A
B D F H
C E G
The retina of the human eye in section. Vertebrates
such as humans and fishes see through the
following layers: nerve fiber layer (A),
ganglion cell layer (B), inner plexiform
layer (C), inner nuclear layer (D),
outer plexiform layer (E), and outer
nuclear layer (F). Only then come the
photoreceptors (G) and the pigment
cell layer (H). The arrows show the
direction of the light. In octopus
eyes, the light passes directly onto
the photoreceptors.
ceive the world through milk glass,
which requires comprehensive con-
trast-enhancing “image processing”
within the retina, as explained in the
sidebar (p. 41). The invertebrate eye is called
an inverse eye because of the inverted position of
the retina.
OCTOPUS EYES
In the octopus, the eye does not develop from the brain
in the embryonic phase, but through an invagination of
the external body surface. This has the effect that the ret-
ina is reversed, so the light coming from the lens hits the
photoreceptors directly without having to pass through
a layer of nerve cells. As a result, the octopus perceives
its environment with much more contrast than humans
do. One might say that the octopus eye is closer to the
ideal than the human eye. Because of the position of its
retina, it is called an “everse eye.”
A VERTEBRATE TRICK: LATERAL INHIBITION
The vertebrate eye uses a complex trick called “lateral
inhibition” to achieve a contrasting image. This term de-
scribes the interconnection principle of photoreceptors,
in which an active cell inhibits neighboring cells. The
photoreceptor receives light of a certain wavelength and
is excited. Most of this excitation signal is transmitted
to the brain. However, a small portion of it is converted
into an inhibitory signal and di-
rected to the adjacent pho-
toreceptors via inhibitory
internal neurons. If a
neighboring receptor
is also illuminated
and fully excited,
this inhibition is
negated. However,
if the neighbor-
ing receptor is only
weakly excited, its
signal to the brain is
even weaker, and the
inhibition increases
contrast enhancement.
Thus, the vertebral eye
compensates for the “milk-
glass” effect of its inverse reti-
nal layer.SEEING SHARPLY
The eyes of all vertebrates function in basically the same
way and go back to the same origin, the common pre-
decessor of fishes and land animals, but the fish eye dif-
fers in having special adaptations to life under water. For
example, it uses a somewhat different principle of focus-
ing. The lens eye of a land animal produces visual acuity
by deforming the lens and changing the refractive power
by flattening. For this, this lens is suspended by zonula fi-
bers, which exert tension via the ciliary body. This works
only because the lens is flat, because in the atmosphere
the refractive power of the lens must be stronger than
one used under water.
The fish eye, on the other hand, still has the original
spherical lens. The fish can afford it because the index
of refraction in the liquid eyeball is almost identical to
that of the surrounding salt water. However, a fish can-
not deform this spherical lens as easily as a land animal
can. Fishes do not produce visual acuity by modifying the
lens shape and thus changing its refractive power, but
by changing the lens position and thus the distance toThe everse eye of the
cephalopod (left) and the
inverse eye of the vertebrate
(right) in comparison; light
incidence in the direction of
the arrow, nerve fibers red,
photoreceptors orange. In
cephalopods, the light strikes
the photoreceptors directly,
while in vertebrates it passes
through a thick cell layer first.
nerve fibers (red) nerve fibers (red)photoreceptors (orange) photoreceptors (orange)