Page 371 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Visual Field, Visual Pathway, Central Objects located in the nasal half of the visual
Processing of Visual Stimuli field of each eye (! B, blue and green) are im-
aged in the temporal half of each retina and
The visual field (! A) is the area visualized by vice versa. Along the visual pathway, fibers of
the immobile eye with the head fixed. the optic nerve from the temporal half of each
retina remain on the same side (! B, blue and
The visual field is examined by perimetry. The sub-
ject’s eye is positioned in the center of the perimeter, green), whereas the fibers from the nasal half
Central Nervous System and Senses Lesions of the retina, or of the central visual pathway Lesions of the left optic nerve for instance cause defi-
of each retina decussate at the optic chiasm
which is a hollow hemispherical instrument. The sub-
(! B, orange and red). Fibers from the fovea
ject must then report when laterally flashed points of
centralis are present on both sides.
light appear in or disappear from the visual field. An
area of lost vision within the visual field is a scotoma.
cits in the entire left visual field (! B, a), whereas le-
can cause scotoma. The blind spot (! A) is a normal
sions of the left optic tract produce deficits in the
scotoma occurring at 15 degrees temporal and is
right halves of both visual fields (! B, c). Damage to
caused by nasal interruption of the retina by the optic
the median optic chiasm results in bilateral temporal
disk (! p. 349 B). In binocular vision (! p. 361 A),
deficits, i.e., bitemporal hemianopia (! B, b).
the blind spot of one eye is compensated for by the
other. The visual field for color stimuli is smaller than
Fibers of the optic tract extend to the lateral
that for light–dark stimuli. If, for example, a red ob-
geniculate body (! B) of the thalamus, the six
ject is slowly moved into the visual field, the move-
manner. Axons of the ipsilateral eye terminate
the object.
on layers 2, 3 and 5, while those of the con-
The retina contains more than 10 photosen-
8
12 ment will be identified more quickly than the color of layers of which are organized in a retinotopic
tralateral eye terminate on layers 1, 4 and
sors connected by retinal neurons (! p. 354) to 6. The M cell axons communicate with cells of
ca. 10 retinal ganglion cells. Their axons form magnocellular layers 1 and 2, which serve as a
6
the optic nerve. The convergence of so many relay station for motion-related stimuli that are
sensors on only a few neurons is particularly rapidly conducted to the motor cortex. The P
evident in the retinal periphery (1000 : 1). In cell axons project to the parvocellular layers
the fovea centralis, however, only one or a few 3–6, the main function of which is to process
cones are connected to one neuron. Due to the colors and shapes. The neurons of all layers
low convergence of impulses from the fovea, then project further through the optic radia-
there is a high level of visual acuity with a low tion (arranged also retinotopically) to the pri-
level of light sensitivity, whereas the high con- mary visual cortex (V 1) and, after decussating,
vergence of signals from the periphery pro- to further areas of the visual cortex (V 2–5) in-
duces the reverse effect (cf. spatial summation; cluding pathways to the parietal and temporal
! p. 353 C3). cortex. Magnocellular input reaches the
Ganglion cells. Three types of ganglion cells parietal cortex also via the superior colliculi
can be found in the retina: (1) 10% are M (or α (see below) and the pulvinar.
or Y) cells of the magnocellular system; their
fast-conducting axons emit short phasic im- The primary visual cortex (V 1) is divided depth-wise
pulses in response to light and are very sensi- (x-axis) into six retinotopic layers numbered I to VI
(! p. 333 A). Cells of the primary visual cortex are ar-
tive to movement; (2) 80% are the P (or " or X) ranged as three-dimensional modular hyper-
cells of the parvicellular system; their thin columns (3 ! 1 ! 1 mm) representing modules for
axons have small receptive fields (high spatial analysis of all sensory information from correspond-
resolution), persistently react to constant light ing areas of both retinas (! p. 360). Adjacent hyper-
(tonic reaction), and therefore permit pattern columns represent neighboring retinal regions. Hy-
and color analysis. Both types have equal den- percolumns contain ocular dominance columns (y-
sities of ON and OFF cells (! p. 354). (3) 10% axis), orientation columns (z-axis), and cylinders (x-
are γ (or W) cells of the coniocellular system; axis). The dominance columns receive alternating
input from the right and left eye, orientation
their very thin axons project to the mesen- columns process direction of stimulus movement
cephalon and regulate pupil diameter (see and cylinders receive information of colors.
358
below) and reflex saccades (! pp. 348, 360).
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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