former the receptors point outwards towards the choroid, whereas in invertebrates they
mostly point inwards towards the lens. But for that obstacle we should have been deluged
with theories on the original evolution of the vertebrate eye from the invertebrate. As it
is, vertebrate visual origins have to be approached with great caution, and
is nothing indisputable which can be used to explain the origins of the vertebrate eye
from an invertebrate organ.30
The common solution,
convergent evolution, suffers from major problems and will be discussed elsewhere.
Functions of Rods and Cones
To understand the
critical function of the retinal pigment epithelium (RPE), the chemical process
required for vision must be briefly summarized. The rods and cones are
photoreceptor cells located in the retina that transduce light into electrical signals.
Rods and cones are cells that contain most of the organelles that cells normally require,
including mitochondria, Golgi complexes, a nucleus etc. So-called black-and-white
transduction occurs in the rod-shaped receptors, and color transduction occurs largely in
the cone shaped receptors.31
The inverted retina
vision system requires light to first pass through the cornea, then through the anterior
chamber filled with aqueous fluid, then through the lens, and then through the vitreous
humor fluid. Finally, before reaching the retina, light passes through the inner cell
layers of the retina, past the rod and cone photoreceptors, until it reaches the far
posterior or distal end of these cells, wherein lie the so called outer cell segments.
See Fig. 1.
The outer segment
membrane in cones folds back and forth in a pleated fashion, and in rods the pleats pinch
off to form close to 1,000 separate disks piled up like neatly stacked pennies. The outer
cell segments contain the photoreceptor light-sensitive structures including the visual
pigment, also called the photopigment. The photopigment is where the
transduction of light into receptor potentials occurs.
consist of a family of proteins that undergo physical changes when they absorb light
energy. The principal component of photopigments is the opsin glycoprotein, a
derivative of retinal (a modified vitamin A molecule). Vitamin A is derived
from carotenoids. For this reason, good vision requires a diet high in foods that
contain abundant carotenoids, such as carrots, spinach, and broccoli. Lack of vitamin A
produces night blindness or nyctalopia. Rods contain a single photopigment type
called rhodopsin (rhodo meaning rose and opsis meaning vision).
The cones contain one of three different kinds of photopigments called iodopsins: (1) erythrolabe
(which is most sensitive to red light), (2) chlorolabe (most sensitive to green
light), and (3) cyanolabe (most sensitive to blue light).32 Color vision occurs due to small variations
in the amino acid sequences of these different iodopsins, which enable the rods and cones
to differentially absorb wavelengths of incoming light.
Vision functions by a
change in the retina photopigments molecule caused by light. The molecule has a bent shape
(cis-retinal) in darkness, and when it absorbs light, isomerization occurs and the
molecule becomes the straight form (trans-retinal). This causes several
unstable intermediate chemicals to form, and after about one minute, trans-retinal
completely separates from opsin, causing the photopigment to appear colorless (for this
reason, the process is called bleaching). So that the disk rods or cones can again
function for vision, retinal must be converted from trans back to the cis
form. This resynthesis process called regeneration is aided by the pigment
epithelium cells located next to the rod and cone outer segments.
Fig. 1. A cross section of the back of the eye showing the
retina pigment epithelium and other structures of the retina. Note the structures that the
light must pass through before striking the rods and cones. Drawing by artist Richard
functions by a change in the retina photopigments molecule caused by light.
regenerate more quickly than do the rhodopsins and consequently are less dependent on the pigment
epithelium.33 A half-life of about five minutes is required for rhodopsin
regeneration in the rods compared to 1.5 minutes for iodopsin regeneration in cones.
Excessive light will cause blindness in the effected rods and cones until this
regeneration process occurs. Common examples of this phenomena include temporary blindness
after watching a very bright light flash, such as from a strobe-light or photocopy
When rods and cones
are stimulated by light, they release neurotransmitters that induce graded, local
potentials in both bipolar and horizontal cells. By this means, the rod and cone outer
segments transduce light into electrical signals. The signals are carried by the central
nervous system neurons to bipolar cells, which in turn synapse with the ganglion cells,
then to the lateral geniculate body of the thalamus and other centers in the brain
stem, and, lastly, to the primary visual center in the occipital lobe of the cortex where
the signals are interpreted by the brain.
