VISUAL CORTEX

 

The visual cortex consists of the primary visual area (Brodmann's area 17) and the secondary visual area (Brodmann’s area 18 & 19).

Primary Visual Area (Area 17):

The primary visual area occupies largely the medial aspect of the occipital lobe.

It is present around and in the calcarine sulcus, with extensions into the cuneus and lingual gyrus.

Posteriorly it extends into the occipital pole, limited by the sulcus lunatus.

A small portion of variable size extends onto the posterolateral aspect of the pole.

Anteriorly the area extends forward above the calcarine sulcus as far as the parieto-occipital sulcus; below the calcarine sulcus, it extends forward a little farther.

On sectioning a fresh brain, the primary visual cortex can be recognized by its thinness and the characteristic presence of a white line or stria (of Gennari) in the gray matter (hence the term striate cortex or area striata). The white line is formed in the fourth layer of the cortex by the presence of myelinated fibers from the optic radiation and association fibers.

In the posterior part of the calcarine sulcus, the stria (lines) appear above and below the sulcus. But in the posterior part they are seen in the part of the cortex below the sulcus.

The striate cortex is bound by the cuneate sulcus above and the collateral sulcus below.

The area of the striate cortex is about 3000mm².

The shape is of an elongated ovoid, the narrow end being close to the splenium of the corpus callosum.

The cerebral cortex (except the allocortex of the hippocampal formation, or archipallium) has a laminar arrangement of neurons and their processes.

The general structure of the visual cortex resembles the primary sensory cortex in having six layers.

The layers of the visual cortex are as follows:

I. Plexiform lamina: external layer; dense interviewing of neurites (axons & dendrites) or neuropil, derived from intrinsic cortical neurons (mostly stellate cells) and pyramidal cells. There are also association fibers from other parts of the cortex.

II. External granular lamina: Has large number of nuclei visible (hence, granular). Contains bodies of neurons.

III. Pyramidal lamina: Contains “pyramidal” nucleus. Stellate interneurons with processes oriented vertically (fusiform cells) and horizontally (‘basket cells’).

The layers I-III are also called “SUPRAGRANULAR LAYERS”.

IV. Internal granular lamina: It is thin and contains stellate interneurons and few pyramidal cells. There is condensation of horizontal processes (known as the EXTERNAL BAND OF BAILLARGER). This layer is subdivided into IVA, IVB, IVC. This layer has the most densely arranged cells of any cortical area. This area is also most wide among all other cortical areas.

V. Ganglionic lamina: Contains stellate and pyramidal cells. This layer has the solitary neurons of Meynert, which pyramidal neurons and the largest seen in any cortical area. The cell bodies project to the superior colliculus and possibly to ocular motor nuclei. Interspersed between the neuronal cell bodies is a dense neutropil of dendrites and axons, extending to other levels of the cortex.

VI. Multiform lamina: Consists of small neurons mostly of ‘granular’ or stellate interneuron type with few pyramidal cells. Some of the latter (Martinotti neurons) send a long centrifugal axon into the plexiform layer and vertically arranged dendrites ramify into deeper layers of the cortex.

Layers V and VI are also called the “INFRAGRANULAR LAYERS”.

The striate cortex is the main receptor area for the optic radiation coming from the lateral geniculate nucleus, but other SECONDARY VISUAL AREAS, such as the surrounding peristriate and parastriate zones (area 18 & 19 of Brodman) also receive such fibers directly or through connections from the striate cortex (area 17). The cells in laminae II and III especially project to the secondary visual area.

The primary visual cortex receives the optic radiation fibers and the right half of the field of vision is represented in the visual cortex of the left cerebral hemisphere. The cells of lamina V project to the superior colliculus; those of lamina VI, to the lateral geniculate body.

The superior retinal quadrants (inferior field of vision) pass to the superior wall of the calcarine sulcus, while the inferior retinal quadrants (superior field of vision) pass to the inferior wall of the calcarine sulcus.

The macula lutea, which is the central area of the retina and the area for most perfect vision, is represented on the cortex in the posterior part of area 17 and accounts for one-third of the visual cortex. The peripheral parts of the retina are represented more anteriorly.