Human Physiology, 14th edition (2016)

308 Chapter 10

A population of ganglion cells termed direction-selective
ganglion cells has recently been discovered. These differ from
the ganglion cells previously described in two respects: (1) a light
shining on the center of the visual field stimulates action poten-
tials when the light is turned on and also when the light is turned
off; and (2) each of these ganglion cells responds best to light
moving in a certain “preferred” direction. There are subtypes of
direction-selective ganglion cells that respond best to light mov-
ing in their specfic directions.

Lateral Geniculate Nuclei

Each lateral geniculate nucleus of the thalamus receives input
from ganglion cells in both eyes. The right lateral geniculate
receives input from the right half of each retina (correspond-
ing to the left half of the visual field); the left lateral geniculate
receives input from the left half of each retina (corresponding
to the right half of the visual field). However, within the lateral
geniculate, each neuron is activated by input from only one eye.
The receptive field of each ganglion cell is the part of the
retina it “sees” through its photoreceptor input. The receptive
field of lateral geniculate neurons, similarly, is the part of the
retina it “sees” through its ganglion cell input. Experiments in
which the lateral geniculate receptive fields are mapped with a
spot of light reveal that they are circular, with an antagonistic
center and surround, much like the ganglion cell receptive fields.

Cerebral Cortex

Projections of axons from the lateral geniculate bodies to area

17 of the occipital lobe form the optic radiation (see fig. 10.45 ).

Because these fiber projections give area 17 a striped or stri-

ated appearance, this area is also known as the striate cortex.

As mentioned earlier, neurons in area 17 project to areas 18 and

19 of the occipital lobe. Cortical neurons in areas 17, 18, and

19 are thus stimulated indirectly by light on the retina. On the

basis of their stimulus requirements, these cortical neurons are

classified as simple, complex, and hypercomplex.

The receptive fields of simple neurons are rectangular

rather than circular. This is because they receive input from lat-

eral geniculate neurons whose receptive fields are aligned in a

particular way (as illustrated in fig. 10.48 ). Simple cortical neu-

rons are best stimulated by a slit or bar of light located in a pre-

cise part of the visual field (of either eye) at a precise orientation.

The striate cortex (area 17) contains simple, complex, and

hypercomplex neurons. The other visual association areas, des-

ignated areas 18 and 19, contain only complex and hypercom-

plex cells. Complex neurons receive input from simple cells,

and hypercomplex neurons receive input from complex cells.

Complex and hypercomplex neurons have different stimulus

requirements than do simple neurons. Different complex or

hypercomplex neurons may be stimulated by different edges,

angles, or curves; they may require that these stimuli be in par-

ticular orientations, and further require that the stimuli be mov-

ing in particular directions. These neurons must then interact

with others to produce the cortical processes needed for the

perception of meaningful visual information.

Figure 10.47 Ganglion cell receptive fields. Each
ganglion cell receives input from photoreceptors in the retina
that are part of the ganglion cell’s receptive field. Because of the
antagonism between the field’s center and its surround, an image
that falls across the entire field has less effect than one that
excites just the center or surround. Because of this, edges of an
image are enhanced, improving the clarity of vision.

Ganglion Cell Receptive Fields

On-center Field Off-center Field

Off

On

Off

On

Light on center stimulates,

light on surround inhibits,

ganglion cell

Light on center inhibits,

light on surround stimulates,

ganglion cell

Spot of light

Bar of light

Light across both center

and surround is less effective

at stimulating ganglion cell

On

Off

On

Off

Figure 10.48 Stimulus requirements for simple

cortical neurons. Cortical neurons called simple cells have

rectangular receptive fields that are best stimulated by slits of

light of particular orientations. This may be due to the fact that

these simple cells receive input from ganglion cells that have

circular receptive fields along a particular line.

On-center receptive fields

Simple cortical

neurons in

cerebral cortex

Ganglion cells

in retina

Bar of light

on retina

Off surround

On center