The first level of
processing visual information actually occurs by the amacrine cells, which transmit
information between adjacent bipolar cells and ganglion cells, allowing lateral
communication in the outer retina for a comparison of information. Retinal amacrine cells
help to process visual information by enhancing certain aspects and discarding other
aspects of visual input. Input from several cells can either converge upon one
postsynaptic neuron or diverge to several post synaptic neurons. Convergence
dominates to the degree that in humans about 126 million photoreceptors send their
information to only about one million ganglion cells.
A single cone
tends to synapse with one bipolar cell whereas between six and 600 rods synapse with a
single bipolar cell. The cones one-to-one synapses give them much higher visual
acuity but lower sensitivity. In contrast, rods are extremely sensitive to light but their
visual acuity is not as sharp. Horizontal cells transmit inhibitory signals
to the bipolar cells, which enhance the contrast between areas of the retina that are
strongly stimulated and adjacent areas that are more weakly stimulated. Lastly, ganglion
cell axons that form the optic nerve carry the information to the brain for processing.
The Function of the Retinal
As noted, vision
depends upon the isomerization of 11-cis-retinal to 11-trans-retinal. Each
light photon striking a photoreceptor isomerizes retinal, and billions of photons can
strike the retina at any one second. The 11-cis-retinal must be regularly replaced
to maintain the cycle, a task for which the retinal pigment epithelium (RPE) is
critical. RPE is a single cell layer thick, consisting of relatively uniform cells whose
apical end is covered with dense microvilli. RPE cells are polygonally shaped and contain
apical microvilli and basal membrane enfoldings. Their tight junction cell connections
help to seal the vitreous humor in the eyeball and contribute to the blood-retina barrier.
Posterior to the RPE is the vascular choroid layer, and posterior to that is the
connective tissue known as the sclera.
Research on the
eyes of different species has found that,
although major differences
among them exist,
the retinal pigment epithelium
shows little variation.
RPE touches the
extremities of the photoreceptors, both the rods and the cones, and the microvilli
interdigitate with their sides.35
matrix contains soluble and insoluble components which are critical to the
photoreceptor/RPE functions. Seemingly simple in appearance, the RPE has a complex
structural and functional polarity that allows them to perform highly specialized
of their major functions is to collect the used retinal from the photoreceptors. They then
use vitamin A to regenerate the retinal, after which it is transferred back to the
Vitamin A regeneration requires the RPE to manufacture retinol
isomerase and other compounds. The RPE also stores large quantities of vitamin A.
Research on the eyes
of different species has found that, although major differences among them exist, the RPE
shows little variation.38
The small variations
that exist in the RPE are due to differences in the retina structure, indicating its
critical role in the vision of all vertebrates. One study found retinol isomerase existed
in all the major vertebrates tested and was lacking in all three cephalopods tested.39 Bridges concluded that reciprocal flow of retinoids between
the retina and the site of isomerase action in the RPE is a feature common to the visual
cycle in all vertebrates.40 Since RPE cells use much energy and
nutrients, they must be in intimate contact with both the photoreceptors and the
blood supply, in this case the choroid, to carry out this critical function.41
Phagocytic Role of the
Retinal Pigment Epithelium
Another role of the
RPE is to recycle the used rod and cone outer segment membranes, the portion closest to
the RPE. This area is often referred to as the business end of the photoreceptor cells,
because it is here where the membranous disks that respond to light are located. Cones
usually contain from 1,000 to 1,200 disks, and rods from 700 to 1,000. The high level of
outer segment activity requires them to be continually replaced.42 New outer segment membranes continually grow at the outer
photoreceptor segment base, adding to the photoreceptor length.
