Re: Digital camera "eye" image?
- --- In firstname.lastname@example.org, "ray scanlon" <rscan@...> wrote:
> feedbackdroids wrote:
> > Actually [and putting aside the issue of intelligent design, or
> > the retina is built inside out because the photoreceptors have anlayer.
> > enormous energy requirement, which is supplied by the choroid
> I would put it another way. When the sensory net of a (flatworm?)
> and went inside (making the first vertebrate), the eyespots thenfaced the
> interior of the neural tube. When the eye cup migrated toward thesurface,
> the retina remained facing the interior. If you wish to be avertebrate, it
> is necessary to put up with things like thatmigrate
> The cephalopod eye forms on the surface and the optical axons
> inside. The eye is right side to. How nice!first
> > Also, in nocturnal animals, photons which miss the rods on the
> > pass are reflected from the closely-positioned pigment epitheliumThis layer
> > back to the rods, which improves vision at low light levels.
> Don't think so. The flatworm eye is covered by a pigmented layer.
> transduces light energy to heat energy. Heat energy is easilysensed. When
> this eye went inside, the pigmented layer ended up facing theinside of the
> neural tube. It is not there to improve low-light vision. It isjust there,
> like the appendix is just there. Evolution can be messy..I don't know much about the "insies and outsies" of flatworm vs
vertebrate evolution, and I was mainly addressing whether the
inverted design is really a blunder, or rather a good solution from
an engineering perspective.
It seems most of the accessible info on vertebrate retinal design is
on ID sites [maybe I am reading too many of those], but here is what
one article says .... I'm afraid they hooked me when they mentioned
Helga Kolb's name ...
The photoreceptors (rods and cones) must also face away from the
front of the eye in order to be in close contact with the pigment
epithelium on the choroid, which supplies the photoreceptors with
blood. This arrangement allows a "steady stream of the vital molecule
retinal" to flow to the rods and cones without which vision would be
impossible (Kolb 2003, p. 28). The verted design, claimed by Miller
to be superior, would place the photoreceptors away from their source
of nutrition, oxygen, and retinal (the choroid). This design would
cause major problems because rods and cones require an enormous
amount of energy for their very high metabolism required in
functioning, maintenance, and repair. In addition, because of
phototoxicity damage, the rods and cones must completely replace
themselves approximately every seven days or so.
They do mention heat dissipation too ...
The photoreceptors and retinal epithelium absorb an enormous amount
of light on a continuous basis when the eyes are open. Because the
light is converted largely into heat, the retina must have a very
effective cooling system, again provided by the choroidal blood
supply directly behind the pigment epithelium. If the pigment
epithelium tissue were placed in front of the retina, sight would be
seriously compromised. Reversing the retina so that it faces away
from the pigment epithelium would also compromise sight to the degree
that sight would be impossible because the photoreceptors must be
embedded in the retinal pigment epithelium to obtain the nutrients
required to function.
Regards scattering off neural elements, they mention ...
In the retina region which has the highest resolution, the central
retina (the fovea and, in particular, the foveola), the neurons in
front of the photoreceptors are shifted to the side so that light has
a direct pathway to them, resulting in the least distortion where it
Regards reflected light off the pigmented layer, they mention ...
.... the retinal pigment epithelium (Martínez-Morales 2004, p. 766).
This monolayered tissue contains the black pigment melanin that
absorbs most of the light not captured by the retina. This design has
the very beneficial effect of preventing light from being reflected
off the back of the eye onto the retina, which would degrade the
However, certain noctural animals do have an additional layer of
tissue covering the pigment epithelium called the "tapetum", and
which does reflect light back to the photoreceptors. Witness how a
cat's eyes "glow" in the dark when a light beam is shined on them,
while humans do not glow. Similarly, the eyes of pesky racoons glow,
such as were living under the shed this winter, and won't go away.
In many species for whom vision in very low levels of illumination is
important, a layer of reflective crystalline material, the tapetum
(Latin: carpet) is incorporated in the RPE or choroid. Acting as
a mirror, the tapetum reflects light which has passed between the
photoreceptors, so augmenting the light bombarding the
photoreceptors. Hence the proverbial `cat's eyes' when caught by a
beam of light in the dark.
- --- In email@example.com, "John J. Gagne"
> --- In firstname.lastname@example.org, "John J. Gagne"
> <fitness4eb@> wrote:
> > Certainly, there's a significant difference between the design of
> > digital camera's CCD array (or CMOS if you prefer) and the humanGiven my proclivities, I very much like the behaviorist tinge you put
> > eye's retina array. While I'm sure there are many differences we
> > could discuss, the one I'm most interested in can basically and
> > simply be stated as, "The retina does not employ the RGB-
> > triad/pixel method of sampling light".
> > Question #1 Why not?
> Because "seeing" doesn't happen in snapshot frames. Seeing is not
> about capturing high resolution snapshots. It's about measuring how
> and when change happens over time.
on this post ;-).
> > Question #2 Why this crazy layout?
> This crazy layout of the retina is optimized for finding
>Whatever this means.
> > Q3 Why not triads/pixels all over the retina?As indicated, the brain moves the eye such that the fovea is centered
> Because it's simply not necessary for the purposes of "seeing". I
> think Chris hit the nail on the head when he said something like
> seeing can't be separated from processing.
upon the region of interest in the visual field. And the tiny part of
the visual field that corresponds to the fovea is greatly magnified
in its mapping/representation in the visual areas of cortex.
This clearly turns out to be an engineering compromise from an
evolutionary perspective. The brain just doesn't have enough
processing power to perform the same degree of processing on each and
every pixel in the visual field, so evolution has traded off temporal
for spatial. IE, high-res processing on a small area of visual space,
with temporal scanning to build up an internal representation, which
no doubt requires a certain amount of short-term memory buffering in
cortical areas in order to create a coherent internal image.
> > Q4 If we built a digital cameras CCD array with this type ofWhatever this means.
> > arrangement, what would a snapshot image look like?
> I think the gray scale image would be really good... But not enough
> information to render any color! But, if we cheated and manually
> colored in the dots at the correct intensity I think we could get a
> fairly good idea from such a CCD.
> > Q5 What type of processing algorithm could transform the image
> > from our modified CCD in Q4 into a photo-realistic triad/pixel
> > image
> None! None! None! The necessary information to render color is just
> not there! Difference is something different though... ;o)
> > and how fast would the computer have to be to transform, 24frames
> > per second (or whatever the rate of persistence is) of suchimages?
>Yes. Due to the spatial vs temporal tradeoff mentioned above, which
> As to this second part of Q5, I don't really know but I think this
> is an important question because it involves/is intimately related
> to the processing part.
> I think the term "resolution" has a very different meaning with
> respect to seeing then it does for photography. For a digital
> camera, resolution is all about number pixels. For a "visual array"
> it's about number of samples over time.
> Chris also mentioned how the eye is in continual motion when
> examining the field of interest. This makes perfect sense
> if "difference" defines "interest".
is no doubt related to the brain's having only a limited amount of
internal processing power, it must temporally scan the hi-res fovea
to bear upon areas of interest in the visual field. No doubt the only
reason the brain can make any sense of all this is due to short-term
memory buffering in the cortical regions.