Sensory Physiology 307The tectal system is also involved in the control of the
intrinsic eye muscles—the iris and the muscles of the ciliary
body. Shining a light into one eye stimulates the pupillary
reflex, in which both pupils constrict. This is caused by acti-
vation of parasympathetic neurons in the superior colliculus.
Postganglionic axons from the ciliary ganglia behind the eyes,
in turn, stimulate constrictor fibers in the iris (see fig. 10.28 ).
Contraction of the ciliary body during accommodation also
involves parasympathetic stimulation by the superior colliculus.
Surprisingly, the ability to constrict the pupils maximally
(by 95% in strong light) depends on light striking the gan-
glion cell layer, as well as the rods and cones. Scientists have
discovered ganglion cells that respond to overall illumina-
tion ( luminance ) rather than to patterns and other details of
seen objects. These ganglion cells make up a small population
(less than 2% of the total) that contains a recently discovered
photosensitive pigment called melanopsin. The melanopsin-
containing ganglion cells depolarize and produce action
potentials in response to light.
The melanopsin-containing ganglion cells seem to be
uniquely responsible for the non-image-forming functions
of the retina. These include: (1) the pupillary reflex (through
ganglion cell projection to the optic tectum; see fig. 10.28 );
(2) the entrainment of circadian rhythms to the light/dark
cycle (through ganglion cell projections to the suprachi-
asmatic nucleus, discussed in chapter 8, section 8.3); and
(3) the suppression of the pineal gland secretion of melatonin
by light (chapter 11, fig. 11.33; this hormone participates in
the regulation of circadian rhythms). The melanopsin in these
ganglion cells allows them to respond directly to light, which
supplements the information they receive from rods and cones.
Experiments with mice that lack melanopsin demonstrate that
the input from rods and cones to the melanopsin-containing
ganglion cells can produce some pupillary constriction and
circadian light/dark entrainment, but pupillary constrictions
greater than 50% appear to require melanopsin.
10.8 Neural Processing of Visual Information
Electrical activity in ganglion cells of the retina and in neu-
rons of the lateral geniculate nucleus and cerebral cortex is
evoked in response to light on the retina. The way in which
each type of neuron responds to light at a particular point
on the retina provides information about how the brain
interprets visual information.| CHECKPOINT
14a. Describe the layers of the retina and trace the path of
light and of nerve activity through these layers.
14b. Describe the photochemical reaction in the rods and
explain how dark adaptation occurs.- Describe the electrical state of photoreceptors in the
dark. Explain how light affects the electrical activity
of retinal cells.
16a. Explain what is meant by the trichromatic theory of
color vision.
16b. Compare the architecture of the fovea centralis with
more peripheral regions of the retina. How does this
architecture relate to visual acuity and sensitivity? - Trace the neural pathways and explain the functions
of the geniculostriate system and the tectal system.
LEARNING OUTCOMESAfter studying this section, you should be able to:- Describe some of the higher processing of visual
information.
Light cast on the retina directly affects the activity of photo-
receptors and indirectly affects the neural activity in bipolar and
ganglion cells. The part of the visual field that affects the activity
of a particular ganglion cell can be considered its receptive field.
As previously mentioned, each cone in the fovea has a private line
to a ganglion cell, and thus the receptive fields of these ganglion
cells are equal to the width of one cone (about 2 m m). By contrast,
ganglion cells in more peripheral parts of the retina receive input
from hundreds of photoreceptors, and are therefore influenced by
a larger area of the retina (about 1 mm in diameter).Ganglion Cell Receptive Fields
Studies of the electrical activity of ganglion cells have yielded
some interesting results. In the dark, each ganglion cell dis-
charges spontaneously at a slow rate. When the room lights are
turned on, the firing rate of many (but not all) ganglion cells
increases slightly. With some ganglion cells, however, a small
spot of light directed at the center of their receptive fields elicits
a large increase in firing rate. Surprisingly, then, a small spot of
light can be a more effective stimulus than a larger area of light!
When the spot of light is moved only a short distance away
from the center of the receptive field, the ganglion cell responds
in the opposite manner. The ganglion cell that was stimulated
with light at the center of its receptive field is inhibited by light
in the periphery of its field. The responses produced by light in
the center and by light in the “surround” of the visual field are
antagonistic. Those ganglion cells that are stimulated by light at
the center of their visual fields are said to have on-center fields;
those that are inhibited by light in the center and stimulated by
light in the surround have off-center fields ( fig. 10.47 ).
The reason wide illumination of the retina has a weaker effect
than pinpoint illumination is now clear; diffuse illumination gives
the ganglion cell conflicting orders—on and off. Because of the
antagonism between the center and surround of ganglion cell
receptive fields, the activity of each ganglion cell is a result of the
difference in light intensity between the center and surround of its
visual field. This is a form of lateral inhibition that helps accentu-
ate the contours of images and improve visual acuity.