segments are renewed at an astonishingly rapid pace.43 A normal rod photoreceptor sheds about 10% of its outer
segment disks at its apex and renews the same amount daily.44 As the outer segment lengthens from its base, the oldest membrane, which is the
distal end, is shed in segments of one to three disks at a time. Those sloughed off are
phagocytized by the RPE in order to recycle its parts.45 This process is continuous, effectively maintaining the high sensitivity of the
photoreceptors.46 A summary of this cycle by Bok and Young is as follows:
pigment epithelium carries out several functions that are crucial for the normal operation
of the visual system. One of these important roles, appreciated for about a decade, is the
phagocytosis of rod outer segment debris. This scavenging activity goes on daily at an
impressive rate in the normal retina. It can be accelerated to extraordinary levels when
outer segments are damaged. Disruption of this phagocytic function may underlie a variety
of clinical disorders, some of which result in blindness.47
interdigitate and surround the photoreceptor outer segments to effectively carry out their
phagocytic and recycling role. The first step in phagocytosis is recognition
(causing their binding to the RPE apical microvilli) followed by invagination and,
lastly, ingestion by phagosomes. Pseudopods that engulf the rod and cone outer receptor
fragments are controlled by actin in much the same way that single-celled animals, such as
the amoeba, use pseudopods to consume food. The RPE then breaks down the ingested material
by enzymes stored in its lysosomes. Lastly, the free radicals and superoxides produced by
enzyme action in the RPE must be neutralized by superoxide dismutase, peroxidase, and
Nutrient Role of the
Retinal Pigment Epithelium
The RPE selectively
transports nutrients from choroidal circulation to both the photoreceptors and retinal
cells. The RPE cell-tight junctions prevent diffusion of even small molecules into the
vitreous humor and insures that the metabolites required by the outer retina can move to
where they are needed.49 The RPE has a function like that of a
placenta to insure that the outer retina is protected from injurious compounds and yet
allows the necessary nutrients to pass into the area of the rods and cones. To insure that
enough of the needed nutrients pass the RPE barrier, the basal membrane is highly enfolded
to produce more surface area. This role is critical because the rods and cones require a
greater blood supply than any other bodily tissue.50
Which compounds pass though are determined by basal membrane receptors. RPE also
synthesizes and secretes extracellular matrix molecules.
interference of light as it traverses several layers of retina before reaching the
photoreceptors in an inverted eye is overcome by visual processing. When bipolar or
amacrine cells transmit excitatory signals to ganglion cells, the ganglion cells
become depolarized, initiating a nerve impulse. Nerve impulses travel along axons of the optic
(II) cranial nerve, leading to the optic chiasm where some fibers cross over to
the opposite side and some remain on the same side (chiasma means cross as shown by
the letter X). On the other side of the optic chiasm, the fibers are named the optic
tract and synapse with neurons in the lateral geniculate nucleus of the
thalamus. The lateral geniculate nucleus neurons then form a passageway called the optic
radiations to carry the information for processing to the primary visual areas
in the occipital lobes of the cerebral cortex.
Each eye sees a
slightly different visual field, and the large overlapping area is called the
binocular visual field used to produce stereo vision. One eye will see a crescent-shaped
peripheral monocular visual field that the other eye cannot see, and the same will occur
on the opposite side with the opposite eye. Also, each eye has a blind spot caused by a
hole in the retina where the optic nerve must pass through in order to travel to the
brain. This blind spot falls on a different place in each retina, and the information from
both eyes is combined so that these visual blind spots are not normally perceived.
interference of light as it traverses several layers of retina before reaching the
photoreceptors in an inverted eye is overcome by visual processing.
Light rays from an
object in the temporal half of the visual field (that facing away from the nose)
will fall in the nasal half of the retina, and conversely light rays from an object in the
nasal half of the visual field will fall on the temporal half of the retina, reversing the
image as occurs when a transparent slide is projected by a slide projector. Also, light
rays at the top of the visual field strike the inferior portion of the
retina, and those at the bottom of the visual field are projected on the superior
portion of the retina, again reversing the image. Both the left-right and up-down reversal
must be corrected by the brain.
