lunes, 26 de diciembre de 2011

The visual cerebral cortex



 
Visual Pathways
-The visual pathway begins in the retina to end in the visual  cortex.
-The visual nerve signals leave the retinas through the optic nerves. At the optic chiasma, the optic nerve fibers from the nasal halves of the retinas cross to the
opposite sides ,where they join the fibers from the
opposite temporal retinas to form the optic tracts.
The fibers of each optic tract then synapse in the dorsal lateral geniculate nucleus of the thalamus, and from there geniculo-calcarine fibers pass by way of the optic radiation (also called the geniculocalcarine tract) to the primary visual cortex in the calcarine fissure area of the medial occipital lobe.
-Visual fibers also pass to several older areas of the brain:
(1) from the optic tracts to the  suprachiasmatic nucleus of the hypothalamus, presumably to controlcircadian rhythms that synchronize various physiologic changes of the body with night and day
(2) into the pretectal nuclei in the midbrain, to elicit reflex movements of the eyes to focus on objects of importance and to activate the pupillary light reflex
(3) into the superior colliculus, to control rapid directional movements of the two eyes (4) into the ventral lateral geniculate nucleus of the thalamus and surrounding basal regions of the brain,presumably to help control some of the body’s behavioral functions.

Thus, the visual pathways can be divided roughly into
1-an  old system to the midbrain and base of the forebrain
2- a new system for direct transmission of visual signals into the visual cortex located in the occipital lobes.
In human beings, the new system is responsible for perception of virtually all
aspects of visual form(colors,and other conscious vision).Conversely,in many
primitive animals,even visual form is detected by the older system,using the
superior colliculus in the same manner that the visual cortex is used in
mammals.
Function of the Dorsal Lateral Geniculate Nucleus
of the Thalamus
The optic nerve fibers of the new visual system terminate in the dorsal lateral
geniculate nucleus, located at the dorsal end of the thalamus and also called
simply the lateral geniculate body, .The dorsal lateral geniculate nucleus serves two principal functions:
First,it relays visual information from the optic tract to the visual cortex by way of the optic radiation(also called the geniculocalcarine tract).This relay function is so accurate that there is exact point-to-point transmission with a high degree of spatial fidelity all the way from the retina to the visual cortex.

The second major function of the dorsal lateral geniculate nucleus is to “gate”the transmission of signals to the visual cortex that is to control how much of the signal is allowed to pass to the cortex.
The nucleus receives gating control signals from two major sources:
(1) corticofugal fibersreturning in a backward direction from the primary visual cortex to the lateral geniculate nucleus
 (2) reticular areas of the mesencephalon. Both of these are inhibitory and,when
stimulated,can turn off transmission through selected portions of the dorsal lateral geniculate nucleus.It is assumed that both of these gating circuits help high-
light the visual information that is allowed to pass.
Finally,the dorsal lateral geniculate nucleus is divided in another way:
(1) Layers I and II are called magnocellular layers because they contain large
neurons.These receive their input almost entirely from the large type Y retinal ganglion cells.This magnocellular system provides a rapidly conducting pathway to
the visual cortex.However,this system is color blind, transmitting only black-and-white information.
Also,its point-to-point transmission is poor because there
are not many Y ganglion cells,and their dendrites spread widely in the retina.
(2) Layers III through VI are called parvocellular layers because they contain
large numbers of small to medium-sized neurons.
These neurons receive their input almost entirely from the type X retinal ganglion cells that transmit color and convey accurate point-to-point spatial information,but at only a moderate velocity of conduction rather than at high velocity.

Organization and Function  of the Visual Cortex
 the visual cortex located primarily on the medial aspect of the occipital lobes
Like the cortical representations of the other sensory systems,the visual cortex is divided into a primary visual cortex and secondary visual areas.
Primary Visual Cortex.
 The primary visual cortex  lies in the calcarine fissure area, extending forward from the  occipital pole on the  medial aspect of each occipital cortex.This area is the terminus of direct visual signals from the eyes.
Signals from the macular area of the retina terminate near the occipital pole
while signals from the more peripheral retina terminate at or in concentric half circles anterior to the pole but still along the calcarine fissure on the medial occipital lobe.
The upper portion of the retina is represented superiorly and the lower portion inferiorly.
Note in the figure the especially large area that represents the macula.It is to this region that the retinal fovea transmits its signals.The fovea is responsible for
the highest degree of visual acuity.Based on retinal area,the fovea has several hundred times as much representation in the primary visual cortex as do the most peripheral portions of the retina.
The primary visual cortex is also called visual area I. Still another name is the striate cortex because this area has a grossly striated appearance.





