Human Physiology, 14th edition (2016)

304 Chapter 10

Visual Acuity and Sensitivity

While reading or similarly viewing objects in daylight, each
eye is oriented so that the image falls within a tiny area of the
retina called the fovea centralis. The fovea is a pinhead-size
pit ( fovea 5 pit) within a yellow area of the retina called the
macula lutea (see fig.  10.30 ). The pit is formed as a result
of the displacement of neural layers around the periphery;
therefore, light falls directly on photoreceptors in the center
( fig. 10.43 ). Light falling on other areas, by contrast, must pass
through several layers of neurons.
There are approximately 120 million rods and 6 million
cones in each retina, but only about 1.2 million axons enter the
optic nerve of each eye. This gives an overall convergence ratio
of photoreceptors on ganglion cells of about 105 to 1. This
is misleading, however, because the degree of convergence
is much lower for cones than for rods. In the fovea, the ratio is
1 to 1.
The photoreceptors are distributed in such a way that the
fovea contains only cones, whereas more peripheral regions of
the retina contain a mixture of rods and cones. Approximately
4,000 cones in the fovea provide input to approximately 4,000
ganglion cells; each ganglion cell in this region, therefore, has
a private line to the visual field. Each ganglion cell in the fovea
thus receives input from an area of retina corresponding to the
diameter of one cone (about 2  m m). Because of this, the only
part of the visual field that is seen very clearly is the tiny part
(about 1%) that falls on the fovea centralis. We are unaware of
this because very rapid eye movements (called saccadic eye
movements ) continually shift different parts of the visual field
onto the fovea.
Peripheral to the fovea, many rods synapse with a single
bipolar cell, and many bipolar cells synapse with a single

ganglion cell. A single ganglion cell outside the fovea thus

may receive input from large numbers of rods, corresponding

to an area of about 1 mm^2 on the retina ( fig. 10.44 ).

Because each cone in the fovea has a private line to a gan-

glion cell, and each ganglion cell receives input from only a

tiny region of the retina, visual acuity is greatest and sensitiv-

ity to low light is poorest when light falls on the fovea. In dim

light only the rods are activated, and vision is best out of the

corners of the eye so that the image falls away from the fovea.

Under these conditions, the convergence of large numbers of

rods on a single bipolar cell and the convergence of large num-

bers of bipolar cells on a single ganglion cell increase sensitiv-

ity to dim light at the expense of visual acuity. Night vision is

therefore less distinct than day vision.

The difference in visual sensitivity between cones in the

fovea centralis and rods in the periphery of the retina can eas-

ily be demonstrated using a technique called averted vision. If

you go out on a clear night and stare hard at a very dim star, it

will disappear. This is because the light falls on the fovea and

is not sufficiently bright to activate the cones. If you then look

slightly off to the side, the star will reappear because the light

falls away from the fovea, onto the rods.

Neural Pathways from the Retina

As a result of light refraction by the cornea and lens, the right

half of the visual field is projected to the left half of the retina of

both eyes (the temporal half of the left retina and the nasal half of

the right retina). The left half of the visual field is projected to the

right half of the retina of both eyes. The temporal half of the left

retina and the nasal half of the right retina therefore see the same

image. Axons from ganglion cells in the left (temporal) half of

Retina

Fovea

Light

Rods

Bipolar cells

Ganglion cells

Cones

in fovea

Rods

Optic disc

Optic nerve

Figure 10.43 The fovea centralis. When the
eyes “track” an object, the image is cast upon the fovea
centralis of the retina. The fovea is literally a “pit” formed
by parting of the neural layers. In this region, light falls
directly on the photoreceptors (cones).