by the brain must be extensively processed in other ways as well. This complex operation
involves at least three separate systems located in the cerebral cortex, each with a
specific function. One system processes information related to shape, another
regarding color, and a third about movement, location, and spatial
organization of the object. Goldsmith concluded that the optical design of the vertebrate
eye approaches optima predicted from physics and that in the real world:
animals have a way of
confounding the assumptions and boundary conditions in hypothesized models of optimal
behavior. In dealing with the interrelated sensory tasks of maximizing spatial acuity and
contrast sensitivity, however, both the camera eyes of Old World primates and
birds, as well as the compound eyes of diurnal insects, present clear examples of
The investigators task in examining the hypothesis
of optimization is therefore to ask how closely the optical performance of eyes of
different optical design approaches the limits set by physics
Despite the very
different modes of design that underlie the construction of the single-lens eyes of
vertebrates and the compound eyes of arthropods, similar considerations determine their
capacities to resolve images.51
An area of the retina
in the central macula called the central fovea is part of the solution to the
problem of light loss due to the reversed retina. The nerve cell bodies in this area are
displaced sideways to provide a clearer path for light to reach the photoreceptor cells.52 The macula area is no larger than pencil lead in diameter
but is about 100 times more sensitive to small features than the rest of the retina.
Vision is the sharpest at the macula, which is critical in providing the brain with
information needed to construct an image. It allows us to read, watch television,
recognize friends, and even walk. Most of the rest of the retina actually is concerned
with peripheral vision. The macula provides information needed to maximize image detail,
and the information obtained by the peripheral areas of the retina helps to provide both
spatial and contextual information.
The peripheral retina
also functions to survey a large visual area for clues to determine where a person should
focus his or her macula for more input. The peripheral area does not need to pick up much
detail because its role is primarily to inform the brain of locations that may need more
informational input. This structure allows the person to be aware of a wide visual field,
yet at the same time not be distracted by it.
An area of the
retina in the central macula called the central fovea
is part of the solution to the problem of light loss
due to the reversed retina.
If the entire retina
were sensitive to the same level of detail as the macula, the brain would suffer from
sensory overload and not function properly. The sensory overload problem is well
understood from research on hyperactivity and auditory sensory overload. If the retina
were reversed so that the rods and cones faced in the direction of the light, the
peripheral area may require a means of lowering the light intensity.
The importance of the
RPE is indicated by the fact that one of the most common causes of blindness in the
developed world, macular degeneration, is a result of RPE deterioration.53 In this disease, the eyes macula
loses its ability to function, causing major central vision loss. In macular degeneration,
not only does the central vision deteriorate but the patient is less able to focus on an
object of interest. The retinal pigment cells do not replace themselves by cell division
as do most cells. Consequently, when they are damaged, the retina cells also soon die.
Demise of the RPE is often caused by intracellular accumulation of excessive levels of
lipofuscin damaged so severely that the cells native enzymes cannot properly degrade
Central serous retinopathy
is also considered to be
a RPE disorder, specifically its ion pump function, and/or a result of choroidal vascular
Detached Retina and the
Role of Pigment Epithelial Cells
The retina is
evidently held to the RPE largely by the interphotoreceptor matrix. When the retina pulls
away from the pigment epithelium at the interphotoreceptor matrix area, a detached
retina results. Fluid that accumulates between the neuron portion of the retina and
the pigment epithelium gradually forces the thin pliable retina to billow out toward the
lens of the eye. Some results of this change are visual field defects, light flashes,
floaters, and distorted vision caused by optical effects resulting from the new position
of the retina in relationship to the lens.55
the retina from the pigment epithelium also causes a drastic reduction in the rhodopsin
regeneration rate. As a result, when the pigment epithelium can no longer function to
regenerate the rods and cones, vision is distorted. Eventually the death of significant
amounts of retina tissue occurs in those areas that have become detached from the RPE. The
retinal detachment can sometimes be halted by migrating pigment epithelium cells that bond
to the separating retina, preventing its progressive separation. When this occurs a scar
is formed on the retina called a pigmentation line. If this system fails, the progressive
detachment can often be halted by laser therapy, a procedure only minimally invasive,
because laser light is able to pass through the cornea and the lens without damaging them.
Laser therapy stimulates the migration of the pigment epithelium cells, inducing the
Functions of the Pigment
The pigment epithelium
sheet consists of epithelial cells that produce organelles containing melanin granules.
Since RPE cells are located between the choroid and the retina, they often are classified
as part of the choroid instead of the retina. The melanin they contain functions to absorb
stray light, preventing the reflection and scattering of light within the eyeball, and
ensuring that the image cast on the retina by the cornea and lens remains sharp and clear.
Another function of
the pigment is to form an opaque screen behind the optical path of the photoreceptors.
