Human Physiology, 14th edition (2016)

306 Chapter 10

the left retina pass to the left lateral geniculate nucleus of the
thalamus. Axons from ganglion cells in the nasal half of the right
retina cross over (decussate) in the X-shaped optic chiasma (see
fig.  10.32 ) , also to synapse in the left lateral geniculate body.
The left lateral geniculate thus receives input from both eyes that
relates to the right half of the visual field ( fig. 10.45 ).
The right lateral geniculate body, similarly, receives input
from both eyes relating to the left half of the visual field. Neu-
rons in both lateral geniculate bodies of the thalamus in turn
project to the striate cortex of the occipital lobe in the cerebral
cortex ( fig. 10.46 ). This area is also called area 17, in reference
to a numbering system developed by K. Brodmann in 1906.
Neurons in area 17 synapse with neurons in areas 18 and 19 of
the occipital lobe ( fig. 10.46 ).
Approximately 70% to 80% of the axons from the retina
pass to the lateral geniculate bodies and to the striate cortex.
This geniculostriate system is involved in perception of the
visual field. Put another way, the geniculostriate system is
needed to answer the question, What is it? Approximately 20%

to 30% of the fibers from the retina, however, follow a differ-

ent path to the superior colliculus of the midbrain (also called

the optic tectum ). Axons from the superior colliculus activate

motor pathways leading to eye and body movements. The

tectal system, in other words, is needed to answer the ques-

tion, Where is it?

Neural Control of Eye Movements

Movements of the eyes are produced by contractions of the

extrinsic eye muscles, innervated by neurons originating in

the brain. For example, vertical saccadic eye movements (dis-

cussed next) are initiated by neurons in the midbrain, whereas

horizontal movements are produced by activity of neurons in

the pons and medulla.

There are three types of eye movements coordinated by

the brain. Saccadic eye movements are very high-velocity

movements (400 8 to 800 8 per second) of both eyes that target

an image on the fovea centralis. For example, saccadic eye

movements keep the images of the words you are now read-

ing on or near the fovea, so the words at the middle and end

of this sentence can be as clearly seen as those at the begin-

ning. Smooth pursuit movements are slower (up to 30 8 per

second), and match the speed of moving objects to keep their

images at or near the fovea. Vergence movements (30 8 to 150 8

per second) cause the eyes to converge so that an image of an

object is brought to the fovea of both eyes, allowing the object

to be seen more clearly three-dimensionally.

Even when we fix our gaze on a stationary object, our eyes

are actually moving. Such fixational movements are very tiny

and imperceptible. These movements are required for vision,

however; sight is lost when fixational movements are pre-

vented under laboratory conditions, as would be expected by

sensory adaptation (due to the bleaching, or photo dissocia-

tion, reaction) in the stimulated photoreceptors. Fading of an

image when we attempt to fix our gaze is prevented mostly by

fixational movements called microsaccades, which are smaller

movements than saccades but generated by the same neural

mechanism.

Figure 10.45 The neural pathway for vision. The
neural pathway leading from the retina to the lateral geniculate
body, and then to the visual cortex, is needed for visual
perception. As a result of the crossing of optic fibers, the visual
cortex of each cerebral hemisphere receives input from the
opposite (contralateral) visual field.

Monocular field

Binocular field

Macular field

Retina

Macula lutea

Optic chiasma

Optic tract

Superior

colliculus

(visual

reflexes)

Optic radiation

Occipital lobe

of cerebrum

(visual cortex)

Lens

Point of fixation

(eyes are focusing

on a close object)

Lateral

geniculate nucleus

Optic nerve

Figure 10.46 The striate cortex (area 17) and the

visual association areas (18 and 19). Neural communication

between the striate cortex, the visual association areas, and

other brain regions is required for normal visual perception.

Frontal lobe

Temporal lobe

Parietal lobe

Occipital lobe

Cerebellum

19

18

17