Download The occipital lobe is the visual processing center of the mammalian

Tracking eye pathway to brain :
FIGURE 1 5 - 1 4 The visual pathways. The solid
blue lines represent nerve fibers that extend from
the retina to the occipital cortex and carry afferent
visual information from the right half of the visual
field. The broken blue lines show the pathway from
the left half of the visualfields. The black lines
represent the efferent pathway for the pupillary light
reflex.
Brain lobes:
The occipital lobe is the visual processing center of the
mammalian brain containing most of the anatomical
region of the visual cortex The primary visual cortex is
Brodmann area 17, commonly called V1 (visual one).
Human V1 is located on the medial side of the occipital
lobe within the calcarine sulcus; the full extent of V1
often continues onto the posterior pole of the occipital
lobe. V1 is often also called striate cortex because it can
be identified by a large stripe of myelin, the Stria of
Gennari. Visually driven regions outside V1 are called
extra striate cortex. There are many extrastriate regions,
and these are specialized for different visual tasks, such
as visuospatial processing, color discrimination and
motion perception.
Anatomy
The occipital lobes are the smallest of four lobes in the
human cerebral cortex. Located in the rearmost portion
of the skull, the occipital lobes are part of the forebrain.
It should be noted that the cortical lobes are not defined
by any internal structural features, but rather by the
bones of the skull that overlie them. Thus, the occipital
lobe is defined as the part of the cerebral cortex that lies
underneath the occipital bone.
The lobes rest on the tentorium cerebelli, a process of
dura mater that separates the cerebrum from the
cerebellum. They are structurally isolated in their
respective cerebral hemispheres by the separation of the
cerebral fissure. At the front edge of the occipital are
several lateral occipital gyri, which are separated by
lateral occipital sulcus.
The occipital aspects along the inside face of each
hemisphere are divided by the calcarine sulcus. Above
the medial, Y-shaped sulcus lies the cuneus, and the area
below the sulcus is the lingual gyrus.
THE VISUAL CORTEX
Visual information is relayed from the lateral geniculate
body to the visual cortex via myelinated axons in the
optic radiations. As described later, multiple retinotopic
maps of the visual
world are present within the cortex. The primary visual
cortex is the main way station for incoming visual
signals. Ultimately,
however, visually responsive neurons within at least
Six parts of the occipital cortex and within the temporal
and
parietal lobes form separate visual areas, each with its
own
map of the retina.
Function
The most important functional aspect of the occipital
lobe is that it contains the primary visual cortex.
Retinal sensors convey stimuli through the optic tracts to
the lateral geniculate bodies, where optic radiations
continue to the visual cortex. Each visual cortex receives
raw sensory information from the outside half of the
retina on the same side of the head and from the inside
half of the retina on the other side of the head.
The cuneus (Brodmann's area 17) receives visual
information from the contralateral superior retina
representing the inferior visual field. The lingula receives
information from the contralateral inferior retina
representing the superior visual field. The retinal inputs
pass through a "way station" in the lateral geniculate
nucleus of the thalamus before projecting to the cortex.
Cells on the posterior aspect of the occipital lobes' gray
matter are arranged as a spatial map of the retinal field.
Functional neuroimaging reveals similar patterns of
response in cortical tissue of the lobes when the retinal
fields are exposed to a strong pattern.
If one occipital lobe is damaged, the result can be
homonomous vision loss from similarly positioned "field
cuts" in each eye. Occipital lesions can cause visual
hallucinations. Lesions in the parietal-temporal-occipital
association area are associated with color agnosia,
movement agnosia, and agraphia.
Functional anatomy
The occipital lobe is divided into several functional
visual areas. Each visual area contains a full map of the
visual world. Although there are no anatomical markers
distinguishing these areas (except for the prominent
striations in the striate cortex), physiologists have used
electrode recordings to divide the cortex into different
functional regions.
The first functional area is the primary visual cortex. It
contains a low-level description of the local orientation,
spatial-frequency and color properties within small
receptive fields. Primary visual cortex projects to the
occipital areas of the ventral stream (visual area V2 and
visual area V4), and the occipital areas of the dorsal
stream - visual area V3, visual area MT (V5), and the
dorsomedial area
Visual cortex:
The term visual cortex refers to the primary visual
cortex (also known as striate cortex or V1) and
extrastriate visual cortical areas such as V2, V3, V4, and
V5. The primary visual cortex is anatomically equivalent
to Brodmann area 17, or BA17.
The primary visual cortex, V1, is the koniocortex
(sensory type) located in and around the calcarine fissure
in the occipital lobe. It receives information directly from
the lateral geniculate nucleus.
The dorsal stream (green) and ventral stream (purple) are
shown. They originate from primary visual cortex.
V1 transmits information to two primary pathways,
called the dorsal stream and the ventral stream:


The dorsal stream begins with V1, goes through
Visual area V2, then to the dorsomedial area and
Visual area MT (also known as V5) and to the
posterior parietal cortex. The dorsal stream,
sometimes called the "Where Pathway" or "How
Pathway", is associated with motion, representation
of object locations, and control of the eyes and
arms, especially when visual information is used to
guide saccades or reaching.
The ventral stream begins with V1, goes through
visual area V2, then through visual area V4, and to
the inferior temporal cortex. The ventral stream,
sometimes called the "What Pathway", is associated
with form recognition and object representation. It
is also associated with storage of long-term
memory.
The dichotomy of the dorsal/ventral pathways (also
called the "where/what" or "action/perception" streams)
was first defined by Ungerleider and Mishkin and is still
contentious among vision scientists and psychologists. It
is probably an over-simplification of the true state of
affairs in the visual cortex. It is based on the findings that
visual illusions such as the Ebbinghaus illusion may
distort judgements of a perceptual nature, but when the
subject responds with an action, such as grasping, no
distortion occurs. However, recent work suggests that
both the action and perception systems are equally fooled
by such illusions.
Neurons in the visual cortex fire action potentials when
visual stimuli appear within their receptive field. By
definition, the receptive field is the region within the
entire visual field which elicits an action potential. But
for any given neuron, it may respond to a subset of
stimuli within its receptive field. This property is called
neuronal tuning. In the earlier visual areas, neurons have
simpler tuning. For example, a neuron in V1 may fire to
any vertical stimulus in its receptive field. In the higher
visual areas, neurons have complex tuning. For example,
in the inferior temporal cortex (IT), a neuron may only
fire when a certain face appears in its receptive field
Primary visual cortex (V1)
The primary visual cortex is the best studied visual area
in the brain. In all mammals studied, it is located in the
posterior pole of the occipital cortex (the occipital cortex
is responsible for processing visual stimuli). It is the
simplest, earliest cortical visual area. It is highly
specialized for processing information about static and
moving objects and is excellent in pattern recognition.
The functionally defined primary visual cortex is
approximately equivalent to the anatomically defined
striate cortex. The name "striate cortex" is derived from
the stria of Gennari, a distinctive stripe visible to the
naked eye that represents myelinated axons from the
lateral geniculate body terminating in layer 4 of the gray
matter.
The primary visual cortex is divided into six functionally
distinct layers, labelled 1 through 6. Layer 4, which
receives most visual input from the lateral geniculate
nucleus (LGN), is further divided into 4 layers, labelled
4A, 4B, 4Cα, and 4Cβ. Sublamina 4Cα receives most
magnocellular input from the LGN, while layer 4Cβ
receives input from parvocellular pathways.
Function
V1 has a very well-defined map of the spatial
information in vision. For example, in humans the upper
bank of the calcarine sulcus responds strongly to the
lower half of visual field (below the center), and the
lower bank of the calcarine to the upper half of visual
field. Conceptually, this retinotopy mapping is a
transformation of the visual image from retina to V1. The
correspondence between a given location in V1 and in
the subjective visual field is very precise: even the blind
spots are mapped into V1. Evolutionarily, this
correspondence is very basic and found in most animals
that possess a V1. In human and animals with a fovea in
the retina, a large portion of V1 is mapped to the small,
central portion of visual field, a phenomenon known as
cortical magnification. Perhaps for the purpose of
accurate spatial encoding, neurons in V1 have the
smallest receptive field size of any visual cortex
microscopic regions.
V2:
Visual area V2, also called prestriate cortex, is the
second major area in the visual cortex, and the first
region within the visual association area. It receives
strong feed forward connections from V1 and sends
strong connections to V3, V4, and V5. It also sends
strong feedback connections to V1.
Anatomically, V2 is split into four quadrants, a dorsal
and ventral representation in the left and the right
hemispheres. Together these four regions provide a
complete map of the visual world. Functionally, V2 has
many properties in common with V1. Cells are tuned to
simple properties such as orientation, spatial frequency,
and color.
Third visual complex, including area V3
The term third visual complex refers to the region of
cortex located immediately in front of V2, which
includes the region named visual area V3 in humans. The
"complex" nomenclature is justified by the fact that some
controversy still exists regarding the exact extent of area
V3, with some researchers proposing that the cortex
located in front of V2 may include two or three
functional subdivisions. For example, David Van Essen
and others (1986) have proposed that the existence of a
"dorsal V3" in the upper part of the cerebral hemisphere,
which is distinct from the "ventral V3" (or ventral
posterior area, VP) located in the lower part of the brain.
Dorsal and ventral V3 have distinct connections with
other parts of the brain, appear different in sections
stained with a variety of methods, and contain neurons
that respond to different combinations of visual stimulus
(for example, colour-selective neurons are more common
in the ventral V3). Additional subdivisions, including
V3A and V3B have also been reported in humans. These
subdivisions are located near dorsal V3, but do not adjoin
V2.
Dorsal V3 is normally considered to be part of the dorsal
stream, receiving inputs from V2 and from the primary
visual area and projecting to the posterior parietal cortex.
It may be anatomically located in Brodmann area 19.
Recent work with fMRI has suggested that area V3/V3A
may play a role in the processing of global motion Other
studies prefer to consider dorsal V3 as part of a larger
area, named the dorsomedial area (DM), which contains
a representation of the entire visual field
V4
Visual area V4 is one of the visual areas in the
extrastriate visual cortex of the macaque monkey. It is
located anterior to V2 and posterior to posterior
inferotemporal area (PIT). It comprises at least four
regions (left and right V4d, left and right V4v), and some
groups report that it contains rostral and caudal
subdivisions as well. It is unknown what the human
homologue of V4 is, and this issue is currently the
subject of much scrutiny.
V4 is the third cortical area in the ventral stream,
receiving strong feedforward input from V2 and sending
strong connections to the PIT. It also receives direct
inputs from V1, especially for central space. In addition,
it has weaker connections to V5 and dorsal prelunate
gyrus (DP).
V5/MT
Visual area V5, also known as visual area MT (middle
temporal), is a region of extrastriate visual cortex that is
thought to play a major role in the perception of motion,
the integration of local motion signals into global
percepts and the guidance of some eye movements.
Blood supply:
The primary visual cortex receives its blood from the
calcarine branch of the posterior cerebral artery. The
remainder
of the occipital lobe is supplied by other branches of
this artery. The arterial supply can be (rarely) interrupted
by
emboli or by compression of the artery between the free
edge of the tentorium and enlarging or herniating
portions of the brain.
CLINICAL CORRELATIONS:
The accurate examination of visual defects in a
patient is of considerable importance in localizing
lesions. Such lesions may be in the eye, retina,
optic nerve, optic chiasm or tracts, or visual cortex.
Impaired vision in one eye is usually due to disorder
involving the eye, retina, or the optic nerve (Fig 1516A).Field defects can affect one or both visual
fields. If the lesion is in the optic chiasm, optic
tracts, or visual cortex, both eyes will show field
defects.A chiasmatic lesion (often owing to a
pituitary tumor or a lesion around the sella turcica)
can injure the decussating axons of retinal ganglion
cells within the optic chiasm. These axons originate
in the nasal halves of the two retinas. Thus, this
type of lesion produces bitemporal hemianopsia,
characterized by blindness in the lateral or temporal
half of the visual field for each eye (Fig 15-16B).
Lesions behind the optic chiasm cause a
fielddefectin the temporal field of one eye, together
with a field defect in the nasal (medial) field of the
other eye. The result is a homonymous
hemianopsia in which the visual field defect is on
the side opposite the lesion (Fig 15-16C, 15-16E,
and 15-17). Because Meyer's loop carries optic
radiation fibers representing the upper part of the
contralateral field, temporal lobe lesions can
produce a visual field deficit involving the
contralateral superior ("pie in the sky") quadrant.
This visual field defect is called a superior
quadrantanopsia (Fig 15-16D). An example is
discussed in Clinical Illustration 15-1.