This light absorptive property of the pigment is critical to maintaining high visual
acuity. Hewitt and Adler concluded that the diverse function of the retinal pigment
epithelium cells is essential for the normal functioning of the outer retina.56 For this reason, normal retinal function requires that the RPE and photoreceptors
be in close proximity. A summary of the role of the RPE is as follows:
The rods and cones are
constantly replacing the visual pigment disks. The old ones are discarded toward the
outside, where the pigment epithelium cells absorb them. Were the disks to be disposed of
toward the incoming light, we would soon expect a murky situation inside the eye. The rods
and cones take no vacation, the disks are constantly being replaced throughout our
The reason for renewal of the disks in the eye
preventive maintenance and a way of providing a fresh supply of visually sensitive
chemicals. It appears that the disks [are]
absorbed at the end of the rods.
Many animals contain a
structure called a tapetum lucidus in addition to the pigmented epithelium. The
layer called the tapetum effectively reflects the incoming light back to the rods, giving
them a second opportunity to absorb light, thus providing much greater visual acuity in
low light levels.58
The tapetum produces the
reflective eyes characteristic of nocturnal animals.59 This structure gives cat, dog, and deep-sea
or turbid-water fish eyes the distinctive glow at night called eyeshine, which
causes them to appear to be lit up. Excellent night vision allows predators to prowl at
night when competition for food and space is less. A cats night vision is estimated
to be six times better than humans. Their eyes are so effective that they can operate in
light that humans perceive as close to pitch black. The tapetums importance is
indicated by the fact that the part of the pigment epithelium which covers the tapetum is
always devoid of pigment so that there is no interference with the back reflection
of the light.60
This structure allows
a cat to see much better in dim light but at a cost of much poorer resolution and less
visual clarity during daylight.61 The cat has more rods than cones compared to humans;
consequently, it is more sensitive to low light, but has much less resolving power
and an inferior ability to detect colors compared to humans. Animals with a tapetum
usually have poor vision during daylight hours and many possess highly contractile pupils
to protect their retina.62
The Retina Pigmented
Epitheliums Role in Development
is also critical for normal vertebrate eye development. Raymond and Jackson conclude from
a series of
reciprocal cellular interactions that determine the fate of the eye components
[exists during development and the] presence of the RPE is required for the normal
development of the eye in vivo. Its presence early in development is necessary for
the correct morphogenesis of the neural retina. After the neural retina has started to
differentiate, the RPE is still necessary, either directly or indirectly, to maintain the
organization of the retinal lamina.63
The RPE actually plays
a succession of roles during embryonic development, including trophic influence, transport
functions, retinomotor response, and phagocytic and inductive interaction.64
Other Possible Designs
If the human retina
were verted, we have no evidence that vision would be better. Most likely it would be
worse. Comparisons of different eyes are difficult to make because, although the quality
of the image projected on the retina can be evaluated by a study of the lens systems
optical traits, we lack direct knowledge about the actual image produced in the brain.
A major concern,
when critiquing the existing vertebrate retina design, involves speculations on the
quality of vision that would result from another design. If the retina were reversed, the
RPE or its analog and its cellular support system would have to be placed either in front
of the photoreceptors or on their side. These approaches are clearly inferior to the
existing vertebrate system that produces superior sight for terrestrial animals. If
located in front of the retina, depending on how transparent those cells were, this design
could prevent most light from reaching the photoreceptors.
If the RPE were
located on each side of the rods and cones, as in the cephalopods, primarily only the
front of the sensory cells would be able to respond to light. Prince even claims the
cephalopods side design is protective and shields the receptors from excess
Opaque wastes would accumulate in the path of light, and
nutrients would have to be plentiful, thereby further diminishing the amount of light
reaching the photoreceptors. Surrounding each photoreceptor RPE retina cell also requires
increasing the space between the photoreceptors, further decreasing the amount of light
able to strike the photoreceptors, consequently lowering vision resolution. Recycling of
the outer segments so photoreceptors can be quickly regenerated would also be a problem,
if the photoreceptors faced the vision light path line. If the eye were designed according
to the Darwinist plan, the following would be the result:
Should the disk end of
the rods and cones be reversed in direction so as to face the light, as some evolutionists
suggest they should, we would probably have a visual disaster. What would perform the
essential function of absorbing the some 10,000 million disks produced each day in each of
our eyes? They would probably accumulate in the vitreous humor region and soon interfere
with light en route to the retina. If the pigment epithelium layer were placed on the
inside of the retina so as to absorb the disks, it would also interfere with light trying
to reach the rods and cones. Furthermore, the pigment epithelium, which is closely
associated with the disk ends of the rods and cones, also provides them with nutrients for
making new disks. The epithelium gets its nutrients from the rich blood supply in the
choroid layer next to it. In order for the pigment epithelium to function properly, it
needs this blood supply. To put both the pigment epithelium and its choroid blood supply
on the inside of the eye, between the light source and the light-sensitive rods and cones,
would severely disrupt the visual process.66
The sensitivity of the existing human
inverted design is so great that only one photon is able to elicit an electrical response.67 Consequently, functional sensitivity of
the verted retina could not be significantly improved. Ferl and Wallace note:
yet to determine how such a negative system of operation might be adaptive, but they
marvel over the acute sensitivity possible in rod cells. Apparently rod cells are
excellent amplifiers. A single photon (unit of light) can produce a detectable electrical
signal in the retina, and the human brain can actually see a cluster of five
photonsa small point of light, indeed.68
Greater sensitivity of
the inverted retina, if this were possible, may result in poorer vision due to sensory
overload. Williams syndrome patients have hearing so superior to that of normal persons
that they can hear a faint whisper. Unfortunately, this causes them serious problems
dealing with loud noises; thunder is actually physically painful.
If the human
retina were verted, we have no evidence that vision would be better. Most likely it would
Though higher visual
acuity may improve night vision, it would surely result in difficultly seeing during
daylight hours.69 This would not be functional for most people
who must work in a normal human environment. Actually, a case can be made that more
light blockage of the retina would be functional. Many people must wear sunglasses because
the outdoor light is too bright. In a review of the literature, Young found that excess
light is now a serious health problem. He notes:
All of the major
cellular and molecular features of age-related cataract have been reproduced in the
solar radiation can with similar cogency explain the distinctive global
pattern of age-related cataract among human populationsthe risk of cataract depends
on where one lives on the surface of the earth
When cataract blindness statistics
from 55 different countries of the world are grouped according to latitude, it is found
that in the tropics there is a fivefold increase in blindness resulting from cataract than
at northern latitudes, whereas intermediate latitudes fall in between. Although many
factors are involved, sunlight is the only one known to vary in a gradient from high in
the tropics to low in the northern latitudes
Current evidence provides the basis
for the design of protective lenses that minimize the hazards of sunlight exposure without
significantly interfering with vision. The prescription has two componentsone to
protect the lens, the other to protect the retina
Use of sunglasses
begin early in childhood and be continued throughout the life span whenever exposure to
bright sunlight is desirable or necessary. Radiation damage to delicate ocular structures
can occur at any age and tends to be cumulative. Even modestly effective preventive
measures may produce highly significant benefits if applied over an extended period.70
pigment in their pigment epithelium cells, and consequently they often suffer from foveal
hypoplasia. As a result, they lack the detailed central vision. They also lack iris
pigment and must wear sunglasses because even moderately bright light may severely
adversely affect their vision.71 Even blue-eyed people are at a disadvantage because the blue
pigment allows more light in than darker iris colors. Consequently, blue-eyed people
suffer from more vision problems and blindness.72 Being able to effectively read with very dim light may be an
improvement in some situations, but since most human activities occur during daylight
hours, and darkness is functional to induce sleep due to pineal gland activity, the
existing secretion system appears to be the most effective.
the light yellow tint of the eye lens filters out some ultraviolet light, the inverted eye
design serves to filter out much of the remaining ultraviolet light. The incoming light
must pass through the overlying neural components and blood vessels and the penetrating
power of ultraviolet light is markedly inferior to white light.73 The verted eye is used in animals, such as
the octopus, that live under water where most of the ultraviolet light is filtered out.
Consequently, they have less need for this protection.
Given the role of the
pigmented epithelium, it is clear that the existing design is an ideal compromise. The
main question is: Is the retina reversal an obstacle to vision? Williams notes
that the vertebrate eye works quite effectively despite the retina reversal because it is
a precise visual instrument designed to function with the rods and cones facing away from
the light. He explains:
intervening between the transparent humors of the eye cavity and the optically sensitive
layer are microscopically thin. The absorption and scatter of light is ordinarily minor,
and functional impairment seldom serious
Red blood cells are poor transmitters of
light, but when moving single file through capillaries can cause only a negligible shading
of the light sensors. In larger venuoles and arterioles they cast dense shadows and blot
out images. That we do not ordinarily perceive these shadows is the result of minute
involuntary eye movements, which keep the blood-vessel shadows moving, and of our brains
recording the flux of images as continuous pictures. The reality of the shadow of the vascular
can be demonstrated with a flashlight and instructions from a visual
Nerve cell fibers and
the small branches of the central retina artery and vein actually produce minimal
hindrance to light reaching the photoreceptors. Most cells are 60 to 70 percent water and
thus are largely transparent. In contrast to most peripheral nerves, nerve fibers in front
of the retina are not mylinated. Myelin, an opaque, whitish lipid that coats the nerves,
would block much light. These facts have forced Dawkins to note:
With one exception,
all the eyes I have so far illustrated have had their photocells in front of the nerves
connecting them to the brain. This is the obvious way to do it, but it is not universal.
keeps its photocells apparently on the wrong side of their connecting
nerves. So does our own vertebrate eye. The photocells point backwards, away from the
light. This is not as silly as it sounds. Since they are very tiny and transparent, it
doesnt much matter which way they point: most photons will go straight through and
then run the gauntlet of pigment-laden baffles waiting to catch them.75
produced by the venules and arterioles are also highly functional because they produce
momentary darkness to aid in the rod and cone regeneration. Constant bright light would
excessively bleach the photopigment, and lower light achieved by the existing design
allows their regeneration. Further, as noted above, the RPE metabolic machinery is
essential for the normal functioning of the outer retina [and] because of the nature
of these interactions, it is essential that the RPE and photoreceptors be in close
proximity for normal retina function.76
Given the role of
the pigmented epithelium, it is clear that the existing design is an ideal compromise.
The inverted eye also
produces the most acute image of all known designs. The eyes of birds not only produce the
sharpest vision of all known animals, but they can form sharp images on all areas of their
inverted retina. In addition, they have two to five times the number of cones per square
millimeter as do humans.77 Birds also rely on a large
structure that protrudes into the retina called a pectin, which most likely
replaces the embedded blood vessels in mammals. This system evidently interferes less with
vision than would a network of blood vessels, and is another reason why birds have
unusually high visual activity.78 Many reptiles have a
structure similar to the pectin called a conus papillaris, which is not pleated, is
more cone-like, and often differs in other ways from the pectin structure.79
Birds are also
sensitive to light in the near ultraviolet spectrum, and have red oil droplets in the lower
part of their eyeball cavity that enhances the contrast of objects, such as animals in a
green foliage background. Furthermore, their eyeball contains yellow droplets in the upper
area of the eyeball that enhances objects seen against the sky by filtering out much of
the blue background. The two different oils are kept separate by density differences.
These modifications help animals to see in their world but would be a major hindrance to
humans in our terrestrial world.
Claims of poor retina
design are often raised by evolutionists to argue against Intelligent Design.80 A review of research on the vertebrate
retina indicates that for vertebrates the existing inverted design is superior to the
verted design, even the system used by the most advanced cephalopods. Its design has been
maximized for life in our environment and no doubt would function poorly in another
environment, such as that experienced by undersea bottom dwellers. This review supports
Instead of being a
great disadvantage, or a curse or being incorrectly constructed, the inverted
retina is a tremendous advance in function and design compared with the simple and less
complicated verted arrangement. One problem amongst many, for evolutionists, is to explain
how this abrupt major retinal transformation from the verted type in invertebrates to the
inverted vertebrate model came about as nothing in paleontology offers any support.81
Rather than being
fired, our camera designer would no doubt be promoted for utilizing a less obvious, but,
as a whole, a far more functional design.
I wish to thank Dr.
Tara Richmond O.D. for her comments on an earlier draft of this manuscript.
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* ASA Fellow