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11/15/2006
Eyes
Last week I closed with mention of a theory of autism involving
a deficient mirror neuron system in an article in the November
Scientific American by Vilayanur Ramachandran and Lindsay
Oberman. I’ve since read the article more thoroughly and was
impressed by a simple experiment that supports their theory.
Remember that last week I described studies in which the pattern
of activity in a monkey’s brain was the same if the monkey
picked up a piece of fruit or if the monkey saw a human pick up
the fruit. The neurons in the monkey’s brain mirror the activity
whether it’s performed by the monkey itself or by the human.
Ramachandran and Oberman measured brain activity using
electroencephalograms (EEGs) of a group of normal and selected
autistic children. An EEG is a plot of amplitude (intensity) of
electrical activity in the brain against frequency. In both the
normal and autistic children’s EEGs, there’s a strong peak
known as the Mu wave, which stands out among the squiggles in
the rest of the EEG. When the children were asked to open and
close one of their hands, the Mu wave was suppressed, falling to
about half of its intensity before the opening and closing of the
hand. This was true for both the normal and autistic children.
The children were then shown a video of a hand opening and
closing. As in the monkey experiments, the normal children’s
EEG patterns were the same as when they opened and closed
their own hands - the Mu wave was suppressed. The autistic
children’s EEGs were unaffected when they watched the video
clip - no suppression of the Mu wave. Their mirror neurons did
not store and/or retrieve any memory of the action. With this
deficiency in the mirror neuron system, the autistic child doesn’t
recognize an action and its consequences. The theory says this
helps explain their failure to interact normally.
While the autistic children were not properly interpreting what
their eyes were seeing, at least they could see. Last week I saw
in the Star-Ledger a report by Rick Weiss of the Washington
Post headlined “Procedure restores vision to blind mice”. Weiss
cited the work of a group led by Robin Ali of London’s Institute
of Ophthalmology published in Nature. The blind mice had a
genetic tendency to develop blindness similar to that of macular
degeneration, the leading cause of blindness in us older humans.
As one who has just a faint touch of macular degeneration, I’m
naturally interested in anything pertaining to the malady.
The researchers took immature rod cells from the retinas of
young mice and injected them into the retinas of the blind mice.
The cells were not stem cells but were cells that had already
committed to becoming rod cells. After a few weeks, the blind
mice were exposed to light. Their brains responded and their
pupils narrowed, indicating a response to the light. Autopsies
showed the implanted cells had indeed connected with nerve
cells that send optical signals to the brain. This work on mice no
doubt has a long way to go before any similar treatment for
human blindness but it seems a major step forward.
How did the eye evolve in the first place? Two recent articles
touch on this subject: a November National Geographic article
“A Fin is a Limb is a Wing” by Carl Zimmer and a September 29
Science article “Casting a Genetic Light on the Evolution of
Eyes” by Russell Fernald. The former article compares embryos
of a 1-day old fish, a 3-day-old chicken and a 32-day-old human.
They all look remarkably alike. However, a 3-day-old fish
embryo is developing fins, a 12-day-old chicken embryo has
rudimentary wings and a 56-day-old human embryo has partially
developed arms, all these in about the same general location.
The main theme of the Geographic article is that evolution
generally occurs in small steps in a way that uses and modifies
existing genes and structures to produce change. Zimmer cites
the single-celled choanoflagellates as an example. These are
commonly found in creeks or marshes and look something like a
tadpole. When Nicole King at the University of California,
Berkeley and her coworkers analyzed them they found proteins
thought to be present only in animals, not in single-celled
critters. Surprisingly, some of these proteins were “adhesive”
proteins that help cells stick together in an orderly fashion in
animals such as us humans with ten trillion cells or so.
This finding suggests that hundreds of millions of years ago,
when conditions were ripe, the proteins were available for more
than one cell to stick together and multi-celled creatures were on
their way. What about the eye? Creationists cite the eye as
impossible to have developed via evolution and even Darwin was
disturbed by its complexity. In the Science article, Fernald says
the eye may have developed independently some 40 different
times. In the Geographic article, Zimmer shows that the
evolution of an eye may not have been all that complex a task
once a chemical compound sensitive to light had formed. A
class of such compounds did form - opsins. Once you had
opsins, proteins that responded differently to light, it would be
nice to have some kind of transparent stuff that could cover and
protect the opsins.
Transparent stuff was available – crystallins, a class of proteins
that had other uses. For example, there’s a crystallin in the
nervous system of the sea squirt. Let’s look at how an eye might
evolve. It turns out that in mollusks, for example, there are a
variety of different types of eyes that can provide a clue or even a
scenario for the development of a complex eye such as our own.
Zimmer shows the eyes of a number of the critters that inhabit
some of the seashells I pick up on the beach on Marco Island.
The limpet’s eye is sort of like a wide old-fashioned champagne
glass that was used before we got smart and realized the bubbles
lasted longer in a taller, narrower glass.
The limpet eye contains a layer of light-sensitive cells lining the
bottom of the “glass” with a layer of transparent cells covering
and protecting the light-sensitive cells. With the wide-open
structure the limpet can sense light but there’s no image and not
much directional sense since the light enters from a wide range
of directions. However, the chambered nautilus eye is more like
a brandy snifter, tapering to a small opening at the top, close to
being a pinhole. Camera buffs know that a pinhole focuses light
and the nautilus presumably can see a dim image. I’ve never
found a nautilus on Marco but I have picked up murex shells.
The murex eye has a covering, a cornea, and the eye cavity is
filled with fluid, which can act as a lens. The murex should see a
sharper image.
With the octopus, you’re talking an eye that has it all – a cornea,
a colored iris and a lens that focuses. It’s not hard to imagine
that the first creatures to develop a rudimentary eye would have a
leg up on other creatures of the sea. When the octopus showed
up with its fancy eye, I imagine it had a ball gobbling up its prey.
The animal kingdom is divided up into over 30 major classes, or
phyla. Fernald says that about a third of those phyla have never
developed the eye. Together with the mollusks, we can see how
over millions of years the eye could evolve.
I couldn’t believe my own eyes the other night when Rutgers’
“Judge” Ito missed, then kicked a field goal with a few seconds
remaining to beat Louisville and advance Rutgers to number 6 in
the BCS ratings. As a distinguished visiting scientist (titles are
cheap when you don’t get paid) at Rutgers, I admit to having had
no interest in its football team but after 54 years in New Jersey,
I’m now a convert. Why did ESPN keep showing the Empire
State Building? As with the Statue of Liberty, the Giants and the
Jets, Rutgers is in NJ!
Allen F. Bortrum
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