Secondary Visual Areas of the Cortex.
 The secondary visual areas,also called visual association areas, lie lateral anterior,superior,and inferior to the primary visual cortex.Most of these areas also fold
outward over the lateral surfaces of the occipital and parietal cortex,.Secondary signals are transmitted to these areas for analysis of visual meanings.For
instance,on all sides of the primary visual cortex is
Brodmann’s area 18 ,which is where virtually all signals from the primary visual cortex pass next.Therefore,Brodmann’s area 18 is called visual
area II, or simply V-2.The other,more distant secondary visual areas have specific designations—V-3,V-4
and so forth up to more than a dozen areas.The
importance of all these areas is that various aspects
of the visual image are progressively dissected and
analyzed.
Layered Structure of the Primary
Visual Cortex
Like almost all other portions of the cerebral cortex
the primary visual cortex has six distinct layers
Also,as is true for the other sensory systems,the geniculocalcarine fibers terminate
mainly in layer IV.But this layer,too,is organized into
subdivisions.The rapidly conducted signals from the Y
retinal ganglion cells terminate in layer IVca, and
from there they are relayed vertically both outward
toward the cortical surface and inward toward deeper
levels.
The visual signals from the medium-sized optic
nerve fibers,derived from the X ganglion cells in the
retina,also terminate in layer IV,but at points differ-
ent from the Y signals.They terminate in layers IVa
and IVcb,the shallowest and deepest portions of layer IV,From there,these signals are transmitted vertically both toward the
surface of the cortex and to deeper layers.It is these
X ganglion pathways that transmit the accurate point-to-point type of vision as well as color vision.
Vertical Neuronal Columns in the Visual Cortex.
The visualcortex is organized structurally into several million vertical columns of neuronal cells,each column having a diameter of 30 to 50 micrometers.The same vertical columnar organization is found throughout the cerebral cortex for the other senses as well (and also in the motor and analytical cortical regions).Each column
represents a functional unit.One can roughly calculate that each of the visual vertical columns has perhaps1000 or more neurons.
After the optic signals terminate in layer IV,they are further processed as they spread both outward and inward along each vertical column unit.This processing is believed to decipher separate bits of visual information at successive stations along the pathway.The signals that pass outward to layers I,II,and III eventually transmit signals for short distances laterally in the cortex.Conversely,the signals that pass inward to
layers V and VI excite neurons that transmit signals much greater distances.

“Color Blobs” in the Visual Cortex.
 Interspersed among the primary visual columns as well as among the columns of some of the secondary visual areas are special column-like areas called  color blobs. They receive lateral signals from adjacent visual columns and are activated specifically by color signals.Therefore,it is presumed that these blobs are the primary areas for deciphering color.
Interaction of Visual Signals from the Two Separate Eyes.
Recall that the visual signals from the two separate eyes are relayed through separate neuronal layers in the lateral geniculate nucleus.These signals still remain separated from each other when they arrive in layer IV of the primary visual cortex.In fact,layer IV is interlaced with stripes of neuronal columns,each stripe about 0.5 millimeter wide;the signals from one eye enter the columns of every other stripe,alternating with signals from the second eye.This cortical area deciphers whether the respective areas of the  two visual images from the two separate eyes are “in register”with each other—that is,whether corresponding points from the two retinas fit with each other.In
turn,the deciphered information is used to adjust the directional gaze of the separate eyes so that they will fuse with each other (be brought into “register”).The information observed about degree of register of images from the two eyes also allows a person to distinguish the distance of objects by the mechanism of
stereopsis.
Two Major Pathways for Analysis of Visual Information
(1) The Fast“Position” and “Motion” Pathway;
(2) The Accurate Color Pathway that after leaving the primary visual cortex,the visual information is analyzed in two major pathways in the secondary visual areas.
1. Analysis of Third-Dimensional Position, Gross Form, and
Motion of Objects.
One of the analytical pathways,
analyzes the third-dimensional positions of visual objects in the space around the body.This pathway also analyzes the gross physical form of the visual scene as well as motion in the scene.In other words,this pathway tells where every object is during each instant and whether it is moving.After leaving the primary visual cortex,the signals flow generally into the posterior midtemporal area and upward into the broad occipitoparietal cortex.At the anterior border of the parietal cortex,the signals overlap with signals from the posterior somatic association areas that analyze three-dimensional aspects of somatosensory signals.The signals transmitted in this  position-form-motion pathway are mainly from the large Y optic nerve fibers of the retinal Y ganglion cells,transmitting rapid signals but depicting only black and white with no color.
2. Analysis of Visual Detail and Color. passing frointo secondary visual areas of the inferior,ventral,and medial regions of the occipital and temporal cortex,show the principal pathway for analysis of visual detail.Separate portions of this pathway specifically dissect out color as well.Therefore,this pathway is concerned with such visual feats as recognizing letters, reading,determining the texture of surfaces,deter-mining detailed colors of objects,and deciphering from all this information what the object is and what it means.m the primary visual cortex

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