Eye, Brain, and Vision

THE EYE

The eye has often been compared to a camera. It would be more appropriate to
compare it to a TV camera attached to an automatically tracking tripod—a
machine that is self-focusing, adjusts automatically for light intensity, has a
self-cleaning lens, and feeds into a computer with parallel-processing capabili-
ties so advanced that engineers are only just starting to consider similar strate-
gies for the hardware they design. The gigantic job of taking the light that falls
on the two retinas and translating it into a meaningful visual scene is often
curiously ignored, as though all we needed in order to see was an image of the
external world perfectly focused on the retina. Although obtaining focused
images is no mean task, it is modest compared with the work of the nervous
system—the retina plus the brain. As we shall see in this chapter, the contribu-
tion of the retina itself is impressive. By translating light into nerve signals, it
begins the job of extracting from the environment what is useful and ignoring
what is redundant. No human inventions, including computer-assisted cam-
eras, can begin to rival the eye. This chapter is mainly about the neural part of
the eye—the retina—but I will begin with a short description of the eyeball,
the apparatus that houses the retina and supplies it with sharp images of the
outside world.

THE EYEBALL
The collective function of the nonretinal parts of the eye is to keep a
focused, clear image of the outside world anchored on the two retinas. Each
eye is positioned in its socket by the six small extraocular muscles mentioned
in Chapter 2. That there are six for each eye is no accident; they consist of three
pairs, with the muscles in each pair working in opposition, so as to take care of
movements in one of three orthogonal (perpendicular) planes. For both eyes,
the job of tracking an object has to be done with a precision of a few minutes of
arc—or else we see double. (To see how distressing that can be, try looking at
something and pressing on the side of one eye with your index finger.) Such
precise movements require a collection of finely tuned reflexes, including
those that control head position.
The cornea (the transparent front part of the eye) and lens together form the
equivalent of the camera lens. About two-thirds of the bending of light neces-
sary for focusing takes place at the air-cornea interface, where the light enters
the eye. The lens of the eye supplies the remaining third of the focusing power,
but its main job is to make the necessary adjustments to focus on objects at
various distances. To focus a camera you change the distance between lens and
film; we focus our eye not by changing the distance between lens and retina
but by changing the shape of the rubbery, jellylike lens—by pulling or relax-
ing the tendons that hold it at its margin—so that it goes from more spherical
for near objects to flatter for far ones. A set of radial muscles called ciliary
muscles produces these changes in shape. (When we get older than about 45, the
lens becomes hard and we lose our power to focus. It was to circumvent this
major irritation of aging that Benjamin Franklin invented bifocal spectacles.)
The reflex that contracts these ciliary muscles in order to make the lens
rounder depends on visual input and is closely linked to the reflex controlling
the concomitant turning in of the eyes.

Two other sets of muscles change the diameter of the pupil and thus adjust
the amount of light entering the eye, just as the iris diaphragm of a camera
determines the f-stop. One set, with radial fibers like the spokes of a wheel,
opens the pupil; the other, arranged in circles, closes it. Finally, the self-clean-
ing of the front of the cornea is achieved by blinking the lids and lubricating
with tear glands. The cornea is richly supplied with nerves subserving touch
and pain, so that the slightest irritation by dust specks sets up a reflex that leads
to blinking and secreting of more tears.

THE RETINA
All this intricate superstructure exists in the interests of the retina,
itself an amazing structure. It translates light into nerve signals, allows us to
see under conditions that range from starlight to sunlight, discriminates wave-
length so that we can see colors, and provides a precision sufficient for us to
detect a human hair or speck of dust a few yards away.
The retina is part of the brain, having been sequestered from it early in
development but having kept its connections with the brain proper through a
bundle of fibers—the optic nerve. Like many other structures in the central
nervous system, the retina has the shape of a plate, in this case one about a
quarter millimeter thick. It consists of three layers of nerve-cell bodies sepa-
rated by two layers containing synapses made by the axons and dendrites of
these cells.
The tier of cells at the back of the retina contains the light receptors, the rods
and cones. Rods, which are far more numerous than cones, are responsible for
our vision in dim light and are out of commission in bright light. Cones do not
respond to dim light but are responsible for our ability to see fine detail and for
our color vision.
The numbers of rods and cones vary markedly over the surface of the retina.
In the very center, where our fine-detail vision is best, we have only cones.
This rod-free area is called thefovea and is about half a millimeter in diameter.
Cones are present throughout the retina but are most densely packed in the
fovea.
Because the rods and cones are at the back of the retina, the incoming light
has to go through the other two layers in order to stimulate them. We do not
fully understand why the retina develops in this curious backward fashion.
One possible reason is the location behind the receptors of a row of cells
containing a black pigment, melanin (also found in skin). Melanin mops up the
light that has passed through the retina, keeping it from being reflected back
and scattering around inside the eye; it has the same function as the black paint
inside a camera. The melanin-containing cells also help chemically restore the
light-sensitive visual pigment in the receptors after it has been bleached by
light (see Chapter 8). For both functions, the melanin pigment must be close to
the receptors. If the receptors were at the front of the retina, the pigment cells
would have to be between them and the next layer of nerve cells, in a region
already packed with axons, dendrites, and synapses.
As it is, the layers in front of the receptors are fairly transparent and proba-
bly do not blur the image much. In the central one millimeter, however,
where our vision is most acute, the consequences of even slight blurring would
be disastrous, and evolution seems to have gone to some pains to alleviate it by
having the other layers displaced to the side to form a ring of thicker retina,
exposing the central cones so that they lie at the very front. The resulting
shallow pit constitutes the fovea.
Moving from back to front, we come to the middle layer of the retina,
between the rods and cones and the retinal ganglion cells. This layer contains
three types of nerve cells: bipolar cells, horizontal cells, and amacrine cells.
Bipolar cells receive input from the receptors, as the diagram on this page
shows, and many of them feed directly into the retinal ganglion cells. Horizon-
tal cells link receptors and bipolar cells by relatively long connections that run
parallel to the retinal layers; similarly, amacrine cells link bipolar cells and retinal
ganglion cells.
The layer of cells at the front of the retina contains the retinal ganglion cells,
whose axons pass across the surface of the retina, collect in a bundle at the
optic disc, and leave the eye to form the optic nerve. Each eye contains about
125 million rods and cones but only i million ganglion cells. In the face of this
discrepancy, we need to ask how detailed visual information can be preserved.
Examining the connections between cells in the retina can help resolve this
problem. You can think of the information flow through the retina as follow-
ing two paths: a direct path, from light receptors to bipolar cells to ganglion
cells, and an indirect path, in which horizontal cells may be